Smart Device Fabrication Strategies for Solution Processed ...Spyropoulos... · Smart Device Fabrication Strategies ... AM1.5G Air Mass 1.5 Global AZO Aluminum doped Zinc Oxide Ba

Smart Device Fabrication Strategies

for Solution Processed Solar Cells

Intelligente Herstellungsstrategien für lösungsprozessierte

Solarzellen

Der Technischen Fakultät

der Friedrich-Alexander- Universität

Erlangen- Nürnberg

zur

Erlangung des Doktorgrades Dr.- Ing.

vorgelegt von

Georgios Spyropoulos

aus Amaroussio, Greece

ii 2016 FAU Erlangen-Nürnberg

Als Dissertation genehmigt

von der Technischen Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg

Tag der mündlichen Prüfung: 13 / 01 / 2017

Vorsitzender des Promotionsorgans: Prof. Dr. –Ing. Reinhard Lerch

1. Gutachter: Prof. Dr. Christoph J. Brabec

2. Gutachter: Prof. Dr. Siegfried Bauer

2016 FAU Erlangen-Nürnberg iii

Dedicated to my parents; Dimitris Spyropoulos and Vassiliki Spyropoulou

my grandparents; Georgios Sypropoulos, Anastasia Spyropoulou and Efrosini Georgara

and a very inspirational person; Babis Nikolaidis

iv 2016 FAU Erlangen-Nürnberg

“People do not decide their futures,

they decide their habits and their habits decide their futures“

− Frederick Matthias Alexander

2016 FAU Erlangen-Nürnberg v

Zusammenfassung Intelligente Herstellungsstrategie für Lösungsprozessierte Solarzellen

Die Dünnschicht-Photovoltaik ist eine Schlüsseltechnologie im Bereich der erneuerbaren

Energien, da sie die Umwandlung von Licht in günstige Energie durch die Anwendung

lösungsprozessierter Drucktechniken wie zum Beispiel Rolle-zu-Rolle Verfahren erlaubt.

Zudem ermöglichen verformbare Substrate ästhetisch ansprechende, mechanisch flexible und

individualisierbare Module. Diese erleitern die elektronischen Anwendungen und die

Einbindung in architektonische Objekte. Der Fokus dieser Dissertation ist die Entwicklung

neuer Materialien und Herstellungsmethoden für lösungsprozessierte Solarzellen, welche

anschließend einfach vom Labormaßstab in den Produktionsmaßstab für Rolle-zu-Rolle

Verfahren übertragen werden können. Bei der Wahl der Materialien und Verfahren wurde

berücksichtigt, dass dabei nicht die Effizienz der Energieumwandlung als auch die Stabilität

und die Flexibilität geopfert wurden. Die physikalischen Eigenschaften von Materialen, die

aus der Lösung prozessiert wurden, wurden untersucht, um stabile, qualitativ hochwertige,

dünne Filme mit einer bestimmten Funktionalität innerhalb der Solarzellenarchitektur zu

erzielen. Dies umfasst Untersuchungen an Loch-/Elektron-Transportschichten, photoaktiven

Schichten und Elektroden. Es wurden gezielt intelligente Fertigungsverfahren entwickelt, um

langsame und kostenintensive Prozesse zu vermeiden. Desweiteren wurde die Möglichkeit

des Hochskalierens von Prototypen, welche die bisherigen wissenschaftlich und experimentell

gesetzten Grenzen überschreiten, anhand der Kombination verschiedener Materialsysteme

und Herstellungsmethoden mit neuartigen Techniken zur Laserstrukturierung demonstriert.

Die bisher beschriebenen Aspekte werden deutlich an Hand meines Kernprojektes ersichtlich,

bei dem zusammen mit Kolleg(inn)en Folgendes gezeigt wurde: i) die Anwendung von Rolle-

zu-Rolle nahen Verfahren für hocheffiziente, flexible, lösungsprozessierte organische

Tandem-Solarzellenmodule; ii) die Gewährleistung von langen Lebensdauern von Solarzelle;

iii) die Entwicklung eines innovativen lösungsprozessierten Herstellungsverfahren für

effiziente organische sowie Perovskite-Solarzellenmodule mittels einer tiefenselektiven

Laserstrukturierung; iv) basierend auf dem zuletzt erwähnten Herstellungsansatz wurde dieser

neuartige Prozess auf Module mit mehrfachen Halbleiter-Halbleiter Übergängen übertragen,

indem zwei individuell getrennt fabrizierte Solarzellen durch Laminieren in Serie geschaltet

wurden. Die sich daraus ergebenden Erkenntnisse sind ausschlaggebende Schritte in Richtung

effizienter, stabiler und flexibler Photovoltaik..

vi 2016 FAU Erlangen-Nürnberg

Acknowledgements

I would like to express my gratitude to my supervisor, Prof. Dr. Christoph Brabec not

only for giving me the opportunity to join a fantastic, multi-cultural group which improved

me as a scientist and human being, but also because he never constricted my scientific

curiosity and creativity.

I am very grateful to Dr. Tayebeh Ameri for her academic support, supervision and her

constant belief on my skills and passion. I would like also to thank Dr. Hans Egelhaaf for our

fruitful discussions and his scientific support. My special thanks go to Dr. Ning Li who

reinforced me with friend support and constructive scientific discussions. Additionally, I

would like to thank Dr. Jens Adams, Dr. Peter Kubis and Yi Hou for their contribution to my

work and their patience when I have been importunate. I could not but thank the people

responsible for the nice environment of my office; Stephan Lagner and Jose Dario Perea.

During my thesis time, I have been very lucky to meet people that made me believe I was

never far from home; Luca Lucera, Derya Baran, Michael Salvador, Nicola Gasparini and

Cesar Omar Ramirez Quiroz. They all contributed to this thesis with both moral and scientific

support. Apart from that, they create for me amazing memories here in Germany and they

have gained specific place in my heart.

Last but not least, I would like to thank my family in Greece for their emotional support

and their philosophy to invest in intellectual property and not in material goods.

2016 FAU Erlangen-Nürnberg vii

List of Abbreviations Ag Silver

AgNW Silver Nanowire

AM1.5G Air Mass 1.5 Global

AZO Aluminum doped Zinc Oxide

Ba(OH)2 Barium Hydroxide

BHJ Bulk-heterojunction

DLIT Dark Lock-In Thermography

Eg Bandgap

EHOMO Energy Level of Highest Occupied Molecular Orbital

EIS Electrochemical Impedance Spectroscopy

ELUMO Energy Level of Lowest Unoccupied Molecular Orbital

ETL Electron Transporting Layer

EQE External Quantum Efficiency

FF Fill Factor

GaAs Gallium Arsenide

GFF Geometric Fill Factor

HOMO Highest Occupied Molecular Orbital

HTL Hole Transporting Layer

I Current

IMI Indium Tin Oxide-Metal-Indium Tin Oxide

IML Intermediate Layer

IMPP Current at Maximum Point

IPA Isopropyl Alcohol

Iph Intensity of spectrum of the light source

IQE Internal Quantum Efficiency

ITO Indium Tin Oxide

JMPP Current Density at Maximum Power Point

Jsc Short Circuit Current Density

J-V Current Density-Voltage

LUMO Lowest Unoccupied Molecular Orbital

MPP Maximum Power Point

MoOx Molybdenum Oxide

NiO Nickel Oxide

OLED Organic Light Emitting Diode

OPV Organic Photovoltaic

OPV12 Polymer Donor received from Polyera

OSC Organic Solar Cell

P3HT Poly(3-hexylthiophene-2,5-diyl)

PBTZT-stat-

BDTT-8 Polymer Donor received from Merck

[60]PCBM [6,6]-phenyl C61 butyric acid methyl ester

[70]PCBM [6,6]-phenyl C71 butyric acid methyl ester

pDPP5T-2 Diketopyrrolopyrrole-quinquethiophene alternating copolymer

PEDOT:PSS Poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate)

PEI Polyethylenimine

Rp Parallel Resistance

viii 2016 FAU Erlangen-Nürnberg

Rs Series Resistance

SEM Scanning Electron Microscopy

T Temperature

TCA Transparent Conductive Adhesive

UV Ultra Violet

VMPP Voltage at Maximum Power Point

ZnO Zinc Oxide

k Boltzmann Constant

h Planck’s constant

ν Frequency

q Elementary Charge

ΔE Energetic Loss

μ Mobility

π – π* pi-pi*

σ – σ* sigma-sigma*

2016 FAU Erlangen-Nürnberg ix

List of Figures Figure 1-1: Average isolation of earth for years 1991-1993. The black disks correspond

to the theoretical area that covered with 8% efficient solar cells would give 18TW yearly,

which corresponds to a value higher than the world’s total primary energy demand.3, 4

........... 1

Figure 1-2: Research cell efficiency records chart presented from National Center for

Photovoltaics(NREL)14

.............................................................................................................. 2

Figure 1-3: a) Roll-to-roll production of OPVs. (source: OPV infinity) b) Modern life

application for flexible OPVs (source: OPV infinity) c) Solar leaf (part of a product from

Belectric OPV GmbH, source: www.solarte.de). d),e) Integration of OPVs in architectural

objects (product from Belectric OPV GmbH appeared in EXPO Milan 2015, source: www.solarte.de). f) Integration of OPVs on a bus stop rooftop in San Francisco (source:

demonstrator from Konarka) ..................................................................................................... 4

Figure 1-4: a) Organo-metal-halide active layer on glass (credit: Boshu Zhang, Wong

Choon Lim Glenn & Mingzhen Liu) b) IMEC presented perovskite photovoltaic modules

with 11% PCE.53

c) Flexible perovskite solar module presented by F.D.Giacomo et al.54 ....... 5

Figure 1-5: a) Types of tandem solar cells separated by the terminal connections. b)AM

1.5 global spectrum and a schematic representation of a multijunction device comprising three

sub-cells with complementary absorption spectra. Note that cell 1, cell 2 and cell 3 correspond

to cells with different Eg. Optimally the light meets the cell with the highest band gap first. .. 6

Figure 1-6: Different geometries for plasmon light trapping in OPVs; a) scattering from

large diameter (>50 nm) metal nanoparticles into high angles inside photoactive layer,

causing increased optical path length. b) Localized surface plasmon resonance induced by

small diameter (5–20 nm) metal particles. c) Excitation of surface plasmon polaritons at the

NPs/photoactive layer interfaces ensures the coupling of incident light to photonic modes

propagating in the semiconductor layer plane. Reproduced with permission.113

..................... 13

Figure 1-7: Illustration of up and down conversion processes. a) Up conversion process.

Two photons with energy 1/2 Eg convert in one photon with energy Eg. The optimal position

of up converting layer is before the photoactive layer (light meets up converter first).b) Down

conversion process. One photon with energy Eg converts in two photons with energy 1/2 Eg.

.................................................................................................................................................. 14

Figure 1-8: Simplistic illustration of split spectrum solar cells. The incident light is split

by spectrally sensitive mirrors and sent to the corresponding solar cell. Cell 1,2 and 3 have

different band gaps and they can be connected in series or in parallel configuration. ............. 15

Figure 1-9: Band diagram of a solar cell with intermediate band. Conduction band (CB),

valence band (VB) and intermediate band (IB) are shown. Intermediate band solar cell can

absorb different photons with different energies (presented here with different colors). ........ 15

Figure 1-10: Schematic illustration of multiple electron-hole pair generation. Blue wave

represents the incident photon, while orange wave represent the heat energy from the relaxing

electron that generate a second exciton. ................................................................................... 16

Figure 1-11: Schematic illustration of a the components of a thermophotovoltaic system.

Narrow bandwidth light is emitted from thermal emitter and control by a spectral control

element. Excess energy is emitted from the cell back to be recycled. ..................................... 17

x 2016 FAU Erlangen-Nürnberg

Figure 1-12: a) ITO replacement market forecast (source: Touch Display Research, ITO replacement: non-ITO Transparent Conductor Technologies and Market Forecast 2015 Report, 2015) b) Cost vs conductivity estimation of ITO-replacement technologies (source: Source: Touch Display Research Inc., ITO-Replacement Report, January 2016) .................. 18

Figure 1-13: Schematic illustration for 0-dimensional and 1-dimensional coating

techniques. Slot die coating and spray coating can produce patterns with shims and shadow

masks correspondingly (details in text). Modified with permission.167

................................... 23

Figure 1-14: Schematic illustrations of 2-dimesional printing techniques. Modified with

permission.167

........................................................................................................................... 25

Figure 1-15: Schematic illustration of the principles behind drop on demand

piezoelectric inkjet printing and continuous inkjet printing. Modified with permission.167

.... 26

Figure 1-16: The three necessary “gears” for any photovoltaic technology. .................. 28

Figure 2-1: a) The formation of σ and π bonding and π, π* orbitals in its simplest form

for a molecule of ehtene. b) The corresponding energy diagram. The illustration shows the

optical excitation from π (HOMO) to π* (LUMO) orbitals. .................................................... 31

Figure 2-2: Bilayer vs bulk heterojunction structures. The exciton separation occurs at

interfaces. Bulk heterojunction is more efficient because of the limited exciton diffusion

length in organic materials. Reproduced with permission.184

.................................................. 32

Figure 2-3: Operating principles of bulk heterojunction solar cell. Left: Simplified

kinetics diagram. Right: Simplified energy diagram.(i) Singlet exciton generation. (ii) Exciton

diffusion. (iii) Exciton dissociation. (iv) Charge separation. (v) Charge transport. (vi) Charge

extraction. Reproduced with permission.183

............................................................................ 33

Figure 2-4: Energy levels present in a donor–acceptor system which are relevant to the

mechanisms of generation, recombination and dissociation of CT complexes. Reproduced

with permission183

.................................................................................................................... 34

Figure 2-5: a) Perovskite crystal structure of the form ABX3. b) The energy diagram of

CH3NH3PbI3 perovskite resulted from the antibonding orbitals of the bonds between Pb (B)

and I (X). The illustration shows the optical excitation highest occupied state to the lowest

unoccupied state. ...................................................................................................................... 36

Figure 2-6: Structural evolution of perovskite solar cells: (a) sensitization concept with

surface adsorption of nanodot perovskite, (b) meso-superstructure concept with non-injecting

scaffold layer, (c) pillared structure with a nano oxide building block, and (d) planar p-i-n

heterojunction concept. Spheres represent TiO2 in (a) and (c) and Al2O3 in (b). Reproduced

with permission.188

................................................................................................................... 38

Figure 2-7: Schematic illustration of energy levels and processes in a perovskite solar

cell employing TiO2 and an HTM. ........................................................................................... 39

Figure 2-8: Normal architecture for single junction (a) and tandem solar cell (c). Inverted

architecture of single junction (b) and tandem solar cells (d). ................................................. 40

Figure 2-9: Transmittance versus sheet resistance for promising solution processed

electrodes.164, 212-214

Transmittance values were obtained at ~550nm. The bulk regime

(described by equation 2.3) is shown with solid line. The percolation regime (described by

equation 2.7) is shown with dashed line.133

............................................................................. 43

Figure 2-10: a) Linear and b) semi-logarithmic presentation of J-V curves und dark and

illuminated conditions. Reproduced with permission.183

......................................................... 43

2016 FAU Erlangen-Nürnberg xi

Figure 2-11: Single diode equivalent circuit model commonly employed in estimating

electrical losses in solar cell. .................................................................................................... 45

Figure 2-12: a) PCE prediction of a bulk heterojunction solar cell with PCBM as

acceptor material. For the calculation Scharber et al. assumed FF of 75%, EQE of 80% and

Voc according to Eq.2.13. b) Simplified energy diagram of a donor acceptor system.

Reproduced with permission.220

............................................................................................... 51

Figure 2-13: Theoretical efficiency of bulk-heterojunction photovoltaic devices with Eg−

qVoc= 0.60 eV (solid line) versus the lowest optical bandgap of the two materials, calculated

using the AM1.5 spectrum, FF = 0.65, and assuming constant EQE = 0.65 between 3.5 eV

and Eg . The dashed lines show the theoretical efficiencies for devices using the larger Eg −

qV oc offsets for (from top to bottom): PF10TBT:[60]PCBM (0.70 eV,circles),

PCPDTBT:[70]PCBM (0.76 eV, down triangles), PBBTDPP2:[70]PCBM (0.80 eV, up

triangles), and P3HT:[60]PCBM (1.09 eV, squares). The closed markers represent the

theoretical efficiency, the open markers the device efficiencies. Reproduced with permission. 186

.............................................................................................................................................. 52

Figure 2-14: Contour plot showing the calculated energy-conversion efficiency (contour

lines and colors) versus the absorption onset and the HOMO level of the donor polymer

according to ref. [217

] assuming an EQE and a FF of 70%; Dots indicate the performance

potential of the investigated polymers. Reproduced with permission. 225

............................... 53

Figure 2-15: Percentage of efficiency increase of a tandem cell over the best single cell

(R) for a device comprising a top (back) sub-cell and a bottom (front) sub-cell based on

donors each having a LUMO level at − 4 eV and each blended with a fullerene acceptor of

LUMO = − 4.3 eV. The variables are the bandgap of both donors. The lines indicate the

efficiency of the tandem devices. Reproduced with permission.91

Copyright 2008, Wiley-

VCH. ........................................................................................................................................ 56

Figure 2-16: PCE prediction of organic tandem solar cell comprising sing cells with

different bandgap energy (Eg). The LUMO level of donor is at –4 eV to keep the LUMO

difference between donor and PCBM to 0.3 eV. The optical simulation was performed based

on previous publication with updated assumptions: EQE = 80% and IQE = 100% for front

cell; EQE = 80% for back cell; FF = 75% for tandem solar cells. Reproduced with permission. 230

Copyright 2014, Wiley-VCH. ............................................................................................. 57

Figure 2-17: Performance comparison of various tandem configurations (2, 3 and 4

terminals) based on idealized SQ-limit calculation vs. bandgap of the top cell. The bottom cell

is Si (1.1 eV) which is filtered by the top cell: (a) J–V curves under AM1.5G 1 sun light for

the top cell with Eg2 = 2.0 eV. (b) Efficiencies of the constituent cells and the tandem cells.

Reproduced with permission.231

............................................................................................... 58

Figure 2-18: Schematic illustration of a solar module comprising three cells

interconnected in series. Red boxes represent the active area of each solar cell. The area of the

interconnection lines (l × w) is called dead area as it does not contribute to the photocurrent.

.................................................................................................................................................. 58

Figure 2-19: Equivalent circuit model commonly employed in estimating electrical

losses in solar module. ............................................................................................................. 60

Figure 3-1: Chemical structure of the photoactive materials used in the thesis .............. 65

Figure 3-2: Architecture of organic solar cell .................................................................. 65

xii 2016 FAU Erlangen-Nürnberg

Figure 3-3: Architecture of tandem solar cell .................................................................. 66

Figure 3-4: a) Architecture of laminated organic solar cell. b) Step-wise fabrication route

of solution-processed roll laminated cells. c) Photograph of the lamination process. The two

substrates bearing the active layers and the top contact are driven through a pre-heated (120

°C) roll laminator consisting of three rolls for intimate electrical contact. d) Photograph of the

finalized substrate. .................................................................................................................... 67

Figure 3-5: Architecture of laminated perovskite solar cell ............................................ 68

Figure 3-6: Simplified architecture of laminated tandem cell. Cell1 and Cell2 are made

simultaneously on different substrates and connected afterwards through lamination. The

combination of two different PV technologies is feasible. ...................................................... 69

Figure 3-7: a) Squared Diameter of ablated area versus laser pulsed energy. b)

Calculated threshold fluence for each functional film. The difference in threshold fluence

allows to successively scribing interconnection lines w/o damaging other active layers of the

device stack. Active layer refers to the organic absorber. The ablation threshold of perovskite

based active layer is generally similar to organic or even slightly higher. .............................. 70

Figure 4-1: Schematic illustration of flexible tandem solar cell architecture. ................. 76

Figure 4-2: a) Optical Properties of PET/IMI substrate. b) Resistivity of PET/IMI

substrate over bending cycles ................................................................................................... 77

Figure 4-3: Absorption spectra of OPV12 and pDPP5T-2 active layers. ........................ 78

Figure 4-4: Efficiency prediction for tandem solar cell based on OPV12: [60]PCBM

(bottom cell) and pDPP5T-2:[70]PCBM (top cell). ................................................................. 79

Figure 4-5: a) Point of our experimental data on the figure of theoretical prediction. J-V

characteristics of the OPV12, pDPP5T-2 based single cells and the corresponding tandem cell

under illumination with an AM1.5G solar simulator and 100 mW/cm2 ................................. 80

Figure 4-6: EQE spectra of OPV12:[60]PCBM and pDPP5T-2:[70]PCBM sub-cells

inside the tandem configuration. .............................................................................................. 81

Figure 4-7: Schematic illustration of the interconnection lines in the organic tandem

module (3-cells module). .......................................................................................................... 82

Figure 4-8: a) Top view microscope photograph of a P1 line on an IMI substrate b) SEM

top view image of a ~23μm P2 line. c) SEM top view image of P3 line. d)Top view SEM

image three patterning lines (narrow P2 line) .......................................................................... 83

Figure 4-9: Photograph of one of the 9 substrates carrying two reference single tandem

cells (center) and two pairs of tandem modules (left and right), with narrow (≈25 μm, left) and

wide (≈325 μm, right) P2 line patterning. The insets represent top views from an optical

microscope displaying the lines P1 – P3. The wide P2 line was realized by laser hatching

(scanning many single lines parallel to each other). As such, due to Gaussian energy

distribution of the laser beam, rests of the absorber material are visible in the overlapping

regions (lines visible in the P2 trench). This process did not affect the electrical

interconnection quality of the P2 line. ..................................................................................... 83

Figure 4-10: Top view illustration of the PET foil and the doctor blading direction (left).

After deposition of top electrode, PET was divided into 9 substrates (area of 2.5×2.5 cm2

) for

characterization. Photograph of PET foil (one substrate was marked with red dotted line)

(right). ....................................................................................................................................... 84

2016 FAU Erlangen-Nürnberg xiii

Figure 4-11: a) The assumed total area of flexible tandem modules is marked with a red

line box on the left. b) Active area is defined as the sum of 3 red line boxes. Dead area of the

module was assumed to be the area that does not contribute to photocurrent between the

active area boxes. ..................................................................................................................... 84

Figure 4-12: a) J-V characteristics of reference tandem cells (black line) and tandem

modules with narrow (≈25 m, red lne) and wide (≈325 m, green line) P2 line under

illumination. b) The corresponding J-V characteristics in the dark. ........................................ 85

Figure 4-13: Photovoltaic parameters distribution of 9 devices. a) Parameters distribution

for reference tandem solar cells. b) Parameters distribution for narrow P2 line modules.

c)Parameters distribution of wide P2 line modules .................................................................. 87

Figure 4-14: Normalized device characteristics of flexible tandem module after 1000,

3000 and 5000 bending cycles. ................................................................................................ 88

Figure 4-15: Schematic device representation of the tandem and single cells investigated

in this photodegradation study. ................................................................................................ 89

Figure 4-16: Initial J-V characteristics of a representative tandem cell and their

respective sub-cells .................................................................................................................. 90

Figure 4-17: Long-term decay of the UV light soaking (LS) state in the dark. Each data

point represents the average value of 5 tandem cells. The filled symbols represent the

condition after immediate light soaking, whereas the hollow symbols represent the temporal

decay of the LS state. The data were extracted from J-V-measurements using an AM1.5

spectrum and an illumination power of 1000W/m². Outside the J-V-measurements the tandem

cells were stored in the dark at room temperature. .................................................................. 91

Figure 4-18: Photoaging of single and tandem OPV cells. The graph show the average

long-term temporal evolution of PCE, Voc, Jsc, and FF for the different single and tandem

cells under continuous white light illumination. The photovoltaic parameters were extracted

from J-V-measurements using an AM1.5 spectrum at 1000W/m². Before each J-V-

measurements the samples were UV treated (365nm, 10 s). Each data point represents the

average value of 5 tandem devices, 5 DPP devices, and 5 P3HT devices. .............................. 92

Figure 4-19: Extrapolated lifetime of inverted OPV tandem cells. Long-term PCE decay

of inverted P3HT:PC[60]BM and pDPP5T-2:PC[70]BM based tandem solar cells. Each data

point represents an average value of 5 tandem devices. For estimating the accelerated lifetime,

we applied a linear fit of the form y = 0.899x – 3.6x10-6

to the data points following the burn-

in period and extended the fit to where the efficiency drops to 80% of the initial value (red

line). For a minimum expectable lifetime of our cells we extrapolated the minimum

(maximum) values of the error bars (dashed lines). The lifetimes were calculated considering

an average 1500 hours of sunshine per year (central Europe). ................................................ 93

Figure 5-1: The effect of patterning in solar cells with laminated top electrode. Left side:

realization of laminated solar cell with unpatterned bottom IMI. Right side: the realization of

laminated solar cell with laser patterned IMI. Typical J-V characteristics under 1 sun

illumination and DLIT images are shown for both architectures. ............................................ 99

Figure 5-2: Cross-section scanning electron microscopy (SEM) image of flexible

laminated organic solar device (left). Top view of AgNWs on TCA after delamination of PET

substrate(right). ...................................................................................................................... 100

xiv 2016 FAU Erlangen-Nürnberg

Figure 5-3: a) Current – Voltage (J-V) characteristics of organic solar cells with

laminated and evaporated top electrode b) EQE spectra of reference OPV solar cell with

evaporated silver top electrode (100 nm, blue dashed line), laminated OPV solar cell with

reflecting mirror in the back (black line), laminated OPV solar cell measured without

reflecting mirror (green line). ................................................................................................. 101

Figure 5-4: Transmittance spectra of PET substrate, charge extraction layers and

laminated electrode ................................................................................................................ 102

Figure 5-5: DLIT images of solar cells with evaporated and laminated top electrode and

c) integrated DLIT signal profile along the long axis of the DLIT image. ............................ 103

Figure 5-6: a) Impedance spectra for devices with evaporated top electrode under

different applied biases. b) Impedance spectra for devices with laminated top electrode under

different applied biases. EIS Spectrum Analyser was used for analysis and simulation of

impedance spectra311

. c) Equivalent circuit used for fitting data obtained by impedance

spectroscopy. Cg and Cμ represent geometrical and chemical capacitance, respectively.. Rrec

denotes the recombination resistance and Rt represents the transport resistance. Rs´ denotes an

additional resistive element due to electrode resistance losses.. For applied biases greater than

Voc the total series resistance in the model is given by Rs = Rs´ + Rt .................................... 104

Figure 5-7: a) Recombination resistance Rrec and b)transport resistance Rt as a function

of applied bias for devices with evaporated and laminated electrode. ................................... 104

Figure 5-8: Mott-Schottky plot (10Hz) for devices with evaporated and laminated top

electrode. The dashed lines represent linear fits to the slope. A scheme of the equivalent

electrical circuit model used for analyzing impedance spectroscopy data is displayed in

Figure 5-6. ............................................................................................................................. 106

Figure 5-9: Normalized device characteristics of a flexible organic laminated solar cell

over successive bending cycles. ............................................................................................. 106

Figure 5-10: Step-wise fabrication route of solution-processed roll laminated modules.

................................................................................................................................................ 108

Figure 5-11: Architecture of laminated organic solar cell/module and illustration of

depth-resolved post patterning of the top electrode (P3) using a femtosecond laser. Inset

shows laser-patterned lines required for interconnection of successive cells, i.e. module

fabrication. .............................................................................................................................. 108

Figure 5-12: The P1 and P2 line are scribed before the lamination process while the P3

line is post-patterned through the top substrate. ..................................................................... 109

Figure 5-13: Top view illustration of the module layout and the preparation road. . .... 109

Figure 5-14: a) (Left) Ablation depth upon laser patterning of adhesive top electrode

versus the laser fluence applied. (Right) Representative ablation depth profiles for different

laser fluences as determined from confocal optical microscopy images. b) Schematic

representation of post-laser ablation of a P3 line through a PET foil after lamination and

corresponding 3D depth profile. ............................................................................................. 111

Figure 5-15: a) Current – Voltage (J-V) characteristics of organic solar cells and

modules with laminated top electrode. b) J-V characteristics under dark conditions for flexible

OPV devices with laminated and evaporated top electrode. .................................................. 112

2016 FAU Erlangen-Nürnberg xv

Figure 5-16: a) Device architecture of laminated perovskite solar cell/module. b) Cross-

section scanning electron microscopy image of laminated perovskite solar device on glass

substrate. ................................................................................................................................. 114

Figure 5-17: a) J-V characteristics of perovskite solar cells and modules with laminated

top electrode. b) J-V characteristics under dark conditions for perovskite devices with

laminated and evaporated top electrode. c) EQE spectra of reference perovskite solar cell with

100 nm evaporated Ag top electrode (blue dashed line), laminated perovskite solar cell

measured with reflecting mirror in the back (black line), laminated OPV cell measured

without reflecting mirror (green line). .................................................................................... 115

Figure 5-18: Step-wise generic fabrication route of laminated tandem solar cell. ETL

(bright yellow), Active layers (bright blue, red), HTL (dark blue), AgNWs (yellow grey),

TCA (purple). ......................................................................................................................... 117

Figure 5-19: Architecture of laminated hybrid tandem solar cell. ................................. 118

Figure 5-20: a) Current – Voltage (J-V) characteristics of hybrid laminated tandem solar

cell and the corresponding single cells with laminated top electrode. b) J-V characteristics

under dark conditions for the same devices. .......................................................................... 119

Figure 6-1: Proposed route for post-fabrication laser patterning of single junction solar

device with laminated top electrode. ...................................................................................... 123

Figure 6-2: Laminated roll-to-roll web design with post laser patterning. .................... 124

xvi 2016 FAU Erlangen-Nürnberg

List of Tables Table 1-1 Comparison of film-forming techniques by printing and coating. Ink waste Ink

waste: 1 (none), 2 (little), 3 (some), 4 (considerable), 5 (significant). Pattern: 0 (0-

dimensional), 1 (1-dimensional), 2 (2-dimensional), 3 (pseudo/quasi 2/3-dimensional), 4

(digital master). Speed: 1 (very slow), 2 (slow<1 m min−1

), 3 (medium 1–10 m min−1

), 4 (fast

10–100 m min−1

), 5 (very fast 100–1000 m min−1

). Ink preparation: 1 (simple), 2 (moderate),

3 (demanding), 4 (difficult), 5 (critical). Ink viscosity: 1 (very low <10 cP) 2 (low 10–100 cP),

3 (medium 100–1000 cP), 4 (high 1000–10,000 cP), 5 (very high 10,000–

100,000 cP).Reproduced with permission.170

........................................................................... 26

Table 3-1: Substrates used in this thesis........................................................................... 63

Table 3-2: Photoactive materials used in this thesis ........................................................ 64

Table 3-3: Interface and electrode materials used in this thesis ....................................... 64

Table 4-1: Photovoltaic parameters of hero flexible tandem solar cells and the

corresponding flexible single-junction solar cells. ................................................................... 81

Table 4-2: Device parameters for OPV12/ pDPP5T-2 reference tandem cells (Device A)

and tandem modules (Device B and C) .................................................................................... 85

Table 5-1: Power conversion efficiencies of laminated organic solar cells due date ...... 96

Table 5-2: Key metrics for organic and perovskites solar devices with evaporated and

laminated top electrode under AM 1.5G illumination (100 mW cm−2). Best performance and

mean values with standard deviation population (shown in parenthesis) were extracted from

10 organic devices and 5 perovskite devices.......................................................................... 113

Table 5-3: Key metrics for hybrid laminated tandem solar cell and the corresponding

single cells with laminated top electrode under AM 1.5G illumination (100 mW cm−2). Best

performance and mean values with standard deviation population (shown in parenthesis) were

extracted from 5 devices. ....................................................................................................... 119

2016 FAU Erlangen-Nürnberg xvii

Table of Contents Zusammenfassung ............................................................................................................... v

Acknowledgements ............................................................................................................ vi

List of Abbreviations ......................................................................................................... vii

List of Figures .................................................................................................................... ix

List of Tables.................................................................................................................... xvi

Table of Contents ............................................................................................................ xvii

Chapter 1 Introduction ........................................................................................................ 1

1.1 Solar Cells ................................................................................................................. 2

1.2 Third generation concepts ......................................................................................... 3

1.2.1 Organic solar cells .............................................................................................. 3

1.2.2 Hybrid perovskite solar cells .............................................................................. 5

1.2.3 Multijunction solar cells (tandem cells) ............................................................. 6

1.2.4 Alternative third generation concepts .............................................................. 12

1.3 Solution Processed Electrodes ................................................................................. 17

1.4 The art of upscaling ................................................................................................. 21

1.5 Motivation and Outline............................................................................................ 27

Chapter 2 Fundamentals .................................................................................................... 30

2.1 The theory behind organic solar cells ...................................................................... 30

2.1.1 Organic semiconductors ................................................................................... 30

2.1.2 Bulk heterojunction .......................................................................................... 31

2.2 The theory behind perovskite solar cells ................................................................. 35

2.2.1 Perovskite light absorbers ................................................................................ 35

2.3 Device architectures ................................................................................................ 39

2.4 Electrodes ................................................................................................................ 41

2.5 Current-voltage characteristics and diode equation ................................................ 43

2.6 Efficiency limits of solar cells ................................................................................. 46

2.6.1 Shockley-Queisser limit for single junction solar cells.................................... 46

2.6.2 Efficiency limits in single-junction organic solar cell ..................................... 50

2.6.3 Efficiency limits in single-junction perovskite solar cell ................................. 53

2.6.4 Efficiency limits in tandem solar cells ............................................................. 55

2.7 Geometrical and Electrical Losses in Solar Modules .............................................. 58

Chapter 3 Materials and Methods ..................................................................................... 63

3.1 Materials .................................................................................................................. 63

3.2 Solar cell fabrication ............................................................................................... 65

xviii 2016 FAU Erlangen-Nürnberg

3.2.1 Organic solar cells ............................................................................................ 65

3.2.2 Organic tandem solar cells ............................................................................... 66

3.2.3 Laminated organic solar cell ............................................................................ 67

3.2.4 Laminated perovskite solar cell fabrication ..................................................... 68

3.2.5 Laminated tandem solar cell fabrication .......................................................... 69

3.3 Solar module fabrication ......................................................................................... 69

3.3.1 Tandem module fabrication ............................................................................. 69

3.3.2 Laminated module fabrication ......................................................................... 70

3.4 Characterization ....................................................................................................... 71

Chapter 4 Flexible tandem solar modules ......................................................................... 73

4.1 Motivation and State of the art ................................................................................ 73

4.2 Flexible organic tandem solar cells ......................................................................... 76

4.2.1 Materials screening .......................................................................................... 76

4.2.2 Optical Simulations .......................................................................................... 78

4.2.3 Roll-to-Roll compatible coating technique ...................................................... 79

4.2.4 Performance and key characteristics ................................................................ 80

4.3 Flexible organic tandem solar modules ................................................................... 81

4.3.1 Design and realization ...................................................................................... 81

4.3.2 Performance and key characteristics ................................................................ 85

4.4 Towards competitive operating lifetimes ................................................................ 88

4.5 Conclusion ............................................................................................................... 94

Chapter 5 Lamination as fabrication strategy ................................................................... 95

5.1 Motivation and State of the art ................................................................................ 95

5.2 Realization of efficient adhesive top electrode ....................................................... 97

5.3 Innovating solution-processed solar modules ....................................................... 107

5.4 Innovating tandem solar cells ................................................................................ 116

5.5 Conclusion ............................................................................................................. 119

Chapter 6 Summary and Outlook .................................................................................... 121

6.1 Summary ............................................................................................................... 121

6.2 Outlook .................................................................................................................. 121

Bibliography .................................................................................................................... 125

Curriculum Vitae ............................................................................................................. 138

2016 FAU Erlangen-Nürnberg 1

Chapter 1 Introduction

As we move through the Information Age, the world faces important challenges resulting

from energy demand growth and a rising population. 1.2 billion people or 17% of the world’s

global population lack access to electricity1. We now need-more than ever low-cost

sustainable energy sources to fight technological and social inequality.

Yet, 150 million kilometers far an energy giant continuously bombards the surface of the

earth with vast amounts of energy which reach the enormous value of 890 million

terawatthours (TWh) yearly! In 2008, this amount was enough to feed ~6000 times the year

energy demands of humankind. While, in the near future of 2035 it would be enough to cover

around ~4000 time the energy demands of humankind.2 However, most of the times plain

numbers of TWh do not excite human mind as we are used to think in empirical magnitudes.

For that reason is noteworthy to translate the above mentioned to simple but powerful

statements that would grasp strongly the attention of the reader. In 1.5 hours the energy that

earth receives from sun is enough to cover a year’s energy demand of humankind!2 So the

only thing that humans should do is harvest this free sustainable energy and distribute it

around the planet. M. Loster presented a very interesting graph that shows how six small areas

around the planet could deliver 18TW/year and power the whole world if they were covered

with solar cells of 8% efficiency (Figure 1-1)!3 The message is undeniably powerful, solar

cells can exclusively supply our energy demands.

Figure 1-1: Average isolation of earth for years 1991-1993. The black disks correspond to the theoretical area that covered with 8% efficient solar cells would give 18TW yearly, which corresponds to a value higher than the world’s total primary energy demand.3, 4

Chapter 1 Intorduction

2 2016 FAU Erlangen-Nürnberg

1.1 Solar Cells

It all started when the French physicist Alexandre-Edmond Becquerel observed the

photovoltaic phenomenon back in 1839.5, 6

This event triggered chain reactions that lead in

devices that would capture the solar energy and transform it into electricity for human’s

benefit.7-11

The inception of the solar cell was well founded by the end of 19th

century when

Adams and Day build the first all-solid cell based on selenium.12

In 1950s the power

conversion efficiency (PCE) would start to become promising with the famous 6% Si solar

cell of Bell Labs.13

Since then technological leaps took the PCE up to 46% (Figure 1-2).

During these years the evolution road of the photovoltaic technology is parted in three

generations highlighting important milestones in the development of novel photovoltaic

materials and device concepts. First generation solar cells are mainly the mature silicon-

wafers based technology (blue colour lined in Figure 1-2). With the record efficiency of

around 26.3 % and a commercial available efficiency of ~20% this technology holds the

major share of the market. The second generation solar cells are based on alternative thin-film

technologies (green in Figure 1-2). Cu(In,Ga)Se2 (CIGS) and CdTe with ~20% efficiency

demonstrate the record for this generation. Aside from efficiency, thin-film technologies can

reduce materials and production cost and present flexible products.

Figure 1-2: Research cell efficiency records chart presented from National Center for Photovoltaics(NREL)14

Third generation solar cells are mainly divided in two sections; the first one aims-cost

independently-at very high efficiencies (purple in Figure 1-2) and the second at low-cost



adequate efficiencies (red in Figure 1-2). The first part includes research on multijunction

device concepts and efficient photovoltaic materials (such as GaAs). However the cost of

these devices is very high making them currently unavailable for commercialization. The

second part which includes emerging PVs and different device concepts raises big hopes on

commercial availability of efficient, conformable, low cost solar cells. On this thesis we are

focused on this second part of the third generation solar cells; specifically on organic

photovoltaics (OPVs), hybrid perovskites and multijunction solar cells.

1.2 Third generation concepts

As outlined in the previous section third generation photovoltaics include different

emerging PV technologies, based either on novel photovoltaic materials or more sophisticated

device concepts, aiming on surpassing the Shockley-Queisser limit with affordable fabrication

cost. OPVs and hybrid perovskites grasp leading roles in sustainable energy production with

short energy payback times15

because they can be processed from solution and deployed on

massive scale while providing excellent form factors16-18

and competitive power conversion

efficiencies19-22

. An important factor that highlights the advantages and the potential impact of

these emerging PV technologies on earth transportation or even space travel, is the power-per-

weight metric. M. Kaltenbrunner et al. presented recently a power-per-weight chart that sets

OPVs and perovskites in the highest positions among other established PV technologies with

~10Wg-1

and ~23Wg-1

correspondingly.18

1.2.1 Organic solar cells

OPVs are in the family of organic electronics- devices that utilize organic semiconductors

(oligomer or polymer based) to perform particular functions. Chemical tailoring of polymers

can create unlimited combination of materials that absorb different areas of the solar

spectrum. Additionally, the charm of this technology springs from the ability to easily process

organic semiconductors from solutions on a variety of substrates. Thus cost-effective

production methods and shape adaptable, colorful, semi-transparent, light products

compensate the lower PCE values compared to the inorganic counterpart (Figure 1-3). In the

next few paragraphs I will try to recur the past of this interesting technology through its main

milestones.

In 1960s the first generation organic solar cells appeared and consisted of a single organic

layer sandwiched between asymmetric work function metals23, 24

. Despite the poor



efficiencies (<1%) it was a ground breaking idea that attracted the attention of research

community. In 1986 Tang et al. demonstrated a 1% bilayer structure of a p-type and n-type

organic semiconductors.25

Saritcifci et al. observed in 1992 the photoinduced transfer from a

conjugated polymer to a C60 molecule which led to polymer-fullerene heterostructure.26

Bulk

heterojunction, a concept that initiated a change in the field of organic solar cells did not take

a lot of time to be proposed.27-29

. Entering the new millennium, the hype that was gained the

previous years led to higher power conversion efficiencies, up to 4.2% for evaporated bilayer

devices30, 31

and up to 3% for bulk heterojunction devices32-39

.

Figure 1-3: a) Roll-to-roll production of OPVs. (source: OPV infinity) b) Modern life application for flexible OPVs (source: OPV infinity) c) Solar leaf (part of a product from Belectric OPV GmbH, source: www.solarte.de). d),e) Integration of OPVs in architectural objects (product from Belectric OPV GmbH appeared in EXPO Milan 2015, source: www.solarte.de). f) Integration of OPVs on a bus stop rooftop in San Francisco (source: demonstrator from Konarka)

Additional research on the bulk hetero-junction concept to influence morphology with

different processing conditions40, 41

and post thermal treatment42, 43

brought us to PCE values

close to 5%.

Even though, nowadays the PCE of single junction organic solar cells have reached 11-

12% 44, 45

the basic science limitations that have been preventing this technology from market

implementation need to be addressed. Particularly, the poor match of the absorption spectrum

of the active blend materials with the solar spectrum limits the photon harvesting capabilities

and, consequently, the photocurrent generation. Additionally, thermalization losses diminish

possible voltage outputs.46, 47

One promising approach for overcoming these limitations is the

tandem concept48, 49

which is introduced and briefly reviewed later in the thesis (sub-chapter

1.2.3 , 2.6.4 ).

a b c

d fe

http://www.solarte.de/

http://www.solarte.de/



1.2.2 Hybrid perovskite solar cells

Hybrid organic-inorganic perovskite solar cells have met a great hype during the last

years taking the reins of research in emerging PV technology. They are promising cost-

effective technology as they employ solution-processed organo-metal-trihalide semiconductor

materials. Figure 1-4 shows some prototype products from different research groups. The

perovskite crystalline structure follows the formula ABX3 where A is an organic cation, B and

inorganic cation and X halogen anion (details in 2.2 ). The most common-used light

harvesters due date are based on (CH3NH3)PbX where X is typically I, Cl or Br.50-52

It is

worthwhile noticing that; i) (in difference with OPVs) it is relatively easy to control the

quality and morphology of the resulting film, ii) different band gaps can be obtained with

different halogen atoms. In the next paragraph, I present the main milestones of this exciting

technology.

Figure 1-4: a) Organo-metal-halide active layer on glass (credit: Boshu Zhang, Wong Choon Lim Glenn & Mingzhen Liu) b) IMEC presented perovskite photovoltaic modules with 11% PCE.53 c) Flexible perovskite solar module presented by F.D.Giacomo et al.54

Since Miyasaka’s group first demonstrated CH3NH3PbBr3 based solar cells with 2.2%

PCE back in 200655

and an updated PCE of 3.8% in 200956

, the interest for this field

exploded. Organic-inorganic halide perovskites start gaining popularity and until today they

grasp the interest of energy research community.51

In 2011, J-H. Im et al. presents a 6.54%

PCE solar cell based on CH3NH3PbI3 nanocrystals by applying a TiO2 surface treatment

before deposition.57

The following year Park/Grätzel’s and Snaith’s group introduced

simultaneously a spiro-MeOTAD hole transporting medium (HTM), and they push PCE

values to 9.7% and 10.9% respectively.58, 59

Until May of 2013 devices that employ TiO2

scaffolding yield 15% PCE.60-63

Meanwhile Snaith’s group reported efficiency of 15.4%

without employing scaffolding.64

In the end of 2013 Seok’s group reports efficiency of 16.2%

with a CH3NH3PbI3-xBrx and poly-triarylamine HTM and boosted it to 17.9% in 2014 (S.I.

Seok, personal communication). Nowadays, the tremendous momentum of perovskite solar



cells continues and record efficiency has reached 21%21, 65

while everything shows that

efficiency can be boosted even higher.66

1.2.3 Multijunction solar cells (tandem cells)

Figure 1-5: a) Types of tandem solar cells separated by the terminal connections. b)AM 1.5 global spectrum and a schematic representation of a multijunction device comprising three sub-cells with complementary absorption spectra. Note that cell 1, cell 2 and cell 3 correspond to cells with different Eg. Optimally the light meets the cell with the highest band gap first.

In the multijunction conept, sub-cells of different band gap (Eg) are stacked in series (2-

terminal), in parallel (3-terminal) or both in a post-production electrical connection (4-

terminal) to absorb a wider range of the solar spectrum and reduce the thermalization losses of

Cell 1

Bottom electrode

Interconnection Layer

Cell 2

Top electrode-

+

Cell 1

Bottom electrode

Interconnection Layer

Cell 2

Top electrode-

+

- Cell 1

Bottom electrode

Cell 2

Top Electrode-

+Bottom electrode

Top Electrode-

+

a

b

2T 4T3T



the high-energy photons (Figure 1-5). 48, 67, 68

In that way efficiencies beyond the single-

junction Shockley-Queisser limit can be achieved (details in sub-chapter 2.6.4 ). The number

of the sub-cells connected can be theoretically infinite, however in sake of simplicity here we

illustrate tandems comprise two sub-cells (Figure 1-5a) and three sub-cells (Figure 1-5b).

The most promising configuration, the two-terminal (2T) tandem device is developed

monolithically on a single substrate by depositing successively modifying layers, photoactive

materials and interconnection layers. In this configuration, the interconnection layer is very

important as it should ensure appropriate charge extraction from both cells and recombination.

A parallel connection between the cells can be achieved with a 3-terminal (3T) configuration.

Here the interconnection layer-equally important as in the 2T devices- should selectively

extract the carriers from each cell but also give the ability for a terminal contact. These

devices even though promising for proving different concepts in a research level, they are

impractical when it comes to large scale fabrication. Lastly, in the four-terminal (4T) concept

devices fabricated with different routes are electrically post-connected. This type of device

comprise no intermediate layer, however bottom and top electrodes should demonstrate high

transparency to minimize the parasitic absorption losses.

In inorganic III-V PV technology, tandem solar cells attracted researcheres’ interest since

1960. 69

However it took around 25 years to develop a 20% efficient device based on

AlGaAs/GaAs.70

Since then the efficiency chart went upwards with the introduction of stable

tunnel junctions, defect free active materials and additional sub-cells (four junction solar

cells) to cover even broader spectrum. Recently, Fraunhofer institute demonstrated a record

efficiency based on III-V semiconductor compounds and a quadruple junction of 46% at 50.8

W/cm2.71

This is a record not only for inorganic multijunction technology but also for the

whole PV field (Figure 1-2). As promising these results as they may be, it is really difficult to

end up on vast commercialization and have a broad social impact because of their extremely

expensive fabrication.72

For this reason, during our decade research on tandem devices

incorporating solution processed materials (such as organics and perovskites) is blooming.

Since this thesis focuses on solution processed photoactive materials, in the next sections we

present an analytical state of the art for organic, perovskite and hybrid technology tandem

solar cells.

i) Organic Tandem Solar Cells

As mentioned in section 1.2.1 solar cells based on organic semiconductors show

numerous processing advantages (solution processing, roll-to-roll processing) and products



with exceptional characteristics (high conformability, low weight, transparency, color).

However, they lack high efficiencies to be competitive against other PV technologies. With

the highest certified single junction efficiency ~11.5% and estimated upper limit of 11-13%

(details in sub-chapter 2.6 ) it is obvious since decades that researchers should explore 3rd

generation device concepts. Organic tandem solar cells is one of the most explored and

promising concept. Below I review the results that brought us to current state of the art.

-Evaporated Small Molecule Tandem Solar Cells

In the field of organic solar cells, the first tandem cells presented were based on

evaporated small molecules. Particularly, Hiramoto et al. first realized in 1990, an organic

tandem cell built by two identical bilayers (perylenetetracarboxylic derivative/Me-PTC)

connected in series with a thin evaporated (2nm) Au interstitial layer that provided

recombination sites for the charges arriving from top and bottom sub-cells.73

Following

similar approach Yakimov and Forrest demonstrated organic tandem solar cells with more

than two thin heterojunction sub-cells. They utilize Cu-phthalocyanine (CuPC) and

perylenetetracarboxylic bis-benzimidazole (PTCBI) as donor and acceptor correspondingly.

The resulting efficiencies showed the highest trend for a two sub cell tandem cell at 2.5%

indicating that parasitic absorption from recombination layers lowered the absorbed light from

later active layers. In 2004, Xue et al. achieved 5.7 % efficiency by fabricating a two bulk

heterojunction tandem cell based on evaporated CuPc and C60. Here, they employed thin

exciton blocking layers of PTCBI and bathocuproine (BCP) to achieve a FF of 0.59%. Until

2012, the efficiency of small molecules evaporated tandem solar cells reached values of ~7%

PCE following similar recipes for intermediate layer (metallic based recombination

centres).74-79

Later, in 2013 and more recently in 2016 the R&D department of Heliatek

presented a corresponding evaporated multi-junction cell based on small molecules with PCE

of 12% and 13.2%, setting new world record for organic photovoltaic cells.80, 81

-Solution Processed Tandem Solar Cells

Despite those indisputably promising high PCE values, to fully exploit the main

advantage of organic solar cells, solution processability and simplification of a production line

is needed. This fact pushed the research community towards solution-processed organic

tandem solar cells. The first solution-processed organic tandem cell was reported by

Kwawano et al. in 2006 and it comprised two sub cells with an sputtered ITO-based

intermediate layer. 82

Both sub-cells were based on two conjugated polymer poly[2-methoxy-



5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and PCBM. The tandem

cell delivered a Voc of 1.34 V, Jsc of 4.1 mA cm-2 and a FF of 0.56 which resulted in 3.1%

efficiency. Even though the efficiency increased 35% compared to the single cells, the tandem

cell suffered from voltage losses due to the energy misalignment of the ITO based

recombination layer. This work highlights the importance of mechanical stability and

electrical characteristics that recombination layer should demonstrate to achieve a final

structure with minimized losses and high efficiency.

Later that year Dennler et al. utilized two active layers with different absorption spectra

to build a solution-processed organic tandem.83

The authors combined a P3HT:PCBM bottom

sub-cell with a ZnPC:C60 top sub-cell using a recombination layer from C60, 1nm thick Au

layer and ZnPC. The final structure demonstrated a full Voc (sum of both sub-cells) but low

FF (0.49%) and Jsc (4.(mAcm-2) resulting in a limited 2.3 % efficiency. Similar approaches

were followed by other groups improving the efficiency.84, 85

On a similar note, Janssen et al.

utilized evaporated WO3 instead of evaporated gold to fabricate an efficient recombination

layer. Meanwhile, Hadipour et al. fabricated in 2006 a tandem cell with two solution

processed active layers based on a wide and low band gap polymer and an intermediate layer

comprising LiF/Al/Au layer/PEDOT:PSS. A relatively thick Au layer (10-50nm) was chosen

to protect LiF/Al from the moisture of PEDOT:PSS. The resulted efficiency was poor.86

Gilot et al. innovated the tandem device processing by presenting not only solution

processed polymer based active layers but also a solution processed intermediate layer based

on modified PEDOT:PSS and ZnO.87

The authors demonstrated double and triple junction

solar cells with full Voc values and pave the way to fully solution processed multijunction

solar cells. In the same direction, Heeger’s group develops a fully solution processed tandem

cell with a recombination layer based on PEDOT:PSS and TiOx and boosts the PCE to

6.5%.88

Since then the solution-processed approach for the intermediate layer prevails and

different combination are tried until the PCE values hits 10.6% for double-junction and 11.5%

for triple-junction tandem design from.89, 90

Those findings from Y.Yang’s group remain the

record efficiency for solution-processed tandem devices until today.

As it becomes clear from the abovementioned, during the last decades organic tandem

solar cells have faced tremendous advancements as one of the most promising concepts to

capture sunlight. Yet, the PCE limits predicted from simulations have not met reality until

now.91, 92



ii) Hybrid Tandem Solar Cells

The tandem concept has been proven very promising for surpassing the Shockley-

Queisser limit of single junction solar cells. Up to date, in terms of performance and

manufacturing, the most promising devices are based on monolithically developed 2T

configuration. 2T requires one transparent electrode which minimizes the parasitic absorption

losses and is made by the lowest amount of processing steps ensuring high quality and low

fabrication cost. However this configuration dramatically reduces the processing window of

the device, making difficult the combination of technologies with incompatible processing.

This becomes even more critical when it comes to combination of 2nd

generation high

temperature processed thin film technologies (such as CIGS) or 3rd

generation solution

processed solar cells (such as perovskites). For this reason, during the previous decade before

the maturity of OPVs and perovskite fields, researchers turned to 4T configuration tandem

cells made out of CIGS, dye sensitized solar cells, CdTe.93-95

Nowadays, the combination of different active layers in a 2T configuration for the fields

of OPVs and inorganic PVs (e.g. Si, GaAs) has been proven very promissing.71, 90

On the

other hand scientists and engineers still struggle on demonstrating an efficient 2T monolithic

tandem entirely made by perovskite active layers (e.g. (CH3NH3)PbI3, (CH3NH3)PbBr3). This

is mainly due to the lack of efficient intermediate layer processed by perovskite compatible

solvents that can also protect from additional perovskite layers. On top of that, the high

sensitivity of perovskite layers to processing steps complicates even more the fabrication

route. With this in mind, Jin Hyuck Heo and Sang Hyuk Im focused their research on

fabricating mechanically stacked perovskite-perovskite 2T tandem solar cell.96

Although their

product did not show high efficiency (10.4%), it clearly demonstrates the potential of a

mechanical stack 2T tandem solar cell that could even incorporate different PV technologies.

During the last years, research community is increasingly focused on hybrid tandem solar

cells, where two or more solar technologies are combined to fabricate an efficient tandem

solar cell with low energy payback time. Hybrid tandem solar cells comprising amorphous

silicon (a-Si:H) as bottom sub-cells and OPVs as top-cells have demonstrated PCE values up

to 10.5%.97-99

Nevertheless, this field exploded when perovskites solar cells reached high

efficiency values (~20%) with band gaps greater than 1.5eV. The last 3 years, there were

many attempts on fabricating homo and hybrid tandem solar cells by connecting in 2T or 4T

configuration perovskite cells with silicon, CIGS, CZTS or OPVs.



-2T monolithic perovskite based tandem cells

In 2014, Todorov et al. demonstrated one of the first trials for development of a 2T

monolithic perovskite-CZTS tandem cell.100

The authors used a sputtered ITO to form a

recombination layer but both of the sub-cells were solution processed. The efficiency was

limited to 4.6% but the starting pistol for the race of perovskite based hybrid tandem solar cell

has sounded. In 2015 Mailoa et al. presented a more efficient 2T monolithically developed

hybrid tandem cell based on (CH3NH3)PbI3 and silicon sub cells.101

In this work, they

achieved interconnection with tunnel junction. The final efficiency even higher than previous

attempts was limited to 13.7%. Later this year, Todorov et al. based on their previous

architecture showed a perovskite-CIGS tandem with 10.9% efficiency.102

The work of F.Jiang

et al. highlighted the aforementioned difficulties on monolithically developed perovskite-

perovskite tandem solar cells.103

With an intermediate layer based on solution processed

materials (PCBM, PEI, PEDOT:PSS and spiro-OMeTAD) the efficiency reached values only

up to 7%. Werner et al. demonstrated a perovskite-crystalline silicon tandem solar cell with

21.2% efficiency.104

The authors connected the two sub cells with a sputtered indium zinc

oxide (IZO) based intermediate layer. Most recently, Y. Liu et al. presented a record

efficiency 16% perovskite-polymer solar cell with a fullerene/ ultrathin Ag/ MoO3 based

intermediate layer.105

Even though the results I highlighted in this section nicely show the

potential of 2T perovskite monolithic devices, it is clear that high efficiencies require

architectures with sputtered or evaporated interconnection layers.

-2T and 4T mechanically stack and split spectrum perovskite based tandem cells

C.D. Bailie et al. mechanically stacked semi-transparent perovskite devices with CIGS

and low quality multicrystalline Si.106

The semi-transparent perovskite solar cells were based

on silver nanowires electrode and the connection was made in a 2T configuration. The final

structures yielded 18.6% and 17.9% efficiency correspondingly. The 4T split spectrum device

of Uzu et al. was based on crystalline silicon and (CH3NH3)PbI3 perovskite cell with 28%

efficiency.107

A 4t terminal configuration based on CIGS and mp-TiO2: (CH3NH3)PbI3

perovskite cells was shown by Kranz et al.108 The single semitransparent cells were based on

evaporated MoO3 and ZnO:Al sputtered contacts and the efficiency reached 19.5%. Recently,

the group of Henry Snaith demonstrated that combining a mixed-cation lead mixed-halide

perovskite solar cell with a crystalline silicon cell in 4T configuration could result in 25.2%

efficient tandem cell.109



To finish this section I would like to underline the work of M. Filipic et al. who

performed optical simulations for 2T and 4T perovskite-silicon tandem solar cells. His

findings inform that an efficiency greater than 30% is achievable with current technology and

increases even more the hopes on the tandem concept.110

1.2.4 Alternative third generation concepts

Except the multijunction photovoltaics, scientists around the world have conceived

various other concepts to surpass the Shockley-Queisser limit of a single junction solar cell.

The most promising device concepts are listed below.

i) Light management

In a solar cell, typical light related losses such as reflection, parasitic absorption and

reduced light pathway inside the active layer, limit the photocurrent generation and reduce the

efficiency of the device. By light management scientists try to tackle these losses.

-Nanophotonics

One of the most famous concepts of light management is the Nanophotonics and have

been used with various PV technologies. Here, scientists utilize the plasmonic and scattering

effects that nanostructures and nanoparticles-with a size smaller or equal to the wavelength of

incident light- can create.111

These structures can be incorporated in different positions inside

solar cell architectures (surface, inside photoactive layer, interface with an electrode or

modifying layer) with the ultimate goal to trap light inside photoactive layer and increase

photocurrent. As a result, for similar or even higher photocurrent generation thinner

photoactive layers can be used leading to further improvement of total performance due to

reduced recombination losses. Additional improvement has been attributed to photovoltage

enhancement by reduced entropic losses.111, 112

The different cases of light trapping in an

organic active layer are shown in Figure 1-6.113



Figure 1-6: Different geometries for plasmon light trapping in OPVs; a) scattering from large diameter (>50 nm) metal nanoparticles into high angles inside photoactive layer, causing increased optical path length. b) Localized surface plasmon resonance induced by small diameter (5–20 nm) metal particles. c) Excitation of surface plasmon polaritons at the NPs/photoactive layer interfaces ensures the coupling of incident light to photonic modes propagating in the semiconductor layer plane. Reproduced with permission.113

-Up and down conversion

In this concept light-converting materials are utilized to absorb in an inactive area for a

corresponding photoactive layer and re-emit inside its absorption spectrum through a non-

linear optical process. The terms up and down conversion refers to the energy conversion of

the incident light. For example, up conversion materials would absorb relatively low energy

light and re-emit in higher energy and lower wavelength. Usually, such a layer would be

incorporated inside solar architecture between the photoactive layer and the back reflector to

capture the sub-bandgap photons.114

Correspondingly, a down conversion layer that absorbs in

UV-region would re-emit through photoluminescence light with lower energy and higher

wavelength.114, 115

Typically, a down conversion layer would be placed in front of the

photoactive layer (light meets first the conversion layer) to increase spectral irradiance and

reduce (in similar manner to the tandem concept) the thermalization losses from high energy

photons. Figure 1-7 illustrates the generic mechanism of up and down conversion processes,

as well as the corresponding optimal architectures.



Figure 1-7: Illustration of up and down conversion processes. a) Up conversion process. Two photons with energy 1/2 Eg convert in one photon with energy Eg. The optimal position of up converting layer is before the photoactive layer (light meets up converter first).b) Down conversion process. One photon with energy Eg converts in two photons with energy 1/2 Eg.

ii) Split spectrum solar cells

Split spectrum solar cells -often categorized as special section of tandem solar cells-

utilize sub-cells with different band gaps to absorb different areas of the solar spectrum. The

main difference to the traditional tandem concept is that light first meets a spectrally sensitive

mirror that splits the spectrum into components with different energies. Each light component

finally ends on a specialized solar cell with the corresponding absorption spectrum (Figure

1-8). This idea was first proposed in 1955 by Edmond Jackson.116

In 2010 Martin Green et.al

demonstrated a 43% efficiency for a split-spectrum concentrator solar cell.117

Recently, it was

shown that efficiencies higher than 50% are feasible with this concept and current PV

technology.118

a b



Figure 1-8: Simplistic illustration of split spectrum solar cells. The incident light is split by spectrally sensitive mirrors and sent to the corresponding solar cell. Cell 1,2 and 3 have different band gaps and they can be connected in series or in parallel configuration.

iii) Intermediate band solar cells

This concept is based on the idea of introducing impurity energy levels inside a

semiconductor band gap to induce additional absorption of lower energy photons (Figure

1-9).119

In 1997 A. Luque et al. demonstrated through theoretical analysis that the efficiency

limit of such a cell can exceed the Shockley-Queisser limit for single junction solar cells.120

Figure 1-9: Band diagram of a solar cell with intermediate band. Conduction band (CB), valence band (VB) and intermediate band (IB) are shown. Intermediate band solar cell can absorb different photons with different energies (presented here with different colors).

Spectral control

Sun Light

IB

VB

CB



iv) Multiple Electron-Hole pairs per Photon

When a photon with energy higher than the bandgap of the semiconductor is absorbed,

the electron jumps to higher states inside CB. Immediately after excitement electron relaxes to

lower energy states in the CB by losing energy via thermalization losses. In this concept this

energy is used to generate more electron-hole pairs per photon and increase the internal

quantum efficiency of the solar cell above unity (Figure 1-10). In 2006, Schaller et al.

demonstrated that with semiconductor nanocrystals this energy loss can be reduced by

producing multiple electron-hole per photon.121

Ten years earlier, P. Würfel has shown that

the theoretical limit of this concept for a photoactive layer with vanishing band gap would be

85%.122

However, as promising as may be, this concept shows tremendous challenges until

practical realization. 123, 124

Figure 1-10: Schematic illustration of multiple electron-hole pair generation. Blue wave represents the incident photon, while orange wave represent the heat energy from the relaxing electron that generate a second exciton.

v) Hot Carrier solar cells

The idea behind hot carrier solar cells is to extract excited hot electrons before they relax

in the lower energy state of CB. This can be practically achievable with selective contacts that

promote fast carrier collection, or with methods that slow down relaxation processes.119

It has

been shown that such devices can deliver efficiencies of ~66% overcoming Shockley-

Queisser limit.125

In 2011 Gabor et al. demonstrated that graphene shows hot carrier transport

properties that can be utilized for an efficient energy-harvesting device.126

VB

CB



vi) Thermophotovltaics (TPV)

Thermophotovoltaics is another concept that tries to tackle the thermalization losses

derive from the absorption of high energy photons. Specifically, in this concept a heated

element (thermal emitter) is placed in front of a solar cell. The thermal emitter heats up and

emits a narrow bandwidth light with energy similar to the band gap of the photovoltaic placed

behind. The light from the emitter hits the solar cell, part of it is reflected and part of it is

absorbed by the photoactive layer where produces photocurrent while some thermalization

losses occur. Reflected light and thermal energy (from the losses) travel back to the emitter

where they are recycled.119, 127-129

With a theoretical limit of more than 80% this interesting

concept outperforms solar cells, but still the field lack high efficiency experimental

demonstrations.

Figure 1-11: Schematic illustration of a the components of a thermophotovoltaic system. Narrow bandwidth light is emitted from thermal emitter and control by a spectral control element. Excess energy is emitted from the cell back to be recycled.

1.3 Solution Processed Electrodes

Displays, light units, smart windows, wearable electronics and of course last generation

solar cells (among other technologies) necessitate break through, cost effective electrodes that

can keep up with fabrication strategies, form factors, optoelectric requirements and other

characteristics of modern electronics. Specifically in PVs, transparency, flexibility and cost

effective processability are essential factors that must be combined with minimal PCE losses.

Thermal emitter Spectral control Solar Cell



Typically, doped metal oxides have conquered the field of transparent conductors with

tin-doped indium oxide (ITO) being the protagonist.130-132

However, the advantageous

characteristics- low sheet resistance (<10Ω/sq) at relatively high transmittance in the visible

regime (>90%), and favorable work functions (4.2-5.3 eV)- are accompanied with strong

disadvantages.133

From the economical point of view, the extreme growth of portable

electronics and displays has led to a vast demand of ITO. The short supply of Indium

inevitably raises the price of the material yearly. Additionally, the vacuum processing of ITO

films pushes fabrication cost even higher.131

However, cost related limitations are not the only

drawback. From engineering point of view, metal oxide films are brittle, a property that

impedes the future potential of flexible and stretchable electronics. 134-136

Figure 1-12: a) ITO replacement market forecast (source: Touch Display Research, ITO replacement: non-ITO Transparent Conductor Technologies and Market Forecast 2015 Report, 2015) b) Cost vs conductivity estimation of ITO-replacement technologies (source: Source: Touch Display Research Inc., ITO-Replacement Report, January 2016)

In order to address these problems researchers struggle to develop new electrodes with

suitable optoelectric and mechanical properties combined with low-cost processing that serve

modern high-tech demands. This can be clearly seen in the market forecast released by Touch

Display Research. Remarkably, ITO-replacement market is estimated to reach ~13 $ billions

revenue by 2023 (Figure 1-12a)

Carbon based materials-such as; graphene, carbon nanotubes (CNTs), conductive

polymers- and metallic nanostructures-such as; metal nanoparticles (NPs) and nanowires

(NWs)- are the most promising emerging materials for ITO replacement (Figure 1-12b)

Whereas, following a more engineering approach to solve this problem metal grids (formed

by metal NPs)-usually in combination with conductive polymers- show promising results.

Below I present a brief overview of those materials with some significant results.

a b



-Graphene

Since its recent discovery this carbon allotrope has heavily attracted the interest of

scientist due to its amazing optoelectrical and mechanical properties.137

Graphene is a two

dimensional honeycomb lattice- and similar to conjugated polymers- its carbon atoms form

sp2 hybridization to make bonds. Thus it shows high in-plain conductivity and transparency.

Sukang Bae et .al reported a chemical vapor deposition (CVD) grown, roll-to-roll produced

graphene with ~125 Ω/sq and 97.4% optical transmittance which outperform ITO.138

Additionally, graphene shows high flexibility (bending radius of 0.8mm has been reported),

and a moderate out of plane stretchability (tensile of up to 6% until mechanical failure).139

Except CVD methods a very appealing technique to fabricate graphene based electrodes, is

graphene oxide (GO) reduction. GO can be made from graphite by successive exfoliation and

oxidization. The GO sheets are hydrophilic and can be dispersed in aquatic or polar solvent

medium. These dispersions can be easily deposited on different substrates with high

throughput coating techniques. The resulted thin films are not conductive and a reduction

process should follow.140

However, solution processed reduced GO-based electrodes are not

mature yet as they demonstrate relative high sheet resistance (hundreds to thousands Ω/sq) for

60-80% transmittance values.141-143

- Carbon Nanotubes (CNTs)

The second carbon allotrope worth highlighting for its promising application in solution

processed transparent electrodes is carbon nanotubes (CNTs). CNTs can be visualized as

“rolled” graphene sheets and depending on the direction of their “rolling” (chirality) they

demonstrate metallic or semiconducting properties. Single-wall CNTs (SWCNTs) and multi-

wall CNTs (MWCNTs) have been created with corresponding diameters from sub-nm to tens

of nm.144

Even though individual CNTs show really low resistivity (in the range of μΩ cm)

the conductivity of a deposited film is limited due to impurities and defects. D. Zhang et al.

has demonstrated films (incorporated in OLEDs architecture) with sheet resistance of ~160

Ω/sq at 87% transmittance.145

A bonus feature is their high flexibility and in-plane

stretchability with unchanged conductivity values under 200% of strain (perpendicular to

drawing direction).139

CNTs can be processed with solution processing techniques and their

fabrication cost is potentially low.144, 146



-Conductive Polymers

Conjugated polymers such as poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonic

acid) (PEDOT:PSS) and polyaniline (PANI) combine good processability with interesting

optoelectronic and mechanical properties. Specifically, PEDOT:PSS has shown high

conductivity values upon doping (up to 1000Scm-1

with 5% addition of dimethylsulfoxide)

and for that reason is extensively used optoelectronic devices such as OLEDs and OPVs.

Impressively, the group of Zhenan Bao in Stanford University has shown fluorosurfactant-

treated PEDOT:PSS films with 46 Ω/sq at 82% transmittance. Additionally, the resulted films

showed no change in series resistance upon successive cycles of 0 to 10 % strain.147

PEDOT:PSS is known for the potentially very low cost electrode fabrication as it can be

easily coated with various solution processed methods.146, 147

-Metal Nanoparticles (NPs)

Metal nanoparticles (NPs) are often used to develop macro-scale structures such as

metal grids, honeycomb structures and rings to promote conductivity and transparency. Fine

structures have been reported with solution processed methods (such as inkjet printing and

printable embedded patterns) and self assembly techniques.144

Remarkably, Zhang et al.

utilized inkjet printing and the coffee stain effect to produce a grid from Ag NPs with um

range line widths.148

The resulted grid showed low line resistivity (10-3

-10-4

Ωcm) but it had

to be annealed at ~200°C for 2h. The same group has shown that similar structure meshes

demonstrate superior mechanical stability compared to ITO electrodes.149

In the upscaling of

OPVs, silver NPs based pastes in combination with conductive polymers and silver precursor

inks have been widely used.150-153

-Metal Nanowires (NWs)

NWs are structures with high length-to-diameter ratio, with diameter of several

nanometers and length of micrometers. Perhaps one of the most promising candidates for ITO

replacement in all electronic areas is silver nanowires (Ag NWs) electrodes. Showing a

combination of exquisite optical and electrical properties and being easily coated from

solution they have proven that can be the future direction of electrodes for different PV

technologies154, 155

and other devices.156-162

Ag NWs layers with 9.7 Ω/sq at 89%

transmittance have been reported which denotes electrical properties similar if not superior to

ITO.146

Their advantages do not stop in optoelectric properties; mechanically they show high

stability with flexibility and in plane stretchability under high strains (>100%).139

Most



importantly, the characteristic that separates them from the other materials of this list is that

all the high standards optoelectronic and mechanical properties reported for AgNWs are

usually fabricated high throughput solution process coating techniques.139, 144, 146

1.4 The art of upscaling

Thin film solar cells are promising devices for green energy in low cost. However,

practical applications require output voltages higher than what single or tandem solar cell of

different active materials can deliver (0.5V-2V). Additionally, building large area devices

from single cells would be completely inefficient as the parasitic electrical resistances over a

large active area would increase dramatically. A form of circumventing these limitations

resides in the possibility of electrically interconnecting multiple solar cells into a photovoltaic

module to further increase the voltage output and reduce the resistances. This step-also known

as upscaling- consists one of the most important steps before commercialization of a

photovoltaic device and hides two major packets of challenges.112, 163

The first packet of challenges that engineers face during up-scaling process is about

deposition methods of every functional layer. Usually, record efficiencies are achieved in

small scale demonstrators with specialized techniques for research purposes and not high-

throughput production. A classic example in the world of solution processed electronics is the

spin coating technique which can produce excellent small-scale results but without any large

scale applicability. Thus, engineers have to translate this processing into a different, up

scalable coating method (e.g. doctor blading) which many times shows complications as

different dry kinetics are involved. Another classical complication derives from the deposition

of electrode. In small-scale devices vacuum evaporation is usually used to form the metal

electrode; a time and energy consuming technique. On the other hand high-throughput

processing requires functional electrodes coated with solution processing techniques

(discussed in section 1.3 ). Additional difficulties can be added when during this transition

engineers overdraw the processing window of a functional material; in this case a close loop

between solution modification and processing must be repeated to end up with the optimal

results.

The second packet of challenges includes the complications derive from solar module

realization. When it comes to the transition from a solar cell to a solar module, geometrical

and electrical losses are inevitable. Specifically, the in-series connection between successive

cells is achieved through interconnection areas. These areas (so called “dead areas”) do not



contribute to the current generation of the module leading to current losses (due to

geometrical losses). In PV community the ration between the active area (area that contributes

to the current generation) and the total area of the module is known as geometric fill-factor

(GFF, details in chapter 2.7 ). Additionally the interconnection areas should ensure excellent

electrical connection between successive cells as poor quality interconnection would mean

ohmic losses that would dramatically decrease voltage and FF outputs. From the

abovementioned it is clear that engineers should achieve the highest quality electrical

interconnection in the smallest achievable areas to decrease the performance gap between

cells and modules. 164-166

On top of these challenges, the final product should be encapsulated to enter the market.

Encapsulation is usually done with resins and barrier foils or glasses (that may reduce

unwanted irradiation from the solar spectrum). Thus, a careful choice must be done also there

to avoid unnecessary chemical reactions (with the resin) and optical losses (from resin and

barrier).

In solution processed thin film solar cells where solar modules are made by coating and

patterning techniques, the interconnection losses (electrical and geometrical) depend on the

resolution of these techniques. In general coating techniques can be divided into 0-

dimensional (coating), 1-dimensional and 2-dimentional (printing) regarding their ability to

coat continuous films or patterns. Additionally, extra patterning techniques exist to make

interconnections through fine structuring of a coated film. Below, I present a brief description

and the main representatives for each set with highlighted the roll-to-roll compatible

-0-dimensional coating techniques

This set refers to coating techniques that can produce only continuous films without

giving the possibility to create patterns. A classic technique that belongs to this set and is

broadly used from scientists to produce small scale devices is spin coating. In spin coating

while a substrate (usually in the range of cm2) is spinning and solution is dropped to form a

uniform layer. The thickness of the film (from several nanometers to micrometer range) can

be controlled with the spinning and acceleration speed, temperature and viscosity of the active

solution. It is an easy and straightforward method but with some serious drawbacks; it

produces a vast waste of materials and it is not up scalable. Additionally, due to the nature of

the drying kinetics it also requires from engineers to perform research on translating high

efficiencies to an up scalable technique such as doctor blading.167



Doctor blading (or knife coating) is another 0-dimesional coating technique but with

high potential for up scaling (Figure 1-13). In doctor blading a knife moves on a regulated

height above a substrate removing the excess of ink to form a wet film under shear

(alternatively the knife can be static and a substrate web moves). Subsequently, the solvent in

the wet film evaporates leaving a thinner homogeneous dry film. The dry film thickness can

be regulated by the knife height, the coating speed, the viscosity and the temperature. Doctor

blading is considered an intermediate step before slot-die coating and can be used for sheet-

to-sheet or roll-to-roll fabrication routes. Slot die coating is a technique governed by similar

rules with a main difference; the continuous ink supply.167, 168

Figure 1-13: Schematic illustration for 0-dimensional and 1-dimensional coating techniques. Slot die coating and spray coating can produce patterns with shims and shadow masks correspondingly (details in text). Modified with permission.167

In contrast to doctor blading where a knife is spreading the solution, in slot die coating a

slot-die head continuously supplies ink to the web. In this technique some additional features

such as the flow rate of ink must be taken into account. Doctor blading is a 0-dimensional

coating technique, however it can be converted to 1-dimensional as discussed in the next

section. 168

Another 0-dimensional coating technique is spray coating (Figure 1-13). In spray coating,

a gun sprays the ink under a pressure. The ink forms micrometer range droplets which they

reach the substrate to form a film. Typically, the resultant films demonstrate increased surface

roughness. Spray coating is a 0-dimensional coating method but shadow masks have been

used to produce specific patterns.168

Substrate

Spin Coating Knife Coating Slot Die Coating

Substrate

Spray head

Spray Coating

Ink supply Ink supply

Meniscus



-1-dimensional coating techniques

As mentioned in previous section, doctor blading can be converted in 1-dimensional

coating method. To do so, special designed shims can be installed inside the slot-die coating

head. These shims are promoting the ink flow in specific areas, enabling defined striped films

to be formed. Although this method can produce patterned films with high throughput, the

resolution is quite low (sub-millimeter range) giving rise to high geometrical losses in a

potential solar module production.168

Nevertheless, this method has been extensively used for

roll-to-roll production of solution processed photovoltaic modules.167

-2-dimesional printing techniques

The 2-dimensional printing techniques are techniques that can produce films with more

complicated patterns. Famous techniques that belong to this set are gravure printing,

flexographic printing, screen printing and rotary screen printing. Gravure and flexographic

printing are governed by similar working principles; ink is transferred from an ink bath to a

patterned cylinder, eventually the cylinder comes in contact with the substrate and the pattern

is continuously transferred to the web (Figure 1-14).167

These techniques show resolution

from 30 to 50 μm and the resulted layer thickness can vary from sub-micrometer to several

micrometer ranges.168

On the other hand, screen printing and rotary screen printing have similar working

principles. In screen printing, a mask (screen) is placed over the substrate and a knife coats

(similar to doctor blade) a viscous paste (0.5-50 Pa s) on top. Eventually, paste passes through

the specific areas that are patterned on the mask leaving behind complicated structures. The

difference with rotary screen printing is the rotating mask (rotating screen); here the paste is

coated inside the roll (Figure 1-14).167

Typically resolution of more than 100 μm and film

thickness of several micrometers can be achieved with these techniques.168



Figure 1-14: Schematic illustrations of 2-dimesional printing techniques. Modified with permission.167

It is worth highlighting that there is another 2-dimesional printing technique with higher

freedom than the aforementioned. Inkjet printing is an advanced printing method that utilizes

nozzles to produce droplets continuously or on demand (DOD) with high accuracy and

resolution (~600 DPI).167

Usually, in DOD inkjet printing the ink flow is regulated by a

piezoelectric system. Additionally, this method enables coating of complicated structures;

characteristic examples such as face-portraits solar cells have been presented.169

Table 1-1 summarizes all the important characteristics for the most prominent coating

and printing techniques. The roll-to-roll compatibility of each technique is also shown.



Figure 1-15: Schematic illustration of the principles behind drop on demand piezoelectric inkjet printing and continuous inkjet printing. Modified with permission.167

Table 1-1 Comparison of film-forming techniques by printing and coating. Ink waste Ink waste: 1 (none), 2 (little), 3 (some), 4 (considerable), 5 (significant). Pattern: 0 (0-dimensional), 1 (1-dimensional), 2 (2-dimensional), 3 (pseudo/quasi 2/3-dimensional), 4 (digital master). Speed: 1 (very slow), 2 (slow<1 m min−1), 3 (medium 1–10 m min−1), 4 (fast 10–100 m min−1), 5 (very fast 100–1000 m min−1). Ink preparation: 1 (simple), 2 (moderate), 3 (demanding), 4 (difficult), 5 (critical). Ink viscosity: 1 (very low <10 cP) 2 (low 10–100 cP), 3 (medium 100–1000 cP), 4 (high 1000–10,000 cP), 5 (very high 10,000–100,000 cP).Reproduced with permission.170

Technique Ink waste Pattern Speed Ink

preparation

Ink

viscosity

(cP)

Wet

thickness

(μm)

R2R

compatible

Spincoating 5 0 – 1 1 0–100 No Doctor blade 2 0 – 1 1 0–100 Yes Casting 1 0 – 2 1 5–500 No Spraying 3 0 1–4 2 2–3 1–500 Yes Knife-over-edge 1 0 2–4 2 3–5 20–700 Yes Meniscus 1 0 3–4 1 1–3 5–500 Yes Curtain 1 3 4–5 5 1–4 5–500 Yes



Slide 1 3 3–5 5 1–3 25–250 Yes Slot-die 1 1 3–5 2 2–5 10–250 Yes Screen 1 2 1–4 3 3–5 10–500 Yes Ink jet 1 4 1–3 2 1 1–500 Yes Gravure 1 2 3–5 4 1–3 5–80 Yes Flexo 1 2 3–5 3 1–3 5–200 Yes Pad 1 2 1–2 5 1 5–250 Yes

-Post-patterning techniques

The realization of a module requires interconnection areas between top and bottom

electrodes of successive solar cells (details in chapter 2.7 ). Thus, patterned structures are

required to reveal bottom electrode and discontinue the top electrode. As highlighted

previously 0-dimensional coating techniques do not enable patterning rather they produce

continuous films. Patterning during coating or depositing films is available with 1-

dimensional and 2-dimensional printing techniques. Nevertheless, there are post-patterning

techniques that can produce different structures by removing material from specific areas after

the coating of a uniform film. Some of the most famous post-patterning techniques in the field

of solution processed solar cells are lithography, embossing, mechanical scribing and laser

patterning. The most promising among them for the production of solution –processed

photovoltaic modules is laser patterning. Ultra-fast laser ablation enables high resolution

structuring both in plane (10-50 μm) and in depth (nm range) without thermal heating of

surrounding material.171

Additionally, high processing speeds (up to 4m/s) ensure roll-to-roll

compatibility. Our group has demonstrated several times that combining 0-dimensional

coating techniques with laser patterning can result in solar modules with minimized losses and

efficiencies comparable to single cells.166, 172, 173

1.5 Motivation and Outline

The bloom and survival of every photovoltaic technology requires the harmonic function

of three necessary “gears” (Figure 1-16). In real life applications, an efficient device stands

no chance if not accompanied with long operating lifetime and cost-effective, ergonomic

processing. During this thesis, I try to strengthen every each “gear” for solution processed

solar cells (e.g. OPVs, Perovskites). Readers will go through achievements in smart

processing techniques; like printing on flexible substrates, laser patterning, lamination of

electrodes and substituent sub-cells for tandem structures accompanied with unprecedented

efficiency values. On top of that, minimization of efficiency losses over time comprising



stable active and charge-selective layers as well as integrated barriers through lamination of

the adhesive top electrode construct the important third “gear” of photovoltaic technology.

In the first Chapter of my thesis readers will have the opportunity to receive valuable but

general information regarding solar energy and the technological advances around solar cells.

I try to highlight the social and environmental impact that the harvesting of an abundant

energy from the sun can have to humanity. In this thesis I focused on developing smart device

fabrication strategies for solution processed solar cells that can be easily translated to large

area, roll-to-roll compatible processes. Thus, I give a brief overview about the solar

technologies on which my experiments were based, solution processed electrode and the

upscaling process.

In Chapter 2, I present some of the very basics of each solar technology used. A brief

theoretical background behind and the materials of organic solar cells and perovskite solar

cells are presented. The motivation for tandem solar cells is empowered when the efficiency

limits of single-junction and tandem concept are highlighted. The J-V characterization method

and the most significant values that can be extracted are demonstrated. In the last part of this

chapter the geometrical and electrical losses are analyzed.

Figure 1-16: The three necessary “gears” for any photovoltaic technology.

Chapter 3 is the classic materials and methods section. All materials used in this thesis

are presented with some of their characteristics and their structure. I reveal all the fabrication

roads for organic and perovskite single junction solar cells with evaporated and adhesive

electrode. The methods for tandem solar cells made monolithically or with lamination are also



shown. Additionally, readers will have the opportunity to go through our proposed fabrication

strategy for producing tandem solar modules and modules with adhesive top electrode.

Finally, all the characterization methods used in this thesis are presented.

In Chapter 4, I guide the readers through the main steps for realizing flexible organic

tandem solar modules with minimized losses. Firstly, a state of the art is demonstrated

concluded with what motivated us to focus on this work. In the second part, I present an

approach to make efficient flexible tandem cells. Materials screening, optical simulations and

roll-to-roll compatible processing lead to 6% efficient OPV tandem cell. Next, we present the

design and realization of flexible tandem solar modules with 5.7% efficiency. In the fourth

part of this chapter, I demonstrate the long operating lifetime of our tandem structure. In

conclusion, I summarize our findings, highlight the importance and point out some

disadvantages.

Lamination as a fabrication strategy, this is the name of the next chapter in which I

demonstrate materials and techniques for making inexpensive, efficient, semitransparent

electrodes for solar cells and potentially other electronics. After discussing a state of the art

and motivation for this work, I show the reinvention of adhesive top electrodes that lead to

highly efficient solution processed solar cells. We take this concept further and demonstrate

for the first time solution processed solar modules utilizing depth-resolved laser patterning.

Before I conclude with a summary of this chapter, I present another very promising

innovation that of lamination of two substituent sub cells into a tandem cell.

In the final chapter we summarize all the important findings of this thesis, highlight the

innovations and the weak points that must be empowered and give new perspectives for future

research in the field of solution-processed solar cells and electronics.


Chapter 2 Fundamentals

2.1 The theory behind organic solar cells

As it becomes clear from the 1st chapter of this thesis, where the state of the art for

organic solar cells was presented, the interest on these photovoltaic devices is increasing

through the years. Why all this interest if the efficiency is still inferior compared to

conventional solar cells? The answer is hiding to their main building blocks. These

photovoltaic devices are based on organic semiconductors, which with their amazing

properties and their solution processability they opened new technological roads. In the next

sub-chapters I discuss briefly the materials and the key concepts of the organic solar cells.

2.1.1 Organic semiconductors

The main building units of OSC are organic semiconductors. Organic semiconductors are

polymers, oligomers or small molecules with alternate single and double bonds across their

main backbone (conjugation). This conjugation prerequisite sp2 hybridized carbon atoms, in

which the three sp-2 orbitals will form s-bonds and the remaining pz orbital will form the p-

bonds. In such a way, the electrons occupy the pz orbitals create the alternative single-double

bonds and give the semiconductor properties to the material.174-176

The π -bonds are delocalized over the entire molecule and then, the quantum mechanical

overlap of pz orbitals on two carbon atoms splits their degeneracy and produces two orbitals, a

bonding (π) orbital and an antibonding (π∗).176

The lower energy π-orbital produces the

valence band, and the higher energy π∗ -orbital forms the conduction band. In a polymer

chain, several electrons contribute to the π system and the bonding and antibonding orbitals

further degenerate, and become broad quasi-continuous energy bands.176

By removing or

adding electrons in the delocalized conjugated backbone, we can achieve p-doping or n-

doping correspondingly. Analogous to the valence and conduction band in inorganic

semiconductors, the occupied π band forms the highest occupied molecular orbital (HOMO)

and the unoccupied π∗ band forms the lowest unoccupied molecular orbital (LUMO) of the

organic semiconductor.

The difference in energy between the HOMO and the LUMO is defined as the optical

bandgap. The bandgap controls the optoelectronic properties of the conjugated polymers and

its value varies among conjugated polymers depending on the geometry and the type of the



monomer units building the polymer. Typically for organic semiconductors, band gaps are in

the range of 1.5 to 3.5 eV, indicating that most of the polymers are active in the visible region.

Exciting an electron from the valence to the conduction band is equivalent to transferring an

electron from a bonding orbital to an antibonding orbital, by supplying energy greater than

the bandgap.176

Figure 2-1: a) The formation of σ and π bonding and π, π* orbitals in its simplest form for a molecule of ehtene. b) The corresponding energy diagram. The illustration shows the optical excitation from π (HOMO) to π* (LUMO) orbitals.

When a photon of the appropriate energy interacts with an electron in the ground state,

the electron is promoted from the HOMO (π-orbital) to the LUMO (π*-orbital) (Figure 2-1).

However, the resulting electron and hole are bound, and their motion through the material is

coupled. These coupled pairs are known as excitons. Excitons in organic semiconductors are

considered as Freknel excitons177

(Coulomb attractions in the order of 0.1-1 eV), in contrast

with inorganic semiconductor in which we define them as Mott-Wannier excitons178

(Coulomb attraction in the order of 0.01eV).179, 180

2.1.2 Bulk heterojunction

Based on these organic material properties, researchers started to build the first OPVs in

1960.23

However, following similar approach to the ones used for inorganic techniques

quickly failed to present high efficiency.25, 26

The reason was the different nature of the

materials on their optoelctronic properties. Organic semiconductors as materials with low

dielectric constant have strongly paired excitons (Frenkel excitons, 0.3-0.5eV) with coulomb

attractions one order higher than kT in room temperature (0.025 eV). That means that the

diffusion length of exciton is limited (in the range of 10nm instead of 100μm for single

crystalline silicon solar cell) and charge separation is not easy to occur at an interface of

En

erg

ysp2

pz

σ*-orbital

π*-orbital

π-orbital

σ-orbizal

optical excitation

antibonding

bonding

C C

H

HH

H

120°

σ-bonding

π-bonding

C C C C

C C

pz-orbitals in phase

pz-orbitals out of phase

C C

π* orbital

π orbital

a b



bilayer structure of n-type and p-type materials or polymer-fullerene heterojunctions.181-183

Thus, the question of efficient charge separation was not yet answered.

The bulk heterojunction concept solved this riddle and increased the yield of charge

separation and consecutively the efficiency of OSC. These advantages combined with the

process simplicity made it famous. Solution blends of p-type and n-type semiconductors are

combined and coated to form thin films with intermixed phases (Figure 2-2). Thus,

photogenerated excitons could find now in vicinity (<50nm) an interface to separate.

Figure 2-2: Bilayer vs bulk heterojunction structures. The exciton separation occurs at interfaces. Bulk heterojunction is more efficient because of the limited exciton diffusion length in organic materials. Reproduced with permission.184

-Operating Principles

A really useful generic description of the operating principles of bulk heterojunction

organic solar cells was presented by Carsten Deibel and Vladimir Dyakonov back in 2010.183

The whole process was separated into the following six steps. When a photon with energy

higher than the bandgap is absorbed (mostly by donor material), a singlet exciton is generated

Figure 2-3(i). The exciton will diffuse to a donor-acceptor interface Figure 2-3(ii) in order to

get dissociated-electron transfer in the acceptor molecule Figure 2-3 (iii). At this state the

electron-hole pairs are still Coulomb bound but in two different materials. The electron is in

the acceptor material forming a negative polaron and the hole remains on a donor material

forming a positive polaron. This Coulomb-bound polaron pair (also called charge transfer

state) has energy lower than exciton’s energy and it is the intermediate step before the

separation of charges. After dissociation, the electron-hole pair is separated by the build in

electric field (Figure 2-3(iv)) to free charges which they are transported to respective contacts

(Figure 2-3(v)). At the contacts the charges are extracted and photocurrent is produced



(Figure 2-3(vi)). It is worthwhile to notice that exciton generation can occur at acceptor

material also. In that case a corresponding procedure for the holes is followed.

Figure 2-3: Operating principles of bulk heterojunction solar cell. Left: Simplified kinetics diagram. Right: Simplified energy diagram.(i) Singlet exciton generation. (ii) Exciton diffusion. (iii) Exciton dissociation. (iv) Charge separation. (v) Charge transport. (vi) Charge extraction. Reproduced with permission.183

-Charge Transfer state (CT)

The charge transfer (CT) state, also known as polaron pair, is the very important

intermediate step between exciton dissociation and charge separation. It plays a crucial role

on organic bulk heterojunction devices as it determines both open circuit voltage and

photocurrent.185

Indeed, recent practical estimations of efficiency limits for single bulk-

heterojunction organic solar cells have incorporated losses associated with CT state (sub-

chapter 2.6.1 ). Thus, it is worthwhile to zoom in the previous generic operating principles

diagram and understand the role of the CT state in the whole operation of photocurrent

generation.

Figure 2-4 shows the energy levels relevant to charge generation, dissociation, separation

and recombination in a donor-acceptor system. An incident photon with energy higher than

the optical band gap (here presented as Eabs) is absorbed and an electron is excited (S0-S1

transition) to form a strongly bound singlet exciton in the donor material (Figure 2-4 (i),(ii)).

The exciton will diffuse in order to find a donor-acceptor interface and dissociate (Figure 2-4

(iii)). In case of absence of acceptor molecule in the vicinity (in the range of ~10nm,but

strongly depending on the materials system) the singlet will recombine radiatively. Otherwise,

dissociation will occur with a charge transfer if it is energetically favorable, meaning that the

singlet exciton energy (Eabs) is higher than the CT state energy (ECT). In this case the CT state

will be populated. At this stage, if the coulomb binding energy (lower than Frenkel exciton



binding energy) of the polaron pair is overcome, the pair separates into free charges (Figure

2-4 (iv)). If this is not the case, recombination of the charges can occur.183

Figure 2-4: Energy levels present in a donor–acceptor system which are relevant to the mechanisms of generation, recombination and dissociation of CT complexes. Reproduced with permission183

Polaron pair can recombine radiatively or non-radiatively; with direct transition to the

ground state or through a back transfer of the electron to a singlet or triplet state. These

oppositely charged polarons can originate from the same S0-S1 transition (geminate) or they

can be formed independently (non-geminate).183

It is worth highlighting that every transition

that leads to charge separation or recombination processes occurs under a probability that

depends on the energy difference between the initial and the final state. For organic solar cells

it has been shown that an ECT lower by 100meV from Eabs is close to optimum for driving a

separation process without inducing other losses.186, 187

On the recombination side, when a

triplet state energy is lower than CT state energy (ΔE≥100meV)186

, then most probably charge

recombination will occur via transition of the electron to the triplet state. These traps should

be avoided as they can diminish photocurrent and open circuit voltage leading to poor solar

cell performance.



2.2 The theory behind perovskite solar cells

In this sub-chapter I will give a brief overview of the basic theoretical background behind

perovskite solar cells. The main building blocks and the key characteristics of this technology

are presented.

2.2.1 Perovskite light absorbers

The heart of a perovskite solar cell is an active layer based on organo-metal

semiconductor materials that follow the formula ABX3; where A is an organic cation, B a

small inorganic cation and X a halogen anion. The resulting crystal structure is illustrated in

Figure 2-5a. Their formability can be estimated by Goldschmidt’s tolerance factor (t) and the

octahedral factor (μ). In perovskites, the Goldschmidt’s tolerance factor is the ratio between

A-X and B-X distances, considering molecules as ideal hard spheres:

where rA , rX, rB are the effective ionic radii of A, X, B. While the octahedral factor is given

by:

For halide perovskites is generally expected 0.813 < t < 1.107 and 0.44 < μ < 0.90 however

for high symmetry cubic structures the window of t becomes smaller (0.89-1.0 with the

highest symmetry demonstrated for highest t values).51, 188

By solving those equations for radii of various molecules and atoms of the A,B,X groups

scientists can study the crystallographic stability of each combination. It has been found that

organic cations with radii between 0.16-0.25 nm are forming stable crystal structures.

Therefore cations such as methylamonium (CH3NH3+, rA = 0.18nm), ethylamonium

(CH3CH2NH3+, rA = 0.23 nm) are giving promising results. As a halogen anion X chloride (Cl

-

), bromide(Br-) or iodide (I

-) are commonly used with 0.181 nm, 0.196 nm and 0.220 nm radii

correspondingly. Traditionally, for the inorganic cation B, Pb (rB = 0.119 nm) or Sn (rB =

0.110 nm)-for lead-free but less stable perovskite solar cells- have prevail. In total, the most

studied perovskite system is methylamonium lead trihalide (CH3NH3PbX3) with a tunable

optical bandgap between 1.5 eV (CH3NH3PbI3) and 2.3 eV (CH3NH3PbBr3) depending on the

halide anion.51, 188

𝑡 =(𝑟𝐴+𝑟𝑋)

√2(𝑟𝐵+𝑟𝑋)

𝜇 =𝑟𝐵

𝑟𝑋



Figure 2-5: a) Perovskite crystal structure of the form ABX3. b) The energy diagram of CH3NH3PbI3 perovskite resulted from the antibonding orbitals of the bonds between Pb (B) and I (X). The illustration shows the optical excitation highest occupied state to the lowest unoccupied state.

-Energy Band structure

Long time before the explosion of the perovskite field, Umebayashi et. al presented

useful insight on the energy band structure of perovskite crystals.189

By investigating 3D

crystals CH3NH3PbI3 and 2D crystals (C4H9NH3)2PbI4 they found that they are

semiconductors with a direct band gap at the R and Γ points respectively. Interestingly, the

band formation is only due to antibonding orbitals. Indeed, for the 3D crystal CH3NH3PbI3 ;

the highest occupied states (or top of the valence band) consists of the Pb 6s- I 5p σ-

antibonding orbital, while the lowest unoccupied state (or bottom of the conduction band)

consists of Pb 6p- I 5s σ-antibonding and Pb 6p- I 5p π-antibonding orbitals Figure 2-5b.

This investigation even though very informative it was simplistic and would give general

information as the authors did not take spin orbit interactions into account.190

More recently,

Even et. al. showed that spin splits in the highest occupied states would decrease the

theoretical band gap close to the experimentally observed values.191

It is worth highlighting the ease of band gap tuning in perovskite absorbers as it can have

great impact in optoelectronic devices. Great changes in band gap (from ~1.5 to ~2.3eV) have

been reported mainly by substituting the halogen anion or by using a mixed halide

composition. For example Sadhanala et al. demonstrated a bromide-iodide lead perovskite

film with a tunable band gap between the extremes of pure iodide (2.23V) and bromide

(1.57V) perovskites.192

One year later they showed similar tuning for bromide-chloride

perovskite films with tunable band gap between ~3.1 to 2.3eV.193

Alternatively, band gap

A

B

X

Ene

rgy

Pb 6s – I 5p *

Pb 6p – I 5p *

Pb 6p – I 5s *

Efoptical excitation

0D 3Dba



tuning has been reported by changing the organic cation, e.g. methylamonium to

formamidinium.194, 195

-Charge Transport

Perovskite films combine high absorption coefficient and balanced charge transport

behavior that ensures high efficiency cells. In 2013 Xing et. al and Stranks et al. investigated

the electron and hole diffusion length for CH3NH3PbI3 (presented by both groups) and

CH3NH3PbI3-xClx (presented from Stranks et. al).196, 197

For CH3NH3PbI3 films the electron

and hole diffusion length were found at ~130nm and ~100nm correspondingly. Nevertheless,

for CH3NH3PbI3-xClx films the diffusion lengths were even larger, with the remarkable values

of ~1069nm and ~1213nm for electron and hole respectively. These extremely large balanced

values –which indicate weakly bound excitons- in combination with high absorption

coefficient ( ~104-10

5cm

-1)

51 ensure sufficient photocurrent generation and charge extraction

in a solar cell based on perovskite absorber.

-Perovskite solar cell structure

The solar cell structure of devices employing perovskite absorber layers evolved over

time leading to the high efficiency perovskite solar cells of today.188

Since a state of the art

with the efficiency rising has been presented earlier (sub-chapter 1.2.2 ) here I will focus on

presenting the structural evolution. Initially, perovskite absorber was used as a sensitizer in a

dye-sensitized solar cell concept substituting the molecular dye (Figure 2-6a). In this concept,

a mesoporous oxide layer (e.g TiO2) forms a scaffold and perovskite is absorbed on the

interfaces, finally the whole structure is covered by an HTM. Here the oxide layer is used as

an electron acceptor and the HTM as a hole acceptor layer with a perovskite dye in their

interface to conclude a heterojunction. Later, a similar approach but with a non-injecting

scaffold (electron injection from perovskite to Al2O3 was prohibited) was followed to lead on

the first indication that electron transfer was possible in perovskite absorber and consequently

the dye-sensitized concept was not necessary (Figure 2-6b). After that a mesoporous structure

of TiO2 was infiltrated solely with perovskite absorber and the structure was concluded with a

thin layer of HTM on top (Figure 2-6c). Finally, a planar structure with p-i-n heterojunction

achieved high efficiency. Here the perovskite film would be the intrinsic layer, the HTM the

p-type layer and the oxide film (TiO2) the n-type layer. PEDOT:PSS and spiro-OMeTAD are



some other widely used p-type materials while metal oxides and PCBM are often used as n-

type materials (Figure 2-6d).

Figure 2-6: Structural evolution of perovskite solar cells: (a) sensitization concept with surface adsorption of nanodot perovskite, (b) meso-superstructure concept with non-injecting scaffold layer, (c) pillared structure with a nano oxide building block, and (d) planar p-i-n heterojunction concept. Spheres represent TiO2 in (a) and (c) and Al2O3 in (b). Reproduced with permission.188

-Operating principles

In structures a) and b) presented in Figure 2-6 the charge transport properties are not

critical. However in pillared structure and planar p-i-n heterojunction concept charge

generation, separation, extraction and recombination have to be in balance for efficient solar

cell. The main mechanisms occurring behind photocurrent generation as proposed by Arianna

Marchioro et al.198 will be described on a CH3NH3PbI3 solar cell with spiro-oMeTAD as hole-

transporting material (HTM) and TiO2 nanoparticles.

After perovskite material absorbs a photon with energy equal or higher than the band gap

energy, an exciton can be formed (Figure 2-7(i)). As outlined before electron and hole

diffusion length has been found large indicating a weakly bound exciton. Charges diffuse to

the corresponding n and p layers and separation can occur through injection of electrons in

TiO2 nanoparticles layer (n-type layer) (Figure 2-7(ii)) or injection of holes in HTM layer (p-

type layer) (Figure 2-7(ii)). Radiative and non-radiative recombination processes (Figure

2-7(iv)), as well as back charge transfer at the interfaces (Figure 2-7(v),(vi)), and between

TiO2 and HTM (Figure 2-7(vii)) can diminish the performance of the device. For an efficient

solar cell these loss processes must occur slower than charge generation and extraction

processes.



Figure 2-7: Schematic illustration of energy levels and processes in a perovskite solar cell employing TiO2 and an HTM.

2.3 Device architectures

OPVs, perovskites and tandem solar cells can be fabricated in normal or inverted

architectures. The names normal and inverted are nothing but a convention among the PV

community and they refer to the fabrication sequence of the device. For different architectures

different electrodes or buffer layers may be used and the photogenerated current travels to

opposite directions. Inverted architectures traditionally show higher stability and they are

extensively used through this thesis. Figure 2-8 illustrates normal and inverted architectures

for single junction and tandem solar cells.

Devices can be developed on various substrates with mechanical properties coherent to

the fabrication route and the utility of the final product. Glass and polyethylene terephthalate

(PET) substrates are the most used, but more exotic substrates have been also reported.17, 18,

199, 200. For solar cells and other optoelectronics, at least one of the electrodes should show

high transparency. The most widely used transparent electrode is ITO but it presents

tremendous disadvantages (the readers are addressed to sub-chapter 1.3 ). If transparency for

the whole device is not a requirement then a metal (usually Ag for inverted and Al for normal

structure due to work function appropriateness) usually forms the second electrode.

Ene

rgy

TiO2 Perovskite HTM

hν

(i)

(ii)

(iii)

(iv)(v)

(vi)(vii)



Alternative electrodes have been discussed earlier (sub-chapter 1.3 ). The important

correlation between transparency and sheet resistance will be discussed in the next section.

Figure 2-8: Normal architecture for single junction (a) and tandem solar cell (c). Inverted architecture of single junction (b) and tandem solar cells (d).

In solution processed solar cells, the most commonly used hole transport layer (HTL) is

PEDOT:PSS. However, solution processed metal oxides such as V2O5, WO3 or MoO3 have

been also reported efficient.201-203

Alternatively, if solution processability is not a requirement

thermal evaporation of MoO3 is frequent. As electron transport layers (ETLs) n-type metal

oxides (such as ZnO and TiOX) and polymer electrolytes (such as PFN,CPE and PEIE) have

been reported for equally high performance.204-209

While for normal architecture thermal

evaporated LiF and Ca are used. It is worthwhile to notice that additional modifying layers for

HTLs and ETLs have been frequently reported to improve the extraction of carriers.210

ba

c d



2.4 Electrodes

In general, when it comes to the performance evaluation of a semi-transparent electrode,

two major aspects have to be linked; the transmittance (T) and the sheet resistance (Rsheet).

Typically, high transmittance can be achieved by reducing the thickness of the electrode.

However, this usually also leads to decreased conductivity. The challenge becomes clear;

materials that exhibit high conductivity combined with low thickness- high transmittance with

low sheet resistance- must be employed.

Several experimental studies have been conducted to show the T vs Rsheet correlation.

Based on them, useful figure of merits (FOMs) that link these two values can been

extracted.133, 211

Expressing transmittance as function of thickness (t), for a film thinner that

the wavelength of light, we end up with the following equation:

where Z0 is the impedance of free space (~376.73 Ω) and σop the optical conductivity (a

fundamental property of the material that is related to the absorption coefficient α through

σop α/ Z0). Sheet resistance as function of thickness can be expressed as follows:

where σDC,B is the DC bulk conductivity of the film. Then by combining equation 2.2 and 2.1

we obtain a relationship between T and Rsheet:

Usually, the ratio σDC,B/ σop is used as one useful FOM to describe the T vs Rsheet of a given

film. For higher values of the ratio, a film can achieve higher transmittance for lower sheet

resistance. To get an indication of the order of magnitude of this number it is worth

mentioning that σDC,B/ σop ≥35 is required to reach T≥90% with Rsheet≤100 Ω/sq.

However, fits to experimental data of T vs Rsheet have shown that very thin layers with

high transmittance (T>50-90%, depending on the material) do not follow a trend described

from equation 2.3. This deviation was explained with percolation effects. According to the

percolation theory the conductivity (σDC) of a thin spread inhomogeneous network of

conductive pathways (such as a thin semitransparent electrode of silver nanowires) should be

𝑇 = (1 +𝑍0

2𝜎𝑜𝑝𝑡)

−2 (2.1)

𝑅𝑠ℎ𝑒𝑒𝑡 = (𝜎𝐷𝐶,𝐵𝑡)−1

(2.2)

𝑇 = (1 +𝑍0

2𝑅𝑠ℎ𝑒𝑒𝑡

𝜎𝑜𝑝

𝜎𝐷𝐶,𝐵)−2

(2.3)



described by a formula that includes a critical thickness (threshold thickness of the film)

associated with the percolation threshold:

where n is the percolation exponent. For a network with high enough conductivity (t >>tc)

equation 2.4 can be changed to:

where tmin is the thickness at which σDC,B = σDC . Now a new description of sheet resistance of

the film for this regime would be:

then by solving 2.6 for t and combining with 2.1 we obtain:

where Π is the percolative FOM:

is worth noticing that for n=0 we go back to the equation 2.3 as it is also defined from σDC.

Taking a closer look to the final equations, we see that the ratio σDC,B/ σop is inside the

percolative FOM but does not control it as previously. In general, a film that shows high Π

values combines high T with low Rsheet.

The semi-transparent solution processed electrodes with high potential were introduced

earlier in sub-chapter 1.3 Graphene, carbon nanotubes (CNTs), conductive polymers, metal

grids and metal nanowires (NWs) have been proven promising as ITO substitutes in solar cell

architectures. Figure 2-9 shows the T as a function of Rsheet for the most famous choices. It is

worth noticing that for solution processed electrodes the highest values for the σDC,B/ σop FOM

have been reached by metallic NWs electrodes (σDC,B/ σop =106-453) and the lowest by

graphene only electrodes (σDC,B/ σop 1.3). 133

𝜎𝐷𝐶 ∝ (𝑡 − 𝑡𝑐)𝑛 (2.4)

𝜎𝐷𝐶 = 𝜎𝐷𝐶,𝐵 (𝑡

𝑡𝑚𝑖𝑛)𝑛

(2.5)

𝑅𝑠ℎ𝑒𝑒𝑡 = (𝜎𝐷𝐶𝑡)−1 = [𝜎𝐷𝐶,𝐵 (

𝑡

𝑡𝑚𝑖𝑛)𝑛𝑡]−1

= (𝑡𝑚𝑖𝑛

𝑛+1

𝑡𝑚𝑖𝑛𝜎𝐷𝐶,𝐵𝑡𝑛+1) (2.6)

𝑇 = [1 +1

𝛱(

𝑍0

𝑅𝑠ℎ𝑒𝑒𝑡)1 (𝑛+1)⁄

]−2

(2.7)

𝛱 = 2 [𝜎𝐷𝐶,𝐵 𝜎𝑜𝑝⁄

(𝑍0𝑡𝑚𝑖𝑛𝜎𝑜𝑝)𝑛]1 (𝑛+1)⁄

(2.8)



Figure 2-9: Transmittance versus sheet resistance for promising solution processed electrodes.164, 212-214 Transmittance values were obtained at ~550nm. The bulk regime (described by equation 2.3) is shown with solid line. The percolation regime (described by equation 2.7) is shown with dashed line.133

2.5 Current-voltage characteristics and diode equation

-Current-voltage characteristics

The most important characteristics of a solar cell can be extracted from a current (or

current density) versus voltage curve under dark and illuminated conditions (Figure 2-10).

Figure 2-10: a) Linear and b) semi-logarithmic presentation of J-V curves und dark and illuminated conditions. Reproduced with permission.183

a b



The measurement occurs by recording the current of the device while a range of voltages is

scanned, usually at room temperature and under specified illumination or in the dark. The

parameters that determine the power conversion efficiency (PCE) and can be extracted from a

J-V curve under illumination are :

o Open circuit voltage (Voc), as its name reveals, is the applied voltage at which the

current density equals to 0. At this point the diode is open and no current flows

through the device

o Short circuit current density (Jsc) is the current density of the device when the applied

voltage equals to 0.

o Maximum power point (MPP) is the point between short-circuit and open-circuit

conditions at which the generated power (P = V x I) reaches the highest value. At this

point V=VMPP and I=IMPP.

o Fill Factor (FF) is the ratio between MPP and the product of Jsc and Voc and is

described by the following formula

o Then, the power conversion efficiency (PCE) of a solar cell is described as:

-Shockley-diode equation

In 1961 Shockley and Queisser estimated the efficiency limits (sub-chapter 2.6 ) for inorganic

solar cells by using the famous Shockley-diode equation. Shockley-diode equation models a

solar cell simply as a parallel connection of a photocurrent generator and a diode. Thus, in the

dark the net current density (J) at a given voltage (V) for an ideal solar cell is given by:

where Jo is the dark saturation current density of the diode, q is the elementary charge, n the

diode ideality factor, k the Boltzman constant and T the temperature (kT describes the thermal

energy). Under illumination the two components generate a current with opposite directions

and the equation changes to:

𝐹𝐹 =𝑉𝑀𝑃𝑃×𝐼𝑀𝑃𝑃

𝑉𝑜𝑐×𝐼𝑠𝑐 (2.9)

𝑃𝐶𝐸 =𝑃𝑜𝑢𝑡

𝑃𝑖𝑛=

𝐹𝐹×𝑉𝑜𝑐×𝐼𝑠𝑐

𝑃𝑖𝑛 (2.10)

𝐽 = 𝐽0 [𝑒𝑥𝑝 (𝑞𝑉

𝑛𝑘𝑇) − 1] (2.11)


𝑛𝑘𝑇) − 1] − 𝐽𝑠𝑐 (2.12)



By solving the equation for J=0 we find the correlation of Voc with the diode and

photogenerator characteristics:

Although a good first approximation the above mentioned equations refers to an ideal solar

cell. In order to describe the J-V behavior of a real solar cell, two resistance elements were

introduced in the ideal Shockley-diode equation. The series resistance (Rs), which is

electrically connected is series with the diode (Figure 2-11), describes charge transport

resistances (contact resistances, injection barriers, sheet resistances etc.). The parallel or shunt

resistance (Rp), which is electrically connected in parallel with the photodiode, describes

shunts between two electrodes and every current flow pathway that bypasses the diode. Thus,

the equation now becomes:

where Jph the optional photocurrent density (shift of J-V under illumination, ~ Jsc). An

efficient system would demonstrate minimized Rs and maximized Rp. These elements can be

extracted from semi logarithmic representation of J-V graphs under dark and illuminated

conditions (Figure 2-10b).

Figure 2-11: Single diode equivalent circuit model commonly employed in estimating electrical losses in solar cell.

Rp

Rs

hνV

+

-

J

Jph

𝑉𝑜𝑐 =𝑛𝑘𝑇

𝑞ln (

𝐽𝑠𝑐

𝐽0+ 1) (2.13)

𝐽 = 𝐽0 [𝑒𝑥𝑝 (𝑞(𝑉−𝐽×𝑅𝑠)

𝑛𝑘𝑇) − 1] −

𝑉−𝐽×𝑅𝑠

𝑅𝑝− 𝐽𝑝ℎ (2.14)



2.6 Efficiency limits of solar cells

What is the efficiency limit of a solar cell? This question is one of the most fundamental

and perhaps vital questions in the field. Not only because it reveals the boundaries of solar

technology but also because it gives us hints on how to circumvent those boundaries. One of

the most famous examples of fighting those boundaries, the multi-junction (or tandem)

concept, has its roots on the deep understanding of the fundamental problems. In the next sub-

chapters all these are discussed.

2.6.1 Shockley-Queisser limit for single junction solar cells

Back in 1961, William Shockley and Hans-Joachim Queisser make one very significant

contribution to the field of solar energy. They calculated the maximum theoretical conversion

efficiency (detailed balance limit) of inorganic single p-n junction solar cells by assuming

several hypothesis and mechanisms based on thermodynamic phenomena.215

The main

hypothesis were;

i) Only photons with higher energy than the bandgap (hvg > Eg) could be absorbed

and contribute in photocurrent generation (step function). Photons with higher

energy than the bandgap would excite charge carriers in higher states than Ec.

These hot carriers would then relax through thermalization process to Ec.

ii) Only radiative recombination occurs (loss mechanism). Cell is assumed to radiate

as an ideal black body with temperature of 300°K. Sun is assumed to radiate as an

ideal black body with a temperature of 6000°K.

iii) Each photon absorbed produces an electron that eventually will be extracted,

meaning IQE values of 100%.

-Ultimate efficiency limit

According to these assumptions and since radiative recombination was assumed the only

loss mechanism, they first calculated the ideal ultimate efficiency of a circular cell with a T=

0°K. For the ideal black body behavior that means that no recombination occurs. The cell was

surrounded by a cavity which was emitting at a T=Ts=6000°K (corresponding to sun). Then

the efficiency would be

𝜂 = 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑝ℎ𝑜𝑡𝑜𝑛 𝑒𝑛𝑒𝑟𝑔𝑦

𝑖𝑛𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦=

ℎ𝑣𝑔𝑄𝑠

𝑃𝑠 (2.15)



where hvg is the energy of a photon required to be absorbed, Qs the number of photons

absorbed and Ps is the input energy containing the spectrum. By using the Plank’s law which

gives the spectral photon flux (ϕ) as a function of wavelength (or frequency, v) and the

temperature of the black body (T):

they calculated Qs and Ps as follows :

where xg is given by

corresponding to the energy of the band gap.

Then eventually by substituting equations 2.18 and 2.16 in 2.15 they found that an ideal

ultimate efficiency of ~44% for a material with a band gap of ~1.08eV. However this

efficiency is far from a realistic scenario since no losses were assumed.

-Realistic efficiency limit

In a more realistic approach, the radiative recombination losses as well as a rectangular

architecture of a solar cell was taken into account. Due to the rectangular architecture the

number of photons absorbed (Qs) was changed by a geometrical factor. To estimate the

radiative recombination losses, they assumed that a cell at a temperature of 300°K is

surrounded by a cavity of an equal temperature (which corresponds to dark ambient). No

voltage is applied to the cell, thus the whole system is in thermodynamic equilibrium. That

means that the radiation absorbed by the cell equals to the radiation emitted by the cell which

results completely from radiative recombination as was hypothesized. Then the radiation

absorbed which equals to the radiation from radiative recombination (RRR ) at V=0 would be:

𝜙(𝑣, 𝑇) =2ℎ𝑣3

𝑐21

𝑒

ℎ𝑣𝑘𝐵𝑇−1

(2.16)

𝑄𝑠 ≡ 𝑄(𝑣𝑔, 𝑇𝑠) =2𝜋

𝑐2∫

𝑣2𝑑𝑣

𝑒ℎ𝑣𝑘𝑇𝑠−1

=2𝜋(𝑘𝑇𝑠)

3

ℎ3𝑐2∫

𝑥2𝑑𝑥

𝑒𝑥−1

∞

𝑥𝑔

∞

𝑣𝑔 (2.17)

𝑥𝑔𝑘𝑇𝑠 = ℎ𝑣𝑔 = 𝑞𝑉𝑔

𝑃𝑠 =2𝜋

𝑐2∫

𝑣2𝑑𝑣

𝑒ℎ𝑣𝑘𝑇𝑠−1

=∞

0

2𝜋(𝑘𝑇𝑠)4

ℎ3𝑐2∫

𝑥3𝑑𝑥

𝑒𝑥−1

∞

0 (2.18)



where A is the area of the cell and Qc is the number of photons absorbed by the cell:

To find the RRR for V≠0 they utilized pn junction theory as follows. If V=0 then:

where n denotes the number of elections and p the number of holes and the indicator the

voltage. If V≠0 then the rate of radiative recombination becomes:

then according to equation 2.19 the main loss mechanism would be described by:

By knowing the amount of absorbed photons Qs, the amount of recombined emitted photons

Qc and the general correlation of the rate of recombination and the applied voltage, they end

up with a current voltage relationship (which is of the same form as the famous Shockley

diode equation, equation 2.12):

where the second term

is independent of voltage. And the first term includes the loss mechanism.

The efficiency of a solar cell can be described also as:

Then by including following equation 2.25 the authors plot the η as a function of xg

(which corresponds to the band gap of the material) to find out a maximum efficiency of

~30% for an energy band gap of 1.1eV. 215

𝑅𝑅𝑅,𝑉=0 = 2𝐴 𝑄𝑐 (2.19)

𝑄𝑐 = 𝑄(𝑣𝑔, 𝑇𝑐) =2𝜋(𝑘𝑇𝑐)

3

ℎ3𝑐2∫

𝑥2𝑑𝑥

𝑒𝑥−1

∞

𝑥𝑔 𝑥𝑐⁄ (2.20)

𝑅𝑅𝑅 = 2𝐴 𝑄𝑐𝑒𝑞𝑉

𝑘𝑇 (2.23)


𝑛𝑘𝑇) − 1] − 𝐽𝑠𝑐 (2.24)

𝐽𝑠𝑐 ∝ 𝑞(𝑄𝑠 − 𝑄𝑐) (2.25)

𝜂 = 𝑃𝑚𝑎𝑥

𝑃𝑖𝑛=

max(𝐽𝑉)

𝑃𝑖𝑛 (2.26)

𝑅𝑅𝑅,𝑉=0 ∝ 𝑛0𝑝0 (2.21)

𝑅𝑅𝑅,𝑉≠0 ∝ 𝑛𝑝 = 𝑛0𝑝0𝑒𝑞𝑉

𝑘𝑇 (2.22)



-Useful equivalent formulas

The phenomena introduced above can be equally described by equivalent formulas which

in some cases are more practical (see next sections). The efficiency of a solar cell

incorporating semiconductor with band gap energy of Eg can be equally described as :

which is the same equation we practically use when measuring current-voltage

characteristics (eub-chapter 2.5 ). Jsc can be expressed as:

where for a step function as assumed in SQ theory α(E) is zero for E<Eg and the

boundaries of the integral can be changed from Eg to ∞. Or equally expressed:

and

where ϕsun is the spectral photon flux density of the sun incident on flat surface, ϕbb is

the spectral flux density emitted by a black body at temperature T ( the environment, which in

case of SQ theory since all recombination processes are radiative it also corresponds to the

spectral flux density emitted by the cell).46

For a more realistic approach where the

recombination processes are not only radiative, Rau showed that a reciprocal relationship

between quantum efficiency of electroluminescence (EQEEL) and external quantum efficiency

(EQE) exists :216

then a practical dark saturation current J0 can be described as function of EQEEL and EQE :

𝜂 =𝐽𝑠𝑐𝑉𝑜𝑐𝐹𝐹

𝑃𝑖𝑛 (2.27)

𝐽𝑠𝑐 = 𝑞 ∫ (𝜙𝑠𝑢𝑛(𝐸) − 𝜙𝑏𝑏(𝐸))𝑎(𝐸)𝑑𝐸∞

0 (2.28)

𝐽𝑠𝑐 = 𝑞 ∫ 𝜙𝑠𝑢𝑛(𝐸)𝐸𝑄𝐸(𝐸)𝑑𝐸∞

0 (2.29)

𝐽0 = 𝑞 ∫ 𝜙𝑏𝑏(𝐸)𝑎(𝐸)𝑑𝐸∞

0 (2.30)

𝐽0 =𝑞

𝐸𝑄𝐸𝐸𝐿∫ 𝐸𝑄𝐸(𝐸)𝜙𝑏𝑏(𝐸)𝑎(𝐸)𝑑𝐸∞

0 (2.32)

𝐸𝑄𝐸𝐸𝐿(𝐸) = 𝐸𝑄𝐸(𝐸)𝜙𝑏𝑏(𝐸) [𝑒𝑥𝑝 (𝑞𝑉

𝑛𝑘𝑇) − 1] (2.31)



2.6.2 Efficiency limits in single-junction organic solar cell

The SQ theory can be applied to organic solar cells basically with a main extension;

charge separation in bulk heterojunction occurs through a CT state and these states determine

Voc losses. Even though ideal materials such the ones described for SQ theory are far from

organics, the OPV community used it as useful tool to extract semi-empirical theoretical

limits. This is beneficial for the community as losses were identified and new design rules for

creating novel materials (appropriate band gap and HOMO levels to increase and efficiency

and Voc ) and device architectures were used to circumvent them.

In 2006, Scharber et al. predicted for the first time the power conversion efficiency limit

for bulk-heterojunction solar cells based on conjugated donor polymer and PCBM

acceptors.217

The maximum limit was found to be ~11% after several empirical estimates.

Initially, the Voc is reduced due to the lowest difference between the LUMO levels of donor

and acceptor that efficient charge separation would occur (as set by the authors, ΔLUMO >

0.3eV) (Figure 2-12b). Additionally it has been empirically proven that Voc is not the exact

difference between donor’s HOMO and acceptor’s LUMO but reduced by an additional factor

of 0.3eV:

On top of those fundamental losses, Scharber et al. assumed charge carrier transport losses

(FF = 65%) and a constant EQE of 65% which brought the limit to its final value. These

values for EQE and FF were chosen according to the best experimental values of the time.

During the following years Koster et al.218 , Minnaert et al.219

and Lunt et al.229 followed

approaches where they assumed different values for Jsc, EQE , FF, Voc and ΔLUMO to

predict a realistic limit close to what presented from Scharber et al. Meanwhile, continuous

development of novel materials led to more efficient devices with higher FFs and EQE values.

In 2011 Lunt et al. assuming that the Voc of single junction nanostructured devices could

reach 80% of the SQ limit, and 75% for both EQE and FF he estimated an practical limit of

17%. While, in 2013 Scharber et al. updated his older prediction to find an achievable PCE of

15%, assuming a FF of 75% and a constant EQE of 80% (Figure 2-12a).220

𝑉𝑜𝑐 =1

𝑒(|𝐸𝐻𝑂𝑀𝑂

𝐷 | − |𝐸𝐿𝑈𝑀𝑂𝐴 |) −0.3V (2.33)



Figure 2-12: a) PCE prediction of a bulk heterojunction solar cell with PCBM as acceptor material. For the calculation Scharber et al. assumed FF of 75%, EQE of 80% and Voc according to Eq.2.13. b) Simplified energy diagram of a donor acceptor system. Reproduced with permission.220

Although good practical estimations of the efficiency limit for the organic solar cells, the

aforementioned approaches do not include the influence of the CT state (sub-chapter 2.1.2 ) in

Voc. Since 2008, Vandewal et al. had already observed experimentally the CT state by

measuring EQE with the high sensitive Fourier-transform photocurrent spectroscopy (FTPS),

and given a first estimation about the losses induced in Voc.221

In consistence with Vandewal

et al., in 2009 Veldman et al. by studying several donor and acceptor materials he obtained

the following relation between Voc and ECT :

Additionally, the authors experimentally showed that energy difference of 100meV between

Eg and ECT is sufficient for charge transfer.186

That would mean that Eg - qVoc 0.6eV. With

these observations in hand and with similar assumptions to the first estimation of maximum

efficiency from Scharber et al. (65% for EQE and FF) Valdeman et al. presented a maximum

efficiency of 11% for bulk heterojunction systems when the lowest optical band bap (Eg)

values lies between 1.37-1.45 eV (Figure 2-13).

a b

𝑞𝑉𝑜𝑐 = 𝐸𝐶𝑇 − 0.47(±0.06)𝑒𝑉 (2.34)



Figure 2-13: Theoretical efficiency of bulk-heterojunction photovoltaic devices with Eg− qVoc= 0.60 eV (solid line) versus the lowest optical bandgap of the two materials, calculated using the AM1.5 spectrum, FF = 0.65, and assuming constant EQE = 0.65 between 3.5 eV and Eg . The dashed lines show the theoretical efficiencies for devices using the larger Eg − qV oc offsets for (from top to bottom): PF10TBT:[60]PCBM (0.70 eV,circles), PCPDTBT:[70]PCBM (0.76 eV, down triangles), PBBTDPP2:[70]PCBM (0.80 eV, up triangles), and P3HT:[60]PCBM (1.09 eV, squares). The closed markers represent the theoretical efficiency, the open markers the device efficiencies. Reproduced with permission. 186

Since then, further investigations on CT state gave new insight on the relation to Voc.

Specifically, Vanderwal et al. utilized effect of the reciprocal relationship by Rau in J0

(equation 2.32) and the Voc formula from Shockley diode equations (equation 2.13) to prove

that:

This formula emerges the three main factors determine the Voc and correlates them with

measurable properties.187, 222, 223

The first term (Ect/q) relates to the energy of the CT state.

The second term (𝑘𝑇

𝑞ln (

𝐽𝑠𝑐ℎ3𝑐2

𝑓𝑞2𝜋𝐸𝐶𝑇)) shows the radiative voltage losses and relates to the CT

absorption strength (𝑓) which results from the density of donor-acceptor contacts and donor-

acceptor electronic coupling. While the third term (𝑘𝑇

𝑞ln(𝐸𝑄𝐸𝐸𝐿)) describes the non-radiative

voltage losses and relates to EQEEL. With approximate values of ~0.25eV and ~0.35eV for the

second and third term respectively a new empirical approximation for VOC and ECT was given:

𝑞𝑉𝑜𝑐 = 𝐸𝐶𝑇 − 0.6𝑒𝑉 (2.36)

𝑉𝑜𝑐 ≈𝐸𝐶𝑇

𝑞+

𝑘𝑇

𝑞ln (

𝐽𝑠𝑐ℎ3𝑐2

𝑓𝑞2𝜋𝐸𝐶𝑇) +

𝑘𝑇

𝑞ln(𝐸𝑄𝐸𝐸𝐿) (2.35)



Although very close to previous on presented by Veldman et al. the theory behind this

approximation gave important insight on radiative and non-radiative losses in organic solar

cells. It is worth highlighting that EQEEL which determines the non-radiative losses, is ~10-6

for organic solar cells and results in ~0.36V losses, while in inorganics is much lower.187

For

example crystalline Si solar cells show EQEEL~10-3

resulting in 0.18V losses, and GaAs solar

cells show EQEEL~10-2

resulting in 0.12V.224

Most recently, Scharber released a new update where he follows a similar approach to

the one presented in 2006 and 2013, but takes into account the CT state and VOC energy losses

of qVOC –Eg= 0.7eV.225

Assuming EQE and FF of 70% he demonstrated that to surpass a

maximum efficiency of ~12% the energy loss (qVOC –Eg ) should be limited to less than

0.7eV (Figure 2-14). However, the work of Li et al. in 2015 suggests that systems with

energy losses lower than 0.7eV are difficult to reach EQE values greater than 70%.226

Figure 2-14: Contour plot showing the calculated energy-conversion efficiency (contour lines and colors) versus the absorption onset and the HOMO level of the donor polymer according to ref. [217] assuming an EQE and a FF of 70%; Dots indicate the performance potential of the investigated polymers. Reproduced with permission. 225

2.6.3 Efficiency limits in single-junction perovskite solar cell

Similarly to organic solar cells, the SQ theory can be also applied to perovskite solar cells

by introducing the main losses of these devices. Additionally, perovskite solar cells exhibit

characteristics closer to the ideal case, making these predictions more realistic.197, 227

The

radiative and non-radiative recombination losses can be studied in open circuit condition and

,as shown in case of organics, they influence the maximum Voc.



In 2014, Tvingstedt et al. followed a similar approach shown by Vandewal et al. (in

OPVs)187

to demonstrate the Voc losses in perovskites. Initially by using the reciprocity

relation between electroluminescence quantum efficiency (EQEEL) and the photovoltaic

quantum efficiency (EQE) described by Rau (equation 2.31) he expressed the dark saturation

current (J0) as:

where as described previously, the ϕbb is the spectral flux density of a black body that

irradiates at temperature, T (the environment). When a solar cell reaches the radiative limit

(meaning that all recombination processes are only radiative) the EQEEL=1 and J0:

In which if we assume also EQE=1 we go back to SQ theory.

Then by using the Shockley diode equation (equation 2.13) for the Voc the authors ended

up with the following expression:

With a close look on that equation we can see the factors determine the Voc. Higher Voc values

can be obtained by lowering J0,Rad (meaning reducing in general the recombination losses),

with a sharper EQE spectral shape, and by increasing EQEEL to unity (meaning that

recombination losses become only radiative). This term that includes EQEEL as previously

discussed determines the non-radiative losses of Voc and thus shows the offset between the

real Voc and the Voc in radiative limit. The authors found that CH3NH3PbI3 solar cells exhibit

EQEEL values of ~10-4

which corresponds to ~0.23V losses in Voc, much lower compared to

OPVs. Finally, they authors proved a strong photoluminescence quenching in case of

perovskite solar cells when going from Voc to Jsc conditions highlighting that perovskite solar

cells are closer to the ideal case when compared to OPVs. These findings show the potential

𝐽0 =𝑞

𝐸𝑄𝐸𝐸𝐿∫ 𝐸𝑄𝐸(𝐸)𝜙𝑏𝑏(𝐸)𝑑𝐸∞

0 (2.37)

𝐽0,𝑅𝑎𝑑 = 𝑞 ∫ 𝐸𝑄𝐸(𝐸)𝜙𝑏𝑏(𝐸)𝑑𝐸∞

0 (2.38)

𝑉𝑜𝑐 =𝑘𝑇

𝑞ln(

𝐽𝑠𝑐𝐸𝑄𝐸𝐸𝐿

𝑞 ∫𝐸𝑄𝐸(𝐸)𝜙𝑏𝑏(𝐸)𝑑𝐸) (2.39)



of perovskite solar cells to achieve a maximum power conversion efficiency limit closer to the

inorganic solar cells predicted by SQ theory.

2.6.4 Efficiency limits in tandem solar cells

The multi-junction concept, in which two or more solar cells of different band gap are

stacked on top of each other, can circumvent the limitations of single-junction solar cell. A

multi-junction device increases the efficiency by broadening the absorption and fighting

thermalization losses. The most promising connection of the two constituent sub cells is in

series.48, 49

De Vos was the first researcher to present the detailed balance limit of the

efficiency for inorganic multi-junction solar cells, in 1980.228

For 1 sun illumination, the 30%

power conversion limit of a single solar cell would become 42% for two-cell tandem cell,

49% for a three cell tandem cell or even higher for combination of more sub cells with

smoothly varying bandgap energies.228

Currently, Fraunhofer Institute holds the world record

with a multi-junction solar cell based on four sub cells with III-V compound semiconductors

and an efficiency of 46% at 50.8 W/cm2.71

-Organic tandem solar cells

In 2007, Minnaert and Burgelman estimated with their approach also the maximum

efficiency of an organic tandem solar cell (except single junctions organic solar cells).219

In an

optimistic scenario for a tandem solar cell comprising two sub-cells with complementary

absorption they assumed for both cells: 400nm absorption window, ΔLUMO=0.2 eV , EQE=

90%, FF=70% and Voc,i =0.7(Eg,i / q), to find a maximum efficiency of 23.2% for Eg,1 =1.7 eV

and Eg,2 =1.1 eV. On a similar note, Lunt et al. assumed EQE and FF of 75% and a Voc = 0.8

Voc,SQ to find an maximum efficiency of 24% for a nanostructured tandem cell with

complementary absorption materials.229

In 2008, Dennler et al. based on the same background demonstrated in single junction

organic solar cells by Scharber et al. estimated the PCE limit of series connected organic

tandem solar cells.91

The authors assumed ΔLUMO>0.2 eV, FF of 65% and EQE and IQE

constant for both sub-cells over the whole absorption area with 65% and 85% values

correspondingly. The Voc and Jsc of the tandem device would derive by Kirchoff’s law,

which for series connection of two sub cells is:

𝑉𝑜𝑐,𝑡𝑎𝑛 = 𝑉𝑜𝑐,1 + 𝑉𝑜𝑐,2 (2.40)



Where the number 1 in the indicator refers to the corresponding value of front sub-cell (or the

sub-cell that the light meets first on the course through the device) and 2 to the values of back

sub-cells (sub-cell that light meets last). To theoretically calculate the Jsc that the two sub-

cells deliver the authors introduced losses regarding to filtering of the light for the second cell

(cell that light meets last) and regarding to reflection losses due to the absence of back

reflecting contact (MirrorLoss 15%) for the first cell (cell that light meets first).49, 91

The

resulting Jsc values for bottom and top cell would then be:

These approximations resulted in a maximum efficiency of ~15% for a tandem cell employing

photoactive layers with 1.6eV and 1.3eV for front and back sub cell respectively (Figure

2-15). This prediction was indicating an improvement of 40% compared to the estimated

value for single-junction solar cells by Scharber et al. ontour plot where efficiency of the

tandem cells was ploted versus the band gap of the top and bottom sub cells.

Figure 2-15: Percentage of efficiency increase of a tandem cell over the best single cell (R) for a device comprising a top (back) sub-cell and a bottom (front) sub-cell based on donors each having a LUMO level at − 4 eV and each blended with a fullerene acceptor of LUMO = − 4.3 eV. The variables are the bandgap of both donors. The lines indicate the efficiency of the tandem devices. Reproduced with permission.91 Copyright 2008, Wiley-VCH.

𝐽𝑠𝑐,𝑡𝑎𝑛 = 𝑚𝑖𝑛[𝐽𝑠𝑐,1, 𝐽𝑠𝑐,2] (2.41)

𝐽𝑠𝑐,1(𝐸𝑔,1) = 𝑞 ∫ 𝜙𝑠𝑢𝑛(𝐸)𝐸𝑄𝐸1(𝐸)(1 − 𝑀𝑖𝑟𝑟𝑜𝑟𝑙𝑜𝑠𝑠)𝑑𝐸∞

𝐸𝑔,1 (2.42)

𝐽𝑠𝑐,2(𝐸𝑔,1) = 𝑞 ∫ 𝜙𝑠𝑢𝑛(𝐸)𝐸𝑄𝐸2(𝐸) [1 −𝐸𝑄𝐸1(𝐸)

𝐼𝑄𝐸1(𝐸)(1 − 𝑀𝑖𝑟𝑟𝑜𝑟𝑙𝑜𝑠𝑠)] 𝑑𝐸

∞

𝐸𝑔,2 (2.43)



Later, our group updated the power conversion limit by following the same approach with

new assumptions that were verified experimentally. With a FF value of 75%, an EQE of 80%

and IQE of 100% the maximum efficiency reached 21%.230

Figure 2-16: PCE prediction of organic tandem solar cell comprising sing cells with different bandgap energy (Eg). The LUMO level of donor is at –4 eV to keep the LUMO difference between donor and PCBM to 0.3 eV. The optical simulation was performed based on previous publication with updated assumptions: EQE = 80% and IQE = 100% for front cell; EQE = 80% for back cell; FF = 75% for tandem solar cells. Reproduced with permission. 230 Copyright 2014, Wiley-VCH.

-Hybrid perovskite tandem solar cells

Very recently, Todorov et al. conducted an interesting study on the efficiency limits of

hybrid tandem solar cells of different terminal configuration (2T, 3T and 4T configuration,

sub-chapter 1.2.3 ) employing a fixed silicon bottom cell (Eg1 = 1.1 eV) and a perovskite top

cell with a varying band gap Eg2 (Figure 2-17).231

The authors followed the SQ theory (as De

Vos previously) and assumed optical losses only regarding to the band gaps of the two active

layers. By doing so they found that 4-T devices yield the best efficiency (~45%) among the

other terminal configurations at an Eg2 of ~1.8eV which is in accordance to the perovskite

energy band gap used by Bailie et al.232 On the other end, a 3T configuration demonstrated a

flat efficiency profile (~33%, due to the parallel connection) lower than the efficiencies of 4T

and the best efficiency of 2T. Interestingly, the 2T configuration reaches the efficiency limit

of the 4T for an Eg2 of ~1.72 eV. Even though these are rough approximations this study

demonstrates clearly the potential of 2T and 4T devices. It is worth highlighting though that in

case of comparison between 2T and 4T these results do not include the potential optical losses

from the electrodes, thus seemingly characterize 4T configuration as superior. This can be



true only when the optical losses rather than the one deriving from photoactive layers, are

minimized.

Figure 2-17: Performance comparison of various tandem configurations (2, 3 and 4 terminals) based on idealized SQ-limit calculation vs. bandgap of the top cell. The bottom cell is Si (1.1 eV) which is filtered by the top cell: (a) J–V curves under AM1.5G 1 sun light for the top cell with Eg2 = 2.0 eV. (b) Efficiencies of the constituent cells and the tandem cells. Reproduced with permission.231

2.7 Geometrical and Electrical Losses in Solar Modules

As mentioned earlier in the thesis, the realization of a solar module inevitably introduces

geometrical and electrical losses as compared to single solar cells. Understanding the

principles behind those losses is a useful tool for scientists and engineers to close the

efficiency gap between solar cells and solar modules.

-Geometrical Losses

Figure 2-18: Schematic illustration of a solar module comprising three cells interconnected in series. Red boxes represent the active area of each solar cell. The area of the interconnection lines (l × w) is called dead area as it does not contribute to the photocurrent.

Bottom electrode

Top Electrode

Absorber

Substrate

L

w

Cell 1 Cell 2 Cell 3

l

P1

P2

P3



The realization of a solution processed module involves monolithic interconnection of

consecutive cells. Figure 2-18 shows a schematic illustration of a solar module comprising

three cells. Typically, this interconnection is achieved with three patterning lines, providing

electrical separation of the bottom electrode (P1), the active layer (P2) and the top electrode

(P3), while the P2 line allows the in-series connection between the top and bottom electrode

of successive cells (Figure 2-18).233

These three patterning lines are achieved either with

printing techniques (1-dimensional, 2-dimensional), or with a combination of coating

techniques (0-dimensional) and patterning (e.g. laser patterning) and their size is determined

by the resolution of each technique. This is important, as the total size of the area defined by

these patterning lines ensures electrical interconnection between successive cells but it is a

“dead area” regarding photocurrent generation (l × w, Figure 2-18). The area marked with

red box is the active area that participates in photocurrent generation (L × w, Figure 2-18).

Thus, the efficiency loss due to these geometrical constrictions can be described as

follows:

where PCEmodule is the power conversion efficiency of the total module and PCEcell the power

conversion efficiency of a L × w single cell. The second factor in the right part of the

mathematical description is known as geometric fill factor (GFF) and it is analytically

described as follows:

where n denotes the number of the cells. As it becomes clear from the aforementioned, the

higher the GFF, the higher the active area that contributes in photocurrent generation and the

lower the geometrical losses in the PCE of a solar module.

-Electrical Losses

Parasitic resistance losses across the device are responsible for further limiting the PCE

of the final module. Figure 2-19 illustrates a cross-section of the solar module depicted in

Figure 2-18 with an equivalent circuit model, commonly employed to describe the electrical

losses. Neglecting the parallel resistance losses inside active layer234

and assuming that

𝑃𝐶𝐸𝑚𝑜𝑑𝑢𝑙𝑒 = 𝑃𝐶𝐸𝑐𝑒𝑙𝑙 × 𝐿

(𝐿+𝑙) (2.44)

𝐺𝐹𝐹 =𝐴𝑐𝑡𝑖𝑣𝑒 𝐴𝑟𝑒𝑎

𝑇𝑜𝑡𝑎𝑙 𝐴𝑟𝑒𝑎=

𝑛×𝑤×𝐿

𝑛×𝑤×(𝐿+𝑙)=

𝐿

𝐿+𝑙 (2.45)



resistance values across P1 and P3 approach infinity (not shown in figure), the main series

resistance contributions are the following:

The sheet resistance of the top electrode (Rs,top)

The contact resistance between top and bottom electrode (Rint)

The sheet resistance of the bottom electrode (Rs,bottom)

Low electrical losses require electrodes with high conductivity and a balanced tradeoff

between the quality and the size of interconnection area (sufficiently wide and defect-free P2

line but not too wide as the sheet resistances of both electrodes become dominant).235

In solar

module architecture, at least one of the electrodes should be transparent; an additional

problem is the balancing between the optical and electrical properties of electrodes which

complicates even more if someone considers only solution processing electrodes for high

throughput production. For a review on the fundamentals of transparent electrodes I address

the reader to sub-chapter 2.4 .

Figure 2-19: Equivalent circuit model commonly employed in estimating electrical losses in solar module.

-Guidelines and experimental results

Below, I would like to draw the attention of the reader to some important studies on the

efficiency losses of thin film solar modules that employ the theoretical background outlined

above. Simulated and experimental results, presented from several groups, reveal guidelines

on closing the efficiency gap between a solar cell and a solar module and have been used

throughout this thesis.

In 2011, Harald Hoppe et. al studied the geometrical and electrical losses of monolithic

thin film solar modules to determine the optimal geometric design.234

Assuming the losses

P1 P2 P3

Cell 1 Cell 2 Cell 3

Rs,top

Rint

Rs,bottom

L l



introduced above, they experimentally determined the electrical characteristics (sheet

resistance of top, bottom electrode and contact resistance between top electrode-bottom

electrodes) to simulate the PCE losses under different geometrical designs. Focusing on ITO,

Aluminum and highly conductive PEDOT:PSS (PH1000) they showed that the highest

efficiency solar module incorporating combinations of these electrodes can be achieved for a

cell length of ~0.5cm and an interconnection area of less than 0.05cm. The window would be

smaller if electrodes with higher sheet resistance were used. In absolute numbers they

demonstrated that P3HT:PCBM based solar modules with such a geometric design would

deliver 3.5% PCE on glass/ITO, 3.1% on PET/ITO and 2.4% on PET/PH1000 substrates. It is

worth highlighting that such a fine structuring requires printing and/or patterning techniques

with relatively high resolution.

The most promising high resolution patterning technique due date has been repeatedly

proven the laser ablation (sub-chapter 1.4 ). With high resolution, processing speeds and

freedom in structuring patterns that is unmatchable to other patterning and 2-dimesnioal

printing techniques, it has been used in thin film photovoltaics to mitigate the efficiency

geometrical and electrical losses of solar modules.

Remarkably, for the field of OPVs, our group demonstrated P3HT:PCBM based modules

with PCE higher than 3% and a total area of 3500 mm2.172 In this work, coherently with the

study of Hoppe et al., Peter Kubis et al. utilized ultrafast laser ablation to connect in series

fourteen cells and minimize the geometric losses achieving GFF values of over 95%.

Additionally, high FF values and voltage outputs highlight the low electrical losses across the

module. To achieve that, a selective laser ablation of the P2-line that lets no residuals without

destroying the bottom electrode, thus minimizing the Rint was possible (Figure 2-19).

Recently, Luca Lucera et al. updated the guidelines for producing efficient thin film solar

modules incorporating most promising inverted architectures based on ITO, Ag, Ag grid,

highly conductive PEDOT and Ag Nanowires; PET or glass substrates and photoactive layers

that can deliver different Jsc. Their simulations took into account optical, geometrical and

electrical losses and resulted in several interesting results; i) Rint is the major loss and can

greatly affect module’s efficiency (losses up to 70%) even for relatively low values (~0.1

Ω/cm2). ii) Decreasing dead area ( l, Figure 2-18, Figure 2-19 ) is the second most important

checkpoint for achieving modules with efficiency comparable to cells. Specifically, they

showed that for high Jsc absorbers and sheet resistances of ~10 Ω/sq for top and bottom

electrode, a cell length of 3-5 mm and a dead area of 150 μm required to limit the losses



below 5%. iii) Finally, the sheet resistance of the electrodes is of high importance also. As a

rule of thumb, 10 Ω/sq for l=150 μm and L=5mm would result in 5% losses for medium Jsc

absorbers (~7mAcm-2

) but they would increase in case of higher Jsc absorber.173


Chapter 3 Materials and Methods

In this chapter I introduce the materials, the experimental and the characterization

methods of the devices presented throughout this thesis.

3.1 Materials

All solar cells were built on substrates based on glass or polyethylene terephthalate

(PET) substrates (Table 3-1). Indium Tin Oxide (ITO) coated glass substrates and IMI (ITO-

Ag-ITO) coated PET substrates were purchased and used as bottom transparent electrodes

(Table 3-1).

Table 3-1: Substrates used in this thesis

Substrates Provider Glass Weidner Glas

Glass/ITO Weidner Glas PET Melinex

® DuPont Tejin

PET/IMI Konarka

Electron donor and electron acceptor materials used to form the photoactive layer of

the organic solar cells are listed in

Table 3-2. The corresponding chemical structures of the photoactive materials are

displayed in Figure 3-1. OPV12 is a proprietary material, thus properties and chemical

structure were not provided by Polyera. All photoactive materials were used without further

purification.

Interface and electrode materials are summarized in Table 3-3. Aluminum doped ZnO

(AZO) produced in i-MEET and ZnO N-10 received from Nanograde were used after filtered

with 0.45μm PTFE filters. ZnO received from DTU was diluted in acetone at a volume ratio

of 1:5. AZO was synthesized according to the route described by Harnack et al. 236 All types

of PEDOT:PSS were diluted 1:3 and 1:5 to IPA before use. In some cases the wetting agent a

Dynol (purchased from Sigma Aldrich) was used in 0.013% concentration to enable better

coating of PEDOT:PSS on top of the active layer. For the laminated electrode the following

materials were used; AgNW ink purchased from Cambrios Technology Corporation,



PEDOT:PSS (Clevios™ PH 1000) received from Heraeus and D-Sorbitol purchased from

Sigma Aldrich.

Table 3-2: Photoactive materials used in this thesis

Type Material

Abreviation Provider

Product Number

MW

(kg/mol)

Purity (%)

Donor

P3HT Merck EE 99602 65.5 - pDPP5T-2 BASF GSID4133-1 47 -

PBTZT-stat-BDTT-8 Merck - - -

OPV12 Polyera - - -

Acceptor [60]PCBM Solenne - - 99.5

[70]PCBM Solenne - - 99

Table 3-3: Interface and electrode materials used in this thesis

Type Material

Abreviation Provider

Product Number

Solvent

P-type

PEDOT:PSS Heraeus AI4083 Water

PEDOT Heraeus HIL 3.3 Water

PEDOT:PSS Heraeus PH1000 Water

MoOx

N-type

ZnO Nanoparticles Nanograde N-10 Ethanol ZnO Nanoparticles DTU - Acetone

Aluminum Doped ZnO (AZO) i-MEET - Ethanol

Electrode AgNWs Cambrios ClearOhm ink Water

Ag evaporated Kurt J. Lesker EVMAG40EXE-A -



Figure 3-1: Chemical structure of the photoactive materials used in the thesis

3.2 Solar cell fabrication

3.2.1 Organic solar cells

All organic solar cells were fabricated with an inverted structure as shown in Figure 3-2.

Solution processed films were coated by doctor-blading under ambient conditions.

Figure 3-2: Architecture of organic solar cell

Pre-patterned (P1 line) PET/IMI foils were cleaned with isopropanol (IPA). Pre-patterned

Glass/ITO substrates were immersed consecutively in acetone and isopropanol and were

ultrasonicated for 10 minutes. After drying with nitrogen pistol, the substrates were coated

with an electron transport layer (ETL), such as ≈40nm thick AZO or ≈30 nm ZnO layer and

dried on a hot plate at 140 °C or 70 °C correspondingly. In some cases, we modified the ZnO

layer by coating a ≈10 nm Ba(OH)2 film on top (7 mg/ml in 2-methoxyethanol). Active layer,

pDPP5T-2P3HT

PCBM [70]PCBM PBTZT-stat-BDTT-8

Substrate/Electrode

ETL

Active Layer

HTL

Electrode



such as P3HT:PCBM, was coated on top of ETL with a typical thickness between 70-350 nm.

Subsequently, as hole transport layer (HTL), a solution processed ≈30 nm thick PEDOT:PSS

(1:5 vol.% in IPA) or a thermally evaporated ≈10nm thick MoOx were deposited on top of the

active layer. Finally, 100nm Ag layer was thermally-evaporated to form the top electrode.

3.2.2 Organic tandem solar cells

All organic tandem solar cells were fabricated by doctor blading under ambient

conditions with an inverted structure as shown in Figure 3-3. Pre-patterned (P1 line) PET/IMI

foils were cleaned with isopropanol (IPA). Pre-patterned Glass/ITO substrates were immersed

consecutively in acetone and isopropanol and were ultrasonicated for 10 minutes. After drying

with nitrogen pistol, the substrates were coated with an electron transport layer (ETL), such as

≈40nm thick AZO or ≈30 nm ZnO layer and dried on a hot plate at 140 °C or 70 °C

correspondingly. Active solution, such as P3HT:PCBM, was coated on top of ETL with a

typical thickness between 70-350 nm to form the bottom active layer. Subsequently, ~40 nm

thick PEDOT HIL3.3 (1:5, diluted in IPA) and ~30 nm thick ZnO layer were bladed and dried

at 70 °C for 5 min in air. In some cases, we modified the ZnO layer by coating a ≈10 nm

Ba(OH)2 film on top (7 mg/ml in 2-methoxyethanol). Afterwards, we coated an 80 nm thick

layer of pDPP5T-2:[70]PCBM (1:2 wt.% on top of the Ba(OH)2, dissolved in a mixed solvent

of 90% chloroform and 10% dichlorobenzene at a total concentration of 24 mg/ml) as the top

active layer. As a final step, a 10 nm MoOx layer and 100 nm Ag layer were evaporated to

form the top electrode.

Figure 3-3: Architecture of tandem solar cell

Substrate/Electrode

ETL

Active Layer

HTL

ETL

Modifying Layer

Active Layer

Electrode



3.2.3 Laminated organic solar cell

All layers were coated by doctor blading in ambient environment. The final structure of

our solar cells is demonstrated in Figure 3-4a. Figure 3-4b illustrates the fabrication route of

laminated cells. IMI-based PET was cleaned with isopropanol (IPA). Subsequently, the

substrates were coated with a ≈30nm ZnO layer and dried on a hotplate at 80 °C for 5

minutes, on top of which an active layer of ≈250 nm was formed by coating a solution of

PBTZT-stat-BDTT-8: [60]PCBM (1:2 wt%, 32mg/ml in total). The solution was based on a

mixture of the solvents xylene: tetrahydronapthaline (9:1). In parallel, the electrode of the

device was prepared by coating a ≈100 nm Ag NWs layer on a clean PET substrate that was

dried for 3 minutes at 100 °C. To conclude the electrode, a conductive glue based on

PEDOT:PSS (Clevios™ PH 1000) and D-Sorbitol was coated on top and the stack was placed

again on hotplate for 3 minutes at 100°C. Finally, we laminated our devices by passing the

Figure 3-4: a) Architecture of laminated organic solar cell. b) Step-wise fabrication route of solution-processed roll laminated cells. c) Photograph of the lamination process. The two substrates bearing the active layers and the top contact are driven through a pre-heated (120 °C) roll laminator consisting of three rolls for intimate electrical contact. d) Photograph of the finalized substrate.

ab

PET

IMI

ZnO

Active layer

TCA

Ag NWs

PET

1. Patterning

of IMI (P1)

2. Succesive

coating of

ETL/active Layer

3. Succesive coating

of top electrode

4. Roll-lamination

120 C5. Annealing 120 C

c d



sandwich structure through a pre-heated (120°C) roll laminator (Figure 3-4c). The final

device was annealed for 10 minutes at 120°C. Solar cells with a 100 nm evaporated Ag

electrode were prepared similarly but with a thin ≈50 nm PEDOT:PSS (Clevios P VP Al408)

layer immediately coated on top of the active layer.

3.2.4 Laminated perovskite solar cell fabrication

Figure 3-5 illustrates the architecture of a laminated perovskite solar cell. Cleaned

substrates were coated with a NiO layer at a speed of 4000 rpm and annealed for 10 minutes

at 140 ºC in air. The DMF-perovskite precursor was prepared by adding PbI2 and CH3NH3I

powders with a molar ratio of 1:1 and a concentration of 40wt% and 20wt%, respectively, to a

vial and mixed with anhydrous dimethylformamide (DMF). The solution was then stirred for

30 minutes at 60 ºC and filtered through a 0.45 µm PTFE syringe filter prior to deposition.

The precursor solution was spin coated inside glovebox at room temperature using 4000 rpm

for 35 seconds. During the last 5 seconds of the spinning process, the layer was treated with

chlorobenzene drop-casting. The substrate was dried on a hotplate for 10 min at 100 ◦C. After

perovskite deposition, a compact 60 nm thick layer of PC[60]BM was spin coated. The 2wt%

solution of PC[60]BM in chlorobenzene was deposited using a three step speed profile with

no subsequent annealing. The ZnO film was spin coated at 2000 rpm and annealed during 5

minutes at 80 ºC. PEI was spin coated at 1000 rpm and annealed for 5 minutes at 80°C. In this

case, the top electrode was pressure laminated at 60 °C inside a glove box. Solar cells with an

electrode consisting of 100 nm of evaporated Ag were prepared the same way.

Figure 3-5: Architecture of laminated perovskite solar cell

Glass/ITO

NiO

Active Layer

PCBM

ZnO

PEI

PET

TCA

Ag NWs



3.2.5 Laminated tandem solar cell fabrication

We present a simplified architecture of a laminated tandem sola cell in Figure 3-6. All

substrates were cleaned and pre-patterned as mentioned on previous sections. Substrate (I) is

coated successively with all functional layers in an inverted architecture. Substrate (II) is

coated with all functional layers in a normal architecture. Similar to the methods described in

sub-chapter 3.2.3 ≈100 nm Ag NW film and TCA are coated to form the adhesive electrode

on the finished device of substrate (II). Finally, we laminated our devices by passing the

sandwich structure through a pre-heated (120°C) roll laminator or by asking pressure and

temperature on a vertical axis. A more analytical description of our methods as well as a proof

of concept with hybrid photovoltaic technologies is presented in sub-chapter 5.4 .

Figure 3-6: Simplified architecture of laminated tandem cell. Cell1 and Cell2 are made simultaneously on different substrates and connected afterwards through lamination. The combination of two different PV technologies is feasible.

3.3 Solar module fabrication

3.3.1 Tandem module fabrication

The laser patterning was done with a LS - 7xxP laser patterning setup built by LS Laser

Systems (München, Germany), consisting of the ultrafast laser femtoREGENTM

UC - 1040 -

8000 fs Yb SHG from High Q Laser (Rankweil, Austria)39

and the beam guiding system (4

mirrors and galvanometer scanner). The scanners objective has a focal length of 330 mm and

a focal spot diameter of 32 ± 2 µm (at 1 / e2 intensity). The alignment of the laser beam was

realized with the camera and the software positioning system developed by LS Laser Systems.

The power of the laser was measured with the VEGA DISPLAY and 30A-BB-SH-18 ROHS

sensor from Ophir Optronics (Jerusalem, Israel). The P1 line in the IMI was done with a laser

fluence of 0.25 J/cm2 and 50 % overlap. The P2 line in the tandem stack was done with a laser

Substrate/Electrode

TCA

Cell 1

Ag NWs

Cell 2

Substrate/Electrode



fluence of 0.085 J/cm2

and an overlap of 94 %. The tandem layer consists of a stack of several

single layers. In order to properly ablate all the material the laser passed the same line 3 times.

In the last laser step evaporated Ag layer was removed with a laser fluence of 1.25 J/cm2 and

66.6 % overlap. All lines were done with 520 nm wavelength.(Figure 3-7)

3.3.2 Laminated module fabrication

Figure 5-10 illustrates the complete fabrication route of laminated modules. The laser

patterning was carried out with a LS - 7xxP laser patterning setup built by LS Laser Systems

(München, Germany), consisting of an ultrafast laser femtoREGENTM

UC - 1040 - 8000 fs

Yb SHG from High Q Laser (Rankweil, Austria)39

and a beam guiding system (4 mirrors and

galvanometer scanner). The scanners objective has a focal length of 330 mm and a focal spot

diameter of 32 ± 2 µm (at 1 / e2 intensity). The alignment of the laser beam was realized using

a CCD camera and the software positioning system developed by LS Laser Systems. The

power of the laser was measured using a VEGA DISPLAY and 30A-BB-SH-18 ROHS sensor

from Ophir Optronics (Jerusalem, Israel). The P1 line in the IMI was scribed with a laser

fluence of 0.25 J/cm2 and 50 %. The P2 line in the active layer was patterned before the

lamination process with a laser fluence of 0.085 J/cm2

and an overlap of 94%. Finally, the P3

line in the AgNWs/TCA was performed by controlled depth selective ablation through the top

PET layer. All lines were scribed with 520 nm laser wavelength (Figure 5-12,Figure 3-7).

Figure 3-7: a) Squared Diameter of ablated area versus laser pulsed energy. b) Calculated threshold fluence for each functional film. The difference in threshold fluence allows to successively scribing interconnection lines w/o damaging other active layers of the device stack. Active layer refers to the organic absorber. The ablation threshold of perovskite based active layer is generally similar to organic or even slightly higher.

a b

Material Threshold fluence (J/cm2)

IMI 0.1

AgNW 0.04

ITO 0.29

Active Layer 0.08



3.4 Characterization

The active area of the organic solar cells was defined by the top electrode, which was

thermally evaporated through a mask with an opening of 10.4 mm2. In case of the laminated

organic solar cells the area was defined by patterning of the bottom electrode (15mm2).

The J-V characteristics were measured using a source measurement unit from BoTest.

Illumination was provided by a solar simulator (Oriel Sol 1A, from Newport) with AM1.5G

spectrum at 100 mW/cm2. In the case of the perovskite-based devices, J-V characterization

was carried out as follows: forward direction, speed: 1 mV ms-1

and a dwell time of 8 ms.

Bending tests were performed by bending the devices on a drum with a diameter of 28 mm.

Optical investigations of the thin films were carried out using an UV-VIS-NIR spectrometer

(Lambda 950, from Perkin). EQE measurements were carried out using a QE-R system from

Enlitech.

Dark lock-in thermography (DLIT) was measured with an EQUUS 327k NM IR camera

system (IRCAM GmbH, Erlangen, Germany), equipped with an indium antimonite (InSb) and

focal plane array detector providing a spatial resolution of 640 x 512 pixels. The IR-camera

was controlled by a computer to guarantee a real-time lock-in calculation of the measured IR

signal. The InSb detector is highly responsive in a spectral range between 1.5 µm and 5 µm

with a noise equivalent temperature difference less than 20mK and frame rate of 100 Hz. For

focusing, a 25 mm focal lens imaging system providing a spectral transparency >90%

(IRCAM GmbH, Erlangen, Germany) was used. The lock-in frequency was set to 10 Hz in

order to minimize implications due to the heat diffusion length. Each test sample was excited

for 120 s using a pulsed injection current of 3.5 mA. As power supply for the pulsed

excitation a source measure unit from Agilent (B2900) in combination with a switch circuit

was used.237

Impedance spectroscopy measurements were conducted using an Agilent HP 4192A

impedance analyzer. For acquiring Nyquist plot the impedance spectra were taken in the dark

by superimposing an harmonic voltage modulation (amplitude of 20mV) and different dc bias,

with frequency ranging from 10 Hz to 1 MHz. The C–V measurements were taken in the dark

at a frequency of 10 Hz.

A FEI Helios Nanolab 660 was used to prepare cross-sections of the solar cells using

focused ion beam (FIB) milling as well as to acquire scanning electron microscopy (SEM)

images of those cross-sections. Before FIB milling the devices were delaminated to expose

the layer stack. For the FIB milling gallium ions accelerated at 30kV were used with a final

current of 80 pA. Before milling a carbon layer was deposited to protect the area of interest



using beam induced deposition (first 50 nm electron beam induced, then ion beam induced).

The SEM images were taken with an acceleration voltage of 2 kV at an electron current of

100 pA.


Chapter 4 Flexible tandem solar modules Parts of this chapter have been adapted or reproduced with permission from:

o G. D. Spyropoulos, P. Kubis, N. Li, D. Baran, L. Lucera, M. Salvador, T. Ameri, M. M. Voigt, F. C.

Krebs and C. J. Brabec, Energy Environ. Sci, 2014, 7, 3284-3290.

o G. D. Spyropoulos, P. Kubis, N. Li, L. Lucera, M. Salvador, D. Baran, F. Machui, T. Ameri, M. M.

Voigt and C. J. Brabec, Flexible organic tandem solar modules: a story of up-scaling, SPIE 9184,

Organic Photovoltaics XV, 91841A (October 6, 2014);

o J. Adams*, G. D. Spyropoulos*, M. Salvador, N. Li, S. Strohm, L. Lucera, S. Langner, F. Machui, H.

Zhang, T. Ameri, M. M. Voigt, F. C. Krebs and C. J. Brabec, Energy Environ. Sci, 2015, 8, 169-176.

4.1 Motivation and State of the art

The pace with which the efficiency of organic photovoltaic devices (OPVs) has been

progressing within the last decade allows for envisioning a significant share of this technology

in the future’s energy mix, thereby alleviating the world’s increasing energy demand in an

environmentally responsible way. 20, 238, 239

Crucially, OPVs provide excellent form factors,

good performance under indoor lighting conditions and potentially very low energy

production costs using solution processable organic semiconductors.205, 240-242

The

combination of these characteristics makes OPVs ideally suited for targeting niche markets

that are incompatible with brittle semiconductors, e.g., off-grid portable charging, electronics

in apparel and smart labels, as well as building and car integrated photovoltaics for non-planar

surfaces,243, 244

while simultaneously enabling production scale-up through roll-to-roll device

fabrication.

Despite the recognized potential for high-throughput manufacturing, basic science

limitations that have been preventing this technology from market implementation need to be

addressed. Particularly, a poor match of the absorption spectrum of the active blend materials

with the solar spectrum limits the photon harvesting capabilities and, consequently, the

photocurrent generation. Additionally, thermalization losses diminish possible voltage

outputs.46, 47

One promising approach for overcoming these limitations is the tandem

concept:48, 49

sub-cells of different band-gap donor materials are typically combined in series

for better matching the absorption of the device to the solar spectrum, while reducing the

thermalization losses of the high-energy photons.67

Chapter 4 Flexible tandem solar modules


The realization of hetero-tandem junction solar cells imposes several challenges from a

material and device fabrication point of view. In addition to the requirement for

complementary absorption of the absorber materials, the intermediate layer represents a

critical link for achieving efficient optical and electrical coupling between the sub-cells, but

also provides protection of the underlying layer against solvents from subsequently processed

layers.245

Furthermore, photocurrent matching between the sub-cells is required, which can be

controlled through the thickness of the active layers of the sub-cells and supported via the

interlayer, which can provide recombination sites for better charge balance.86, 246

While

current champion organic tandem solar cells have been reported to deliver 10.6% power

conversion efficiency (PCE) for solution-processed247

and 12% for vacuum processed (triple

junction) devices (press release by Heliatek)80

these results still lack demonstration of the

potential for high photovoltaic performance combined with truly low-cost, high-volume

processing using roll-to-roll compatible techniques.

Although series connected tandem cells deliver open-circuit voltages as a result of the

sum of the potentials of the sub-cells, the voltage typically falls short compared to the

requirements as imposed by practical applications. A form of circumventing this limitation

resides in the possibility of electrically interconnecting the solar cells into a photovoltaic

module to further increase the voltage output.

Several groups have tackled the problems involved in demonstrating efficient OPV

module fabrication while aiming for facile processablitiy using one and more-dimensional

coating and/or printing technologies. For instance, Niggemann et al. presented 2% efficient

single cell modules on glass (46.2 cm2

active area) by utilizing photolithography or laser

scribing for ITO structuring, a self-assembled monolayer (SAM) process for PEDOT:PSS

patterning and mechanical scribing for active layer patterning.248

More recently, Etxebaria et

al. introduced a solution processed roll-to-roll compatible patterning technology for the active

layer based on an ink-jet printed SAM process (60 – 80% GFF).249

This is a promising

patterning technology as patterned lines with a width of 120 μm were demonstrated for

inverted structure modules. One drawback could be the choice of SAM for guaranteeing

chemical specificity towards the active layer and sufficient electrical interconnectivity. For

demonstrating flexible OPV modules Kopola et al. used gravure printing, a 1-dimensional

printing method which utilizes engraved cylinders.250

Modules consisting of 5, 7 and 8 in

series connected cells with power conversion efficiencies of 1.92%, 1.79% and 1.68%,

respectively, were demonstrated based on ITO and evaporated metal back electrodes. One of

the most technology relevant approaches for upscaling OPV modules on flexible substrates



utilizing solution based roll-to-roll processing has been presented by the Krebs group.251

252

253

254

255 In these works, roll-to-roll coating methods based on slot-die coating and

screen/flexo printing were used to produce monolithically, in series interconnected modules

on ITO/polyethylenterephthalat (PET), ITO/polyethyleneternaphthalate (PEN) based

substrates as well as ITO free substrates. Large area (360 cm2) modules suffered mainly from

charge carrier extraction and performed at 1.7% while small area (4.8 cm2) modules produced

an efficiency of 2.3% as the Ohmic losses were reduced.251

The highest power conversion

efficiency with this structure was 2.75% for a total active area of 35.5 cm2.253

Based on this

progress on module fabrication, multilayer tandem polymer solar modules were demonstrated

using roll-to-roll processing methods.256

257

258

The efficiency of these tandem modules was

comparable to the efficiency of single junction modules (1.7% for 52.2 cm2), indicating the

need for further material and method optimization, but doubtlessly demonstrating the

feasibility of roll-to-roll printing for large-area OPV tandem production.

The approach developed in our research group aims at demonstrating high efficiency

and virtually loss free OPV solar modules, when compared to the individual solar cells, with

high GFF.233

The concept was recently introduced by Kubis et al. who presented laser

patterned OPV single junction modules based on ITO – metal (silver) – ITO (IMI)

electrodes.259

Here the serial interconnection between individual cells is accomplished with

ultrafast depth-resolved laser patterning. This procedure takes advantage of the high spatial

resolution intrinsic to laser scribing (typically in the μm range233

, as determined by the laser

focal spot size of laser beam) as opposed to the limited, sub-mm resolution of most 1-D and

2-D coating techniques, where on site structuring of the OPV cells, e.g., through lateral

displacement of the coating head, leads to rather low GFFs.252

255

260

As a result, we

demonstrated GFFs of over 90% for P3HT based modules on glass with over 3% PCE. For

comparison, reported GFFs for modules realized via slot-die coating and screen printing are

typically in the range of 70% or lower.170, 261

It is worth emphasizing the high roll-to-roll

compatibility of our laser approach due to high processing speeds (up to 4 m/s).259

Note that

other gropus have previously demonstrated practical examples of ultra-fast laser processing in

combination with transaprent, flexible electrodes.262

In this chapter, we demonstrate the design and realization of organic tandem solar

modules on flexible substrate fabricated by adopting fully roll-to-roll compatible processing.

With appropriate material choice, architecture, processing techniques and theoretical studies

we achieved flexible OPV tandem cells with PCE values of over 6% via doctor-blading.

Doctor-blading is governed by similar working principles as slot die coating, and can be



considered as an intermediate step between lab and mass production. By utilizing ultrafast

laser patterning we realized flexible tandem solar modules with reduced “dead area” and

geometric fill factors beyond 90%. The modules reveal very low interconnection-resistance

compared to the single tandem cells and exhibit a power conversion efficiency of up to 5.7%.

On our final devices, we performed bending tests to demonstrate their flexibility and stability

when considering future applications.

4.2 Flexible organic tandem solar cells

In order to fabricate flexible OPV tandem cells with PCE of over 6% we had to choose

appropriate materials that can deliver high efficiencies when combined in a tandem

configuration. Substrates, buffer layers, active layers and electrodes are all important

components that they have to be considered for the final result. Optical simulations predict the

efficiency limits for different thickness of electrodes and set the bases for time-efficient

device fabrication. We optimized materials and processing techniques not only considering

the efficiency but also according to the potentiality of being translated into an efficient

module.

4.2.1 Materials screening

The complete architecture of our flexible tandem solar cells is illustrated in Figure 4-1.

We chose an inverted architecture as it shows better stability. As transparent bottom electrode

we chose an ITO-Ag-ITO (IMI) coated PET foil.

Figure 4-1: Schematic illustration of flexible tandem solar cell architecture.



The PET/IMI foil shows appropriate optical properties (Figure 4-2a) combined with

superior electrical and mechanical properties than PET/ITO substrates. Resistivity values of

~7.7 Ω/sq are achievable. Remarkably, substrate’s resistivity remains almost unchanged over

a large number of bending deformation (Figure 4-2b). The photoactive layers were based on

two semiconducting polymers with complementary UV-vis spectra (Figure 4-3). For bottom

sub-cell, we used the medium band gap conjugated polymer OPV12246

(obtained from

Polyera) blended with the fullerene [60]PCBM. The top sub-cell was based on a blend

consisting of the low bandgap co-polymer diketopyrrolopyrrole-quinquethiophene pDPP5T-

2245

(BASF, batch no.: GKS1-001) and [70]PCBM. The optical band gaps for OPV12 and

pDPP5T-2 are 1.76 and 1.41 eV respectively, obtained from absorption measurements

(Figure 4-3). As intermediate layer, we coated PEDOT and ZnO layers consecutively to form

a good ohmic contact and provide efficient recombination to the charges extracted from both

sub-cells. Both materials were easily casted from water based and IPA based solutions at

temperatures lower than 70°C making them really easy to apply on large scale applications.

We further modified the ZnO surface with a thin electron selecting Ba(OH)2.92, 263

Finally, a

molybdenum oxide layer (MoOx) and a Ag electrode were thermally evaporated.

Figure 4-2: a) Optical Properties of PET/IMI substrate. b) Resistivity of PET/IMI substrate over bending cycles

a b



Figure 4-3: Absorption spectra of OPV12 and pDPP5T-2 active layers.

4.2.2 Optical Simulations

In a motive of a double feedback loop we performed optical simulations to predict

potential PCE values for different combinations of photoactive materials and gain fabrication

guidelines for the most efficient ones. Based on the same approach presented by Dennler et al.

(described in 2.6.4 ) we assume constant values for EQE, IQE and FF. Given the absorbed

photons and the IQE of each active layer, we can predict the photo-generated current from

each sub-cell. For the final PCE prediction Voc, FF values-verified by experimental

procedures are given to the program. The optical constants of the materials used for the

interlayers and active layers were measured by spectroscopic ellipsometry and verified by

transmission measurements.

For the PCE prediction of a tandem solar cell based on OPV12: [60]PCBM as a front

cell and pDPP5T-2:[70]PCBM as back cell we assumed Voc of 1.35 V, FF of 65% and IQE

of 80% and 65% for the bottom and top cell respectively (Figure 4-4) . Simulations showed

that for the given assumptions a power conversion efficiency of 9.5% is feasible however the

obtaining of high FF values for thicker sub cells is quite challenging especially for flexible

substrates.



Figure 4-4: Efficiency prediction for tandem solar cell based on OPV12: [60]PCBM (bottom cell) and pDPP5T-2:[70]PCBM (top cell).

4.2.3 Roll-to-Roll compatible coating technique

We fabricated flexible OPV tandem cells via doctor-blading. Doctor-blading (or knife

coating) is a zero-dimensional roll-to-roll compatible coating technique, which is governed by

similar working principles as knife-over-edge (KOE) and slot die coating, and it is considered

the link between lab and mass production.264

The only minor difference to KOE is that the

knife is moving over a stationary substrate. On the other hand compared to slot die coating the

main difference is the non-continuous ink supply. Figure 1-13 illustrates the 0 and 1-

dimensional coating techniques and the roll-to-roll compatibility of the knife coating as well

as the similarity with slot-die coating emerges.

In Doctor-blading and KOE the knife stands on a regulated height above substrate

removing the excess of ink to form a wet film under shear. Subsequently, the solvent in the

wet film evaporates leaving a thinner homogeneous dry film. The dry film thickness (d) can

be estimated by the following empirical equation:

4.1

where g is the distance between the substrate and the knife given in cm, c is the concentration

of the ink and ρ the density of the dry film both given in gcm-³.



4.2.4 Performance and key characteristics

Figure 4-5: a) Point of our experimental data on the figure of theoretical prediction. J-V characteristics of the OPV12, pDPP5T-2 based single cells and the corresponding tandem cell under illumination with an AM1.5G solar simulator and 100 mW/cm2

Following these design rules and fabrication methods, we delivered tandem cells with a

hero PCE of 6.12%. This value is in good agreement with PCE prediction for 160nm bottom

and 80nm thick top active layers (Figure 4-5a). A comparison of the J-V characteristics

between tandem cells and the corresponding single sub-cell is demonstrated in Figure 4-5b.

Voc of 1.35 V- sum of Voc delivered by the two sub-cells as indicated from Kirchoff’s law- and

FF of 60% indicate an efficient and fully functional intermediate layer which serves perfectly

as a recombination point and protects the bottom sub cell from subsequent coated layers.

(Figure 4-5b, Table 4-1). The relatively high Jsc in the tandem cells is attributed to the

efficient current matching between the bottom and top sub cells, which we later supported

with EQE measurements (Figure 4-6). In order to obtain the EQE spectra of the two sub-cells

inside a tandem structure we had to use light bias excitation to saturate the one cell while

scanning the whole wavelength area of the second sub-cell. For OPV12:[60]PCBM 750 nm

light bias was used to saturate the top sub-cell. Correspondingly, for pDPP5T-2:[70]PCBM a

550nm light bias was used.

*

a b



Table 4-1: Photovoltaic parameters of hero flexible tandem solar cells and the corresponding flexible single-junction solar cells.

Thickness of

active layer (nm)

Voc

(V) J

sc

(mA/cm2)

FF (%)

PCE (%)

pDPP5T-2:[70]PCBM 80 0.54 11.82 61 3.89 OPV12:[60]PCBM 160 0.80 8.54 70 4.79

Tandem Cell 160/80 1.35 7.61 60 6.12

Figure 4-6: EQE spectra of OPV12:[60]PCBM and pDPP5T-2:[70]PCBM sub-cells inside the tandem configuration.

4.3 Flexible organic tandem solar modules

4.3.1 Design and realization

Following these design rules, we fabricated flexible tandem modules by interconnecting 3

single tandem solar cells in series. The interconnection procedure consists of laser ablation of

the three patterning lines P1 – P3 (Figure 4-7). 265

Selective laser ablation of the patterning

lines is possible through precise adjustment of the laser fluence and overlap. First, the P1 line

is scribed into the bottom 90 nm thick IMI layer without damaging the PET substrate to

electrically isolate the three cells. Then, after depositing the active, intermediate and buffer

layers the P2 interconnection line is patterned into the structure. Achieving a clean P2 line

represents the key challenge of the laser patterning procedure for our tandem OPV modules.



Figure 4-7: Schematic illustration of the interconnection lines in the organic tandem module (3-cells module).

In this step, up to six organic and inorganic layers need to be ablated by the laser, without

destroying or reducing the functionality of the bottom electrode and without creating

substantial amounts of protrusion around the patterning line. Finally, after thermal

evaporation, the silver electrode was laser structured, without negatively affecting the

subjacent layers, to form the P3 line and conclude the electrical interconnection of the tandem

cells. The architecture of the whole module is schematically depicted in Figure 4-7.

Analytically, in Figure 4-8a) we display top view microscope images of P1 line. The high

quality ablation ensures electrical separation without damaging the PET surface. An SEM top

view images of a ~23μm P2 is shown in Figure 4-8b). A stack of seven layers was

successfully patterned without destroying the IMI layer. This area is later filled with silver in

order to succeed an electrical connection of the cells. In Figure 4-8c) the reader can see the

SEM top view image of P3 line. The final silver layer is ablated in order to conclude the

electrical separation of the sub-cells.

The dead area of the cell, which directly affects the GFF, is intrinsically related to the

resolution of the patterning. We optimized the lateral width of the laser-ablation line P2, so

that efficient interconnection between the rather thick tandem cells was possible while

minimizing the dead area of the module. This requires a z-axis resolution in the nanometer

regime and an x-y resolution in the micrometer scale. Both can be accomplished with ultrafast

laser patterning, as proven by SEM (Figure 4-8), optical imaging (Figure 4-9, Figure 4-11)

as well as J-V characterization (sub-chapter 4.2.4 ).

Ag

top sub-cell

intermediate layer

bottom sub-cell

IMI

PET

P3

P2

P1

dead area



Figure 4-8: a) Top view microscope photograph of a P1 line on an IMI substrate b) SEM top view image of a ~23μm P2 line. c) SEM top view image of P3 line. d)Top view SEM image three patterning lines (narrow P2 line)

Figure 4-9: Photograph of one of the 9 substrates carrying two reference single tandem cells (center) and two pairs of tandem modules (left and right), with narrow (≈25 μm, left) and wide (≈325 μm, right) P2 line patterning. The insets represent top views from an optical microscope displaying the lines P1 – P3. The wide P2 line was realized by laser hatching (scanning many single lines parallel to each other). As such, due to Gaussian energy distribution of the laser beam, rests of the absorber material are visible in the overlapping regions (lines visible in the P2 trench). This process did not affect the electrical interconnection quality of the P2 line.

PET

IMI

IMIa b

c d



The IMI foil was pre-patterned to accommodate nine substrates (Figure 4-10). Each

substrate contained two reference single tandem cells and four tandem modules for best

possible direct comparison. Figure 4-9 shows a photograph of one of the substrates and

optical microscope images capturing the P1 – P3 lines. The GFF was derived by calculating

the ratio of the photoactive area and the total area (sum of the active and dead areas) (Figure

4-11), neglecting possible bus bar losses.

Figure 4-10: Top view illustration of the PET foil and the doctor blading direction (left). After deposition of top electrode, PET was divided into 9 substrates (area of 2.5×2.5 cm2 ) for characterization. Photograph of PET foil (one substrate was marked with red dotted line) (right).

Figure 4-11: a) The assumed total area of flexible tandem modules is marked with a red line box on the left. b) Active area is defined as the sum of 3 red line boxes. Dead area of the module was assumed to be the area that does not contribute to photocurrent between the active area boxes.

a b



4.3.2 Performance and key characteristics

The J-V performance of a representative reference tandem cell and the corresponding

tandem modules are shown in Figure 4-12. Additionally, Table 4-2 summarizes the mean and

best photovoltaic parameters.

Figure 4-12: a) J-V characteristics of reference tandem cells (black line) and tandem modules with narrow (≈25

m, red lne) and wide (≈325 m, green line) P2 line under illumination. b) The corresponding J-V characteristics

in the dark.

Table 4-2: Device parameters for OPV12/ pDPP5T-2 reference tandem cells (Device A) and tandem modules (Device B and C)

Voc

(V)

Jsc

(mA/cm2)

FF (%)

PCE (%)

Rs

( Ωcm2)

Rsh

( KΩcm2)

Device Area

( mm2)

Device A

1.35

(1.32a)

7.61

(7.20a)

60

(59a)

6.12

(5.65a)

10

160

10.4

Device B (≈25 μm P2 line module)

3.92

(3.90a)

2.25

(2.18a)

65

(60a)

5.70

(5.10a)

30

1181

10.0

Device C (≈325 μm P2 line module)

3.90

2.15

64

5.38

32

813

10.0

a)Mean values of photovoltaic parameters were extracted from Figure 4-13.

In order to relate possible device losses in the module to the limitations given by the laser

patterning processing, we varied the width of the P2 line (Figure 4-9). We fabricated tandem

modules with a narrow P2 line of ≈25 µm (Device B) and with a wide P2 line of ≈325 µm

a b



(Device C, see Table 4-2). In both cases, the P2 line is fully functional and the devices exhibit

similarly high interconnection quality, as documented by the J-V performance (Figure 4-12a).

As a result, we were able to fabricate tandem modules with a champion efficiency of 5.70%

(Device B). Device B with the narrower P2 line performed slightly better due to the smaller

dead area, demonstrating the benefit of a laser controlled patterning approach. Remarkably,

the loss in PCE was below 7% compared to the reference tandem cell (Device A) and is

mainly determined by losses in Jsc. The highest Jsc value (2.25 mA/cm²) is ≈11% lower than

the value corresponding to 1/3 of the reference cell, which represents the maximum limit for

an in series connected module. Furthermore, Voc values for the tandem module in the order of

3.90 V represent small voltage losses in the range of 3% compared with the combined total

voltage given by three reference tandem cells. Moreover, encouraging FFs of 65% and 64%

were observed in the case of device B and C, respectively. The increase in FF when compared

to the reference tandem cell can be most likely attributed to smaller sub-area partitioning in

the case of the modules. We highlight that by reducing the width of the P2 line from 325 µm

to 25 µm in Device B the GFF of the module was significantly increased from 80% to 94%

without affecting the interconnection quality, resulting in improved short-circuit current (2.25

mA/cm²). The photovoltaic parameters distribution and the mean values for 9 devices of each

set (Figure 4-13) indicate the very good reproducibility of our methods.

From the dark J-V characteristic (Figure 4-12b), we extracted the series resistance (Rs)

and shunt resistance (Rsh) of our devices (Table 4-2). Comparison of the 3-cell modules with

the single tandem devices reveals that the average Rs value for each sub-cell is almost the

same as the Rs value of the reference device. This indicates that the interconnections

generated by laser-patterning do not lead to additional ohmic losses. In addition, the laser

patterning does not deteriorate the shunt resistance of the modules compared to the reference

tandem device.

In 2014, Kubis et al. demonstrated P2 lines in the range of 25 µm for OPV single cell

modules.259

In a module the tandem configuration in series typically generates more voltage

and less current, therefore the requirements on the current capacity of the P2 line are in part

alleviated. However, precise control over the patterning of a multi-stack device is decisive for

a successful integration of laser scribing into up-scaling technologies such as roll-to-roll

coating.

Although flexible OPV single cells on IMI substrates have been demonstrated before,259

the increased thickness of the tandem structure imposes additional strain on the devices.

Tandem modules are particularly susceptible to device failure under mechanical stress, which



can lead, amongst others, to delamination of the interconnection lines/areas and the

recombination layer. Therefore, bending tests of flexible tandem modules are of outmost

significance.

Figure 4-13: Photovoltaic parameters distribution of 9 devices. a) Parameters distribution for reference tandem solar cells. b) Parameters distribution for narrow P2 line modules. c)Parameters distribution of wide P2 line modules

Figure 4-14 depicts the normalized change in photovoltaic performance throughout 5000

bending cycles (drum radius 28 mm) for our tandem modules. Interestingly, we observed high

mechanical resilience with a loss in PCE in the order of 2-7% that can be mainly attributed to

losses in Voc and Jsc. This suggests that the main cause is likely to be related to damage

imposed on the charge-selecting and/or collecting layers and interfaces.

a b

c



Figure 4-14: Normalized device characteristics of flexible tandem module after 1000, 3000 and 5000 bending cycles.

4.4 Towards competitive operating lifetimes

In order to investigate and improve the lifetime of our proposed tandem architecture we

run one of the first and longest photo-degradation studies (duty cycle of 100%). In this sub-

chapter we present evidence for the photostability of encapsulated organic tandem solar cells

with a loss in PCE of only 11% within the first 2000h of continuous irradiation under

ambient conditions when exposed to 1000 W/m² of incident white light. When extrapolating

to 80% of the initial PCE, which is a common metric 266

, we find an accelerated lifetime of 18

years under open-circuit conditions, which represents the longest reported lifetime for organic

tandem cells to date. Additionally, we provide an improved understanding of the transient

UV-light soaking behavior of ZnO-based organic tandem solar cells and sub-cells by

recording the photostability of photovoltaic parameters after initial UV light treatment.

The solar cells for this study are based on the inverted tandem structure presented

previously in this chapter. The tandem cells comprise P3HT:[60]PCBM and pDPP5T-

2:[70]PCBM as active layers for bottom and top sub cells. We use zink oxide (ZnO) in

combination with poly(3,4-ethylenedioxythiophene (PEDOT) as recombination layer and

aluminum-doped ZnO (AZO) as electron extraction layer.267

The interface between ZnO and

pDPP5T-2:PCBM was modified by coating a thin barium hydroxide (Ba(OH)2) layer on top

of ZnO to enhance the photovoltaic performance.268

A detailed description of the device

geometry of the tandem and reference solar cells is depicted in Figure 4-15. Each single cell

was made in the same way as the corresponding sub-cell of the tandem device for intimate



comparison. For obtaining representative variations of device performance vs. time we

prepared and encapsulated five devices of each solar cell type.

Figure 4-15: Schematic device representation of the tandem and single cells investigated in this photodegradation study.

To encapsulate our devices and ensure maximum reproducibility, a dispenser robot I&J

4100-LF from I&J Fisnar Inc. was used to distribute the adhesive Katiobond LP655 from

DELO GmbH & Co KGaA on top of the completed devices, which were overcoated with a

1.5mm glass barrier. The epoxy was cured for one minute inside a UVACUBE 100 from

Hönle AG equipped with an iron doped lamp.

The tandem cells and the corresponding single sub-cells were aged under continuous

white light irradiation using an array of high power LEDs with a spectral range between 400

nm and 750 nm at 1000 W/m².237

The absence of UV light during photodegradation was

essential for studying the effect of UV light treatment on the lifetime of the devices. During

photodegradation the cells were maintained under open circuit. The J-V-characteristics of the

individual cells was measured under AM1.5 conditions. The initial device performance of the

tandem and single cells was 4.4% ±0.2% (tandem), 2.8%±0.1% (P3HT), and 4.1%±0.2%

(pDPP5T-2), respectively (Figure 4-16).



Figure 4-16: Initial J-V characteristics of a representative tandem cell and their respective sub-cells

To gain insight into the effect of the UV light-soaking process, we looked into the

transient behavior of the UV treatment. Figure 4-17 shows the periodically measured change

of the photovoltaic parameters upon UV irradiation using the tandem cells. In between J-V

characterization, the cells were stored in the dark at room temperature in order to discard any

impact on the UV light soaking state due to exposure to ambient light. Upon UV light

soaking, the FF increases dramatically (~45%) while Jsc increases by about 5% and Voc barely

changes. It is well documented in the literature that UV radiation can improve the electronic

properties (conductivity) of the ZnO layer as well as the contact at the ZnO interface.269-271

This is most likely the reason for the J-V characteristics translating from a double-diode type

behavior (S-shape) to a diode behavior with high FF. According to Figure 4-17 the light

soaking state remains constant for about 10 h after which the photovoltaic performance

decays sharply. Moreover, Figure 4-17 reveals that the light soaking state features a half-time

of about 200 h and is, therefore, expected to contribute decisively to the burn-in period in

OPVs containing ZnO.



Figure 4-17: Long-term decay of the UV light soaking (LS) state in the dark. Each data point represents the

average value of 5 tandem cells. The filled symbols represent the condition after immediate light soaking, whereas the hollow symbols represent the temporal decay of the LS state. The data were extracted from J-V-measurements using an AM1.5 spectrum and an illumination power of 1000W/m². Outside the J-V-measurements the tandem cells were stored in the dark at room temperature.

From the previous result we can deduce the importance of UV light exposure during

continuous 1-sun irradiation. We, therefore, designed a long-term aging test, in which the

solar cells were exposed to UV light for 10 s prior to each J-V measurement. Figure 4-18

shows the average long-term temporal evolution of Voc, Jsc, FF, and PCE for the single and

tandem cells under continuous illumination with intermittent UV treatment. The burn-in

period extends to about 1100 h, after which the decay of the PCE follows a close to linear

trend (Figure 4-18d). Remarkably, in the long-term measurements the tandem cells showed

the most stable behavior by losing only 11% of the initial value after 2000 h. The PCE of

P3HT and pDPP5T-2 based single cells followed a similar decay with losses of 16% and

15%, respectively. Overall, the loss in PCE is mainly determined by a loss in FF (5 – 11%)

and a modest decay in Jsc (0 – 8%). Impressively, in the case of the tandem and pDPP5T-2-

based single cells we observe no current losses throughout 2000 h of light exposure. To put

our results into perspective, PCE drops in the range of 10 – 20% for P3HT single cells and

25% for PCDTBT-based single cell devices under 1-sun exposure and within similar periods

of time have been reported in the past.272-274



Figure 4-18: Photoaging of single and tandem OPV cells. The graph show the average long-term temporal evolution of PCE, Voc, Jsc, and FF for the different single and tandem cells under continuous white light illumination. The photovoltaic parameters were extracted from J-V-measurements using an AM1.5 spectrum at 1000W/m². Before each J-V-measurements the samples were UV treated (365nm, 10 s). Each data point represents the average value of 5 tandem devices, 5 DPP devices, and 5 P3HT devices.

We speculate that the reason for the enhanced long-term stability of our solar cells is

manifold. Specifically, we used an inverted device structure, thereby eliminating reactive

metal interfaces.275

Additionally, we replaced the widely used but moisture sensitive and

reactive PEDOT:PSS with MoOx as the top buffer layer. The benefit of using the chemically

more inert MoOx for better device stability has been shown before.276, 277

Moreover, we chose

to modify the ETL/pDPP5T-2 (ETL: electron transporting layer) interface with a hole

blocking Ba(OH)2 layer in the case of the more stable tandem and pDPP5T-2 single cells. The

Ba(OH)2 layer has been proven to reduce exciton quenching at cathode interfaces and

improve device efficiency in the case of OLEDs and OPVs.263, 268, 278

Based on our findings,

we hypothesize that Ba(OH)2 could also contribute to stabilizing the ETL/pDPP5T-2 interface

by reducing electronic trap formation and oxygen adsorption. A more elaborate study in this

direction is currently underway. Furthermore, periodic UV light soaking during prolonged

operation is expected to desorb oxygen trapped at the surface of ZnO and AZO. This step is

likely to prevent significant conductivity losses of the ETLs ZnO and AZO, contributing to a

larger FF throughout the lifetime measurements. 270, 279

For an estimation of the lifetime of the tandem solar cells, we applied a linear regression

to the slowly, linearly decreasing PCE data points and extended this line up to 80% of the

initial value (Figure 4-19). In doing so, an extrapolated operating lifetime of ≈27 000 h can

be extracted. Considering an average 1500 hours of sunshine per year in central Europe, this



represents, under the current conditions, a best case lifetime of ≈18 years. We further derived

a more conservative lifetime for our cells by accounting for the error bars of our

measurement, which still resulted in a lifetime of ≈8 years (Figure 4-19). It is important to

note that the presented operating lifetime was extrapolated from cells, which were aged under

open circuit and under indoor conditions using a LED based solar simulator that does not emit

radiation in the 180–400 nm wavelength range. We recognize that the absence of UV light,

which has been shown to accelerate degradation through bond scission and free radical

formation in OPV semiconducting polymers,280

may artificially increase the lifetime of our

cells. Furthermore, for outdoor conditions in the field, there are influences from other sources

such as natural thermal cycling, shading, and humidity cycling, which need to be taken into

account.

Figure 4-19: Extrapolated lifetime of inverted OPV tandem cells. Long-term PCE decay of inverted P3HT:PC[60]BM and pDPP5T-2:PC[70]BM based tandem solar cells. Each data point represents an average value of 5 tandem devices. For estimating the accelerated lifetime, we applied a linear fit of the form y = 0.899x – 3.6x10-6 to the data points following the burn-in period and extended the fit to where the efficiency drops to 80% of the initial value (red line). For a minimum expectable lifetime of our cells we extrapolated the minimum (maximum) values of the error bars (dashed lines). The lifetimes were calculated considering an average 1500 hours of sunshine per year (central Europe).

Given that we used a glass-on-glass encapsulation architecture and an evaporated top

electrode, the lifetime presented here certainly reflects an upper bound for the lifetime of this

type of organic tandem solar cells. However, if high quality packaging is used, including UV

filters, combined with better interface materials, e.g., towards blue light soaking, similar

lifetimes are perhaps not impossible but need to be documented by thorough outdoor studies.



4.5 Conclusion

We demonstrated the complete design and fabrication route for realizing flexible organic

tandem modules. Starting from the choice of solution processed buffer layers with the

appropriate solvents and complementary absorption photoactive materials we performed

optical simulations to predict the efficiency and we developed a process recipe that can be

transferred to flexible substrates. Flexible tandem cells with efficiency of 6.1% were

produced. Then, we used a fs-laser as a means for separating and interconnecting the cells.

The laser features high speed and high precision patterning in lateral and in z-directions. As a

result, tandem modules with geometric fill factors beyond 90% and high-quality electrical

interconnects are feasible, leading to minor photovoltaic losses compared to the non-patterned

tandem cells. Considering the low temperatures involved (<70 °C) throughout device

fabrication and possible laser writing speeds, the process is fully roll-to-roll, large scale

compatible. Our champion tandem modules, consisting of three series connected cells, deliver

a power conversion efficiency of 5.7% and a voltage output of 3.9 V. Bending tests manifest

high mechanical resilience, demonstrating that this type of flexible cells could potentially be

implemented as a power source on non-planar, foldable and movable surfaces of textiles,

mechatronics and buildings. Additionally, we proved that our device geometry shows high

stability under continuous white light illumination with PCE loss of only 11% within the first

2000h of operation. By extrapolation we found operating lifetime in excess of one decade.

Even though the presented devices show high applicability and process that can be easily

adapted in a production line, there are some last stairs to climb in order to fully exploit the

potential throughput of solution process technology. That of the solution process electrodes.


Chapter 5 Lamination as fabrication strategy Parts of this chapter have been adapted or reproduced with permission from:

o G. D. Spyropoulos, C. O. Ramirez Quiroz, M. Salvador, Y. Hou, N. Gasparini, P. Schweizer, J. Adams,

P. Kubis, N. Li, E. Spiecker, T. Ameri, H.-J. Egelhaaf and C. J. Brabec, Energy Environ. Sci., 2016,

DOI: 10.1039/C1036EE01555G

5.1 Motivation and State of the art

Thin-film photovoltaics using high throughput solution-based printing techniques such as

roll-to-roll processing is a key technology for inexpensive and sustainable light-to-energy

conversion. However, to fully exploit the economical and engineering advantages with which

the roll-to-roll printing technology could benefit an industrial scenario for thin-film

photovoltaics, fully solution-processable photoactive and electrode materials are required.

Solution-processed metal electrodes are particularly difficult to realize because of often

inferior optical (e.g., reflectivity) and electrical properties (e.g., conductivity) as compared to

the more common vacuum deposited metal electrodes. Moreover, processing a metal based

electrode from solution on top of a semi-finished stack is challenging in terms of solvent

compatibility, surface energy and even surface induced damaging. Nevertheless, important

advances have been demonstrated lately using for instance silver ink167

, silver nanowire154, 155,

161, carbon allotrope

262, 281 and hybrid electrodes, e.g., conductive polymer/metal grids

282, 283,

without significant sacrifices in efficiency.

An alternative concept for realizing high-quality metal electrodes in a low-cost, large-

area compatible fashion is roll lamination. In general, a transparent, conductive film with

adhesion properties (TCA; transparent conducting adhesive) coated on a substrate provides

electrical and mechanical functionality and is activated by applying temperature and/or

pressure. This method is particularly attractive because it allows decoupling the processing of

the top metal electrode from the rest of the device fabrication procedure. This could greatly

simplify a production line, for instance, by allowing switching between the fabrication of

single cells and tandem solar cells, the latter via post-connection of two sub-cells.

Furthermore, the substrate of the laminated top electrode could function as barrier and thus

enable encapsulation at the earliest possible fabrication stage.

Previous work has focused on demonstrating lamination as a potentially simple

fabrication route for bilayer structures284, 285

, organic light-emitting diodes286

, semi-

transparent organic solar cells287

and metal electrodes in general288-290

. In this context, a

Chapter 5 Lamination as fabrication strategy


conductive adhesive consisting of PEDOT:PSS and sorbitol has proven to be particularly

effective, but other conductive glues have been explored as well291-296

. Organic solar cells

produced with this method have shown power conversion efficiencies up to 4% (Table 5-1).

More recently alternative materials and lamination techniques to produce single or tandem

organic and perovskite solar cells were presented96, 297-299

.

Table 5-1: Power conversion efficiencies of laminated organic solar cells due date

Research group PCE (%)

J. Huang et al. 2008287 3.00

B. A. Bailey et al. 2011291 3.19

Y. Yuan et al. 2011292 4.00

C. Shimada et al. 2013293 2.41

D. Kaduwal et al. 2014294 2.50

R. Steim et al. 2015290 1.60

G.D. Spyroppoulos et al. 5.88

While previous results are indicative of the future potential of lamination technology, at

present the implementation of this technology as efficient method for producing up-scalable

electrodes and devices hinges on several hurdles. First, thick electrode materials (micron

range) that efficiently extract charge carriers upon being coated using simplified solution-

based methods are not easily accessible. The main challenges associated with the realization

of such a functional electrode are often limited by the trade-off between the adhesion and the

electrical properties as well as by the quality of the contact between the TCA and the active

layer on one side and the TCA and the current collection electrode on the other side.

Additionally, a proof of concept by transferring this technology to a solar module that

delivers tunable voltage output is needed to demonstrate the feasibility and applicability of the

concept. Importantly, the fabrication of a module using a laminated electrode bears several

key challenges associated with the realization of an efficient electrical connection between

two successive cells (P2 line) and the separation of the top electrode (P3 line) for defining

electric current pathways, in both cases without sacrificing the active area. Specifically, high

performance devices call for a top electrode that fills the P2 area and forms an ohmic contact

with the bottom electrode upon lamination (Figure 5-11, Figure 5-12). Furthermore, a second

important challenge lies in aligning the P3 line relative to the P2 line when laminating the

electrode on top of the active layer (Figure 5-11, Figure 5-12). This may induce major



photocurrent losses because standard procedure requires the typically pre-patterned P3 line to

be wide enough so that the electrical separation is not jeopardised upon lamination, which

necessarily translates into a loss of photoactive area (dead area).

In this chapter, we report on a roll lamination process to demonstrate fully solution-

processed, laminated organic solar modules with 5.3% power conversion efficiency. This was

accomplished by developing a highly functional adhesive electrode consisting of embedded

silver nanowires and a transparent conductive adhesive (TCA). The latter is based on a blend

of highly conductive PEDOT:PSS and D-sorbitol. We investigate the optoelectrical quality of

this laminated top electrode using full-frame dark lock-in thermography (DLIT), impedance

spectroscopy (IS) and capacitance versus voltage measurements (C-2

-V). A key innovation of

the flexible modules with laminated top electrode is depth-resolved post-laser patterning using

a pulsed femtosecond laser. The laser beam penetrates the top substrate after the lamination

process and ablates the composite electrode beneath without damaging the top plastic

substrate. This step eliminates common alignment constraints of traditional module coating

and allows for large geometrical fill factors (>90%). We further confirm the general

application of this concept by fabricating laminated perovskite solar cells and modules with

9.80% and 9.75% efficiency, respectively. We thus anticipate that the adhesive

AgNW:PEDOT composite electrode by itself or combined with depth-resolved laser

patterning is likely to be compatible with and of benefit to many thin film photovoltaic

technologies.

5.2 Realization of efficient adhesive top electrode

In order to find and optimize an efficient adhesive top electrode we cycled over

optoelectrical analysis and fabrication. First, we fabricated flexible photovoltaic devices with

laminated top electrode by adopting a solution process that is based on doctor blade coating in

air (Figure 3-4). Once again, we chose an IMI coated PET foil as a transparent bottom

electrode. On top of IMI, we processed commercial ZnO nanoparticles (Nanograde) as

electron selective electrode. As photoactive layer we coated a medium band gap conjugated

polymer PBTZT-stat-BDTT-8165, 300

blended with phenyl-C61-butyric acid methyl ester

([60]PCBM) on top of ZnO (see Chapter 3). The active layer was contacted with a laminated

electrode that was prepared in a separate step.



-Challenges

The road towards an efficient adhesive top electrode concealed several challenges

regarding materials and device architecture design. Literature survey showed similarities for

all work presented due that date; a transparent conductive adhesive (TCA) was deposited on

top of a more conductive layer. Initially, we scanned several materials to realize a TCA by

measuring the conductivity of a sandwich structure where two conductive substrates (IMI or

ITO) were connected with a TCA by pressed through a laminator. Combinations such as

acrylic adhesives-highly conductive PEDOT:PSS, acrylic adhesive- metallic NPs/AgNWs,

UV curable adhesive (such adhesives are used for encapsulation of optoelectronics e.g. Dello

Katiobond)- metallic NPs/AgNWs. For the time of our investigation we found-in accordance

to the literature- that a combination of a highly conductive PEDOT:PSS with D-sorbitol gave

the best balance between adhesion and electronic characteristics. On the one hand, when D-

sorbitol is introduced in water based solution- such as PEDOT:PSS- promotes a gel formation

that upon pressing and applied temperature activates strong collective hydrogen bonding. On

the other hand, it has been extensively proven that D-sorbitol and other substances of the

same family (e.g sugar alcohols, EG, PEG) can improve electronic conductivity and promote

ionic conductivity in PEDOT:PSS.293, 301-304

Thus, initially we combined a TCA based on

mixture of PEDOT:PSS and D-sorbitol with IMI substrate to form a top laminated electrode.

Then, to realize an organic solar cell, employing the laminated electrode described in the

previous paragraph, we followed an architecture design similar to the one with evaporated

electrode (Figure 5-1). In such architecture, the total active area of the cell (the overlap of top

and bottom electrode) is defined by the top adhesive electrode. However, following such a

structure, we encountered serious shunting problems over the whole active area. These shunts

were mainly occurring on the sides of the laminated electrodes as we verified with DLIT

imaging (Figure 5-1). To circumvent this problem we laser structured our bottom IMI

electrode to electrically isolate the active area from the edges of the substrate (Figure 5-1).

By doing so we found consistently higher shunt resistance (Rp) values which resulted in

higher FF (~45%) and efficiencies. The substantial mitigation of the shunts was verified by

DLIT imaging, where a more uniform- without bright spots- flow of current reveals the

superiority of the device (Figure 5-1). Even though a critical step towards efficient solar cells

with laminated top electrode, still our devices suffered from high series resistance (Rs). These

resistance losses led to limited FF, Jsc and Voc values. Optimization on TCA thickness,

lamination pressure and temperature could not take us any further on the efficiency roadmap.



All these problems were solved with the introduction of AgNWs in our structure, as will be

described in the next section.

Figure 5-1: The effect of patterning in solar cells with laminated top electrode. Left side: realization of laminated solar cell with unpatterned bottom IMI. Right side: the realization of laminated solar cell with laser patterned IMI. Typical J-V characteristics under 1 sun illumination and DLIT images are shown for both architectures.

-Efficient adhesive electrode

In contrast to previous work, where primarily the combination of a transparent

conductive adhesive with sputtered electrodes287

and evaporated294

or complex mesh

structures293, 295

has been explored, the laminated electrode designed in this work consists of a

bilayer of successively coated TCA and AgNWs on PET foil (Figure 3-4b, Figure 5-2). As

TCA we optimized a mixture of highly conductive PEDOT:PSS (~850 S/cm) and D-

Succesive

coating of both

substrates

Lamination

Electrical &

optical

characterization+

-

PET / IMI

ZnO / Act. Layer

TCA

PET / IMI

+

-



sorbitol287

. The highly conductive PEDOT:PSS ensures sufficient electrical connection

between the active layer and the AgNWs, while D-sorbitol provides the necessary adhesive

force for establishing a robust connection between the two substrates. Significantly, the

combination of TCA with AgNWs affords process simplicity, semitransparency and high

quality electrical contacts. AgNWs are easily coated from a water-based solution, requiring

only a simple and short drying process at 100 °C for 3 min. Aside from the ease of processing,

AgNWs are known to form macroscopic mesh electrodes (Figure 5-2b) with low sheet

resistance (10 Ω/sq) and average optical transmittance of better than 70% (Figure 5-4).

Notably, we found good ohmic contact with the conductive adhesive, enabling charge carrier

extraction similar to evaporated silver, as discussed later on. Importantly, the AgNWs

electrode is compatible with the technical requirements of many photovoltaic technologies

and can be easily exported to, e.g., CIGS (copper indium gallium selenide solar cells) or a-

Si:H (amorphous silicon) , facilitating the implementation of hybrid tandem technology.

Figure 5-2a depicts a macroscopic cross sectional scanning electron micrograph of a device

with laminated top electrode. It is apparent that the TCA (~2 m) is substantially thicker than

the active layer (280-290 nm). The overall thickness of the TCA is the result of the

combination of optimized coating of the glue, applied pressure and resulting adhesion. We

further note that polymer blend layers featuring PBTZT-stat-BDTT-8 can be coated as thick

as 500 nm without significant roll off in photovoltaic performance300

.

Figure 5-2: Cross-section scanning electron microscopy (SEM) image of flexible laminated organic solar device (left). Top view of AgNWs on TCA after delamination of PET substrate(right).

The J-V characteristics of flexible OPV solar cells with laminated and evaporated

electrode under 1000 W m-2

solar simulator light are shown in Figure 5-3a. The Optimized

solar cells with laminated top electrode deliver an open circuit voltage (Voc) of 810 mV, a



short circuit current density (Jsc) of 12.10 mA cm-2

and a fill factor (FF) of ≈59%. This

corresponds to a power conversion efficiency of 5.9%. Comparison between OPV devices

with laminated and evaporated silver electrode (the latter consisting of IMI/ZnO/PBTZT-stat-

BDTT-8:PC60BM/PEDOT:PSS(≈50nm)/Ag, evaporated) reveals that the losses in PCE are

mainly dominated by Jsc losses (≈5%), which is in good agreement with the integrated

photocurrent from corresponding EQE spectra (Figure 5-3b). The difference in Jsc is likely to

be associated with differences in electromagnetic field distribution and resulting interference

profile.

Figure 5-3: a) Current – Voltage (J-V) characteristics of organic solar cells with laminated and evaporated top electrode b) EQE spectra of reference OPV solar cell with evaporated silver top electrode (100 nm, blue dashed line), laminated OPV solar cell with reflecting mirror in the back (black line), laminated OPV solar cell measured without reflecting mirror (green line).

It is intriguing that such a thick polymer/adhesive/AgNW-based laminated electrode as

the one shown in Figure 5-2a allows efficient photovoltaic performance. In the following we

analyze the optical and electrical properties of the laminated composite electrode. Figure 5-4

shows typical transmission spectra of the laminated electrode, the PET substrate and relevant

charge extraction layers. The transmission spectrum of the TCA electrode shows the

characteristic localized surface plasmon resonance of AgNWs around 380 nm and an average

transmission of ≈70% between 400 nm and 800 nm. The final OPV devices with adhesive top

electrode and an active layer of 290 nm show an average transmittance of ≈31% for the same

wavelength range.

a b



Figure 5-4: Transmittance spectra of PET substrate, charge extraction layers and laminated electrode

To investigate the electrical characteristics and homogeneity of the laminated

electrode we applied impedance spectroscopy and dark lock-in thermography (DLIT). DLIT

is a sensitive full-frame infrared imaging technique that reveals local thermal loss

processes305

. Figure 5-5a compares two devices with laminated and evaporated top electrode

but with otherwise identical layer stack. For DLIT measurements, the devices were operated

under 1V forward bias (charge carrier injection regime). Both electrodes show a typical

temperature gradient behavior with higher temperatures closer to where the IMI electrode was

contacted (Figure 5-5a,b). This observation originates from the difference in sheet resistance

between the bottom electrode (IMI) and the top electrode of the solar cell (evaporated or

laminated)305, 306

. The laminated electrode reveals a DLIT image with similar temperature

profile gradient and homogeneity than the device with evaporated silver, suggesting a

homogenous and fully functional electrode with no major irregularities in current transport

across the IMI and laminated AgNWs electrode.



Figure 5-5: DLIT images of solar cells with evaporated and laminated top electrode and c) integrated DLIT signal profile along the long axis of the DLIT image.

In order to further investigate the effect of the laminated electrode on the device

behavior, we conducted AC impedance spectroscopy and capacitance versus voltage

measurements307-309

. The Nyquist plots in the dark for varied applied bias are represented in

Figure 5-6a,b. Similar to previous reports on OPV devices, the data is best fitted using an

equivalent circuit model that includes two resistor-capacitor sub-circuits and an additional

resistive element due to electrode resistance losses (Figure 5-6c) , which accounts for the

total resistance in the device 307, 308

. Using this model, we extract the resistance elements of

our devices over the range of different applied bias. It has been previously established that the

high-frequency impedance response is primarily related to the transport resistance (Rt) of

photo-generated carriers being extracted at the contacts while the low-frequency response can

be associated with recombination resistances in the bulk (Rrec)307, 308

. Qualitatively, for

optimum device performance Rrec should be large and Rt small for voltages corresponding to

the fourth quadrant of the J-V graph. The recombination resistance of devices with evaporated

electrode is almost identical to the ones with, laminated electrode as demonstrated by the

similar exponential decay with applied bias (Figure 5-7a). Minor variations in the regime V <

Voc may reflect small variations in device thickness or in the electrical field distribution. This

is not surprising considering the usage of the same photoactive blend. For voltages greater

than Voc the recombination resistance approaches zero in both cases, reflecting the higher

probability for the generated carriers to vanish through bimolecular recombination.

Conversely, a comparison between the transport resistance (Rt) shows values of similar order

of magnitude (two orders of magnitude lower than Rrec) but higher over the whole scan of

voltages for the device with a laminated top electrode (Figure 5-7b). The increased Rt is

representative for a higher contact resistance of the laminated electrode and probably further

Evaporated Electrode

Laminated Electrode

a b



dominated by the outstanding large thickness of the TCA in the laminated devices. Since the

probability for charge extraction depends on the correlation between Rrec and Rt310

, the

observation of comparatively low Rt, even in the case of laminated devices, grants efficient

carrier extraction in both cases and thus comparable device performance.

Figure 5-6: a) Impedance spectra for devices with evaporated top electrode under different applied biases. b) Impedance spectra for devices with laminated top electrode under different applied biases. EIS Spectrum Analyser was used for analysis and simulation of impedance spectra311. c) Equivalent circuit used for fitting data obtained by impedance spectroscopy. Cg and Cμ represent geometrical and chemical capacitance, respectively.. Rrec denotes the recombination resistance and Rt represents the transport resistance. Rs´ denotes an additional resistive element due to electrode resistance losses.. For applied biases greater than Voc the total series resistance in the model is given by Rs = Rs´ + Rt

Figure 5-7: a) Recombination resistance Rrec and b)transport resistance Rt as a function of applied bias for devices with evaporated and laminated electrode.

a b

c

a b



Mott-Schottky (C-V) measurements can reveal important insight regarding the quality

of the electrode and electrode-bulk interface309

(Figure 5-8), particularly when comparing

laminated and evaporated electrodes. By fitting a straight line to the region of C-2

vs. V with

the steepest slope it is possible to extract the built-in potential (Vbi)309

. Importantly, both

devices demonstrate similar values of Vbi (~0.57 V), which evidences qualitatively similar

energetic offsets at the electrode-bulk interface. In addition, in the high reverse bias regime

(V<<Vbi) both samples show dielectric capacitor behavior, approaching a constant capacitance

that corresponds to the geometrical capacitance309

. In the forward bias regime (V > Vbi), the

capacitance is mainly determined by the chemical capacitance309

. Different Mott-Schottky

behavior under low reverse and forward bias (V < Vbi) is observed. For devices with

evaporated electrode the capacitance starts increasing at more negative voltages, denoting an

early starting point of a more gradual decrease of the depletion width and high doping density

N=1.04 × 1016

cm-3

. The latter is determined from the corresponding slopes by N= - 2(dC(x)-

2/dV)

-1 / qε0εrA

2, where q denotes elementary charge, ε0 dielectric constant of vacuum and εr=

dielectric constant of the semiconductor (assumed value of 3.5)312

. In contrast, laminated

devices show an inclination point at relatively lower reverse (more positive) bias, revealing a

faster depletion zone collapse over the voltage scan and a lower doping density N=5.09 × 1015

cm-3

. The higher doping density for devices with evaporated electrode may arise due to the

diffusion of small molecules such as oxygen and water migration or evaporated “hot” silver

atoms into the photoactive layer313-315

. These phenomena have been reported before and in the

case of oxygen and water could be even propelled due to intimate contact of the active layer

with a wet, water based PEDOT:PSS solution during device fabrication. For laminated

electrode devices, the TCA was coated on a different substrate and applied on top of the

active layer after a drying step. The difference in doping density highlights the possible

advantage of a procedure, in which electrodes or additional layers are processed on different

substrates and assembled afterwards.



Figure 5-8: Mott-Schottky plot (10Hz) for devices with evaporated and laminated top electrode. The dashed lines represent linear fits to the slope. A scheme of the equivalent electrical circuit model used for analyzing impedance spectroscopy data is displayed in Figure 5-6.

In order to characterize the performance of our devices as a function of mechanical stress,

we performed bending tests on a drum with 28 mm radius (Figure 5-9). We reinforced the

edges of the devices using a commercial adhesive tape (ScotchTM

tape). After 1000 bending

cycles the performance decreases to ~59% of initial value mainly due to Jsc and FF losses.

This suggests that the main cause for mechanically induced performance roll off is damages

imposed to the interface between active layer and adhesive electrode.

Figure 5-9: Normalized device characteristics of a flexible organic laminated solar cell over successive bending cycles.



The above-mentioned properties grant tremendous inherent advantages to the laminated

electrode: low temperature (≤120°C), minimized contamination (solutions printed on different

substrates ensure reduced contamination of the active layer) and vacuum free (easy, fast and

inexpensive lamination) processing are the most important attributes leading to a high quality

semi-transparent electrode with the additional benefit of simultaneously acting as a protective

barrier towards the environment.

5.3 Innovating solution-processed solar modules

Our proposed fabrication route for realizing solar modules with laminated top electrode

involves an innovative depth-resolved post-patterning technique (Figure 5-10). The

fabrication is fully roll-to-roll compatible due to the high processing speed of our laser

patterning approach (up to 4 m s-1

) and solution processing of all functional layers under

ambient conditions. The device fabrication consists of seven consecutive coating, lamination

and patterning steps. Generally, in order to achieve electrical series interconnection between

individual cells in a solar module, three patterning lines P1 – P3 are necessary (Figure 5-11,

Figure 5-12). We utilize femtosecond–laser patterning with optimized ablation thresholds

(Figure 3-7) for each interconnection line to take advantage of high spatial resolution and low

geometrical fill factor (GFF, ratio between the photoactive area and the total area of the

module) losses.

The advantages and challenges associated with laser patterning of thin-film solar modules

have been elucidated before 316

. Typically, a laser source irradiates the active layer material of

a device with ultra-short laser pulses, inducing a phase transition from solid to vapor. The

minimum energy required for this phase transition is an intrinsic material property and known

as the ablation threshold. Given the differences in the ablation threshold for the electrode,

interface and photoactive materials chosen here (reported in Figure 3-7) and by tuning the

laser fluence, we can ablate functional layers of a device individually. Crucially, we

demonstrate that controlled ablation of functional layers is even feasible through a plastic

barrier, i.e., upon lamination of the top electrode, extending the concept of laser patterning to

depth-resolved laser patterning. This allows effectuating the last patterning step upon

protecting the susceptible device structure from environmental contaminants.



Figure 5-10: Step-wise fabrication route of solution-processed roll laminated modules.

Figure 5-11: Architecture of laminated organic solar cell/module and illustration of depth-resolved post patterning of the top electrode (P3) using a femtosecond laser. Inset shows laser-patterned lines required for interconnection of successive cells, i.e. module fabrication.

PET

Ag NWs

TCA

PBTZT-stat-BDTT-8:PCBM

ZnO

PET/IMI

Depth-Resolved Post Patterning

P3 P2 P1



Figure 5-12: The P1 and P2 line are scribed before the lamination process while the P3 line is post-patterned through the top substrate.

We now turn to the laser patterning steps for module fabrication based on laminated

devices. Figure 5-13 illustrates the top view of fabrication route revealing also the module

layout of our devices. Following our experience from single solar cells we laser structure the

bottom IMI electrode for electrically isolating the edges of the substrate, in an opposite case

we would face dramatic losses form shunting as described in sub-chapter 5.2 , and define the

sub-cells of the module (P1 line) (Figure 5-13). The electron extraction ZnO layer and

photoactive layer are successively coated. During the third step of the module fabrication the

active and ZnO layers are ablated to form the P2 line (Figure 5-13).

Figure 5-13: Top view illustration of the module layout and the preparation road. .

This requires a laser fluence smaller than the ablation threshold of the bottom electrode

(Figure 3-7). It is critically important that the P2 line is free of residuals and debris as this is

the area that will later determine the quality of the electrical interconnection (between top and

successive bottom electrode). In a separate process, the TCA and AgNWs are consecutively

coated on a different substrate to form the top electrode, which is simply assembled on top of

dead area

P1 P2 P3

Substrate

ITO/IMI

ETL

Active Layer

TCA

AgNWs

Substrate

P1 laser

patterning

Coating &

P2 laser patterningLamination &

P3 laser patterningCharacterization



the active layer by passing through a pre-heated (120 °C) roll-laminator (pressure ~2 bar,

Figure 3-4c). The final patterning of the top electrode is then performed through the plastic

substrate using depth-resolved laser structuring. We emphasize that the laser fluence can be

fine tuned such that the TCA/AgNWs electrode is selectively ablated -without notably

damaging neither the top PET substrate nor the bottom electrode. The depth selectivity of this

process is demonstrated by plotting the ablation depth as a function of the laser fluence

(Figure 5-14a, left) as well as the corresponding depth profiles (Figure 5-14a, right). The

latter was extracted from confocal microscopy images upon laser structuring of the

TCA/AgNWs layer (Figure 5-14b). We observe that after a relatively slow increase of the

ablation depth with increasing laser fluence the depth increases rapidly after ~2.5 J/cm2. We

associate the slow initial increase with the melting and consecutive ablation of the top

AgNWs film while the steeper slope is likely related to the removal of the TCA. A ~2 m

thick TCA layer can be fully ablated at a laser fluence of ~3 J/cm2. While this laser fluence

exceeds the threshold fluence for the bottom IMI (Figure 3-7) the 2 m thick TCA acts as a

buffer layer and protects the rest of the device from potential damage. Figure 5-14b

visualizes the topography of a representative P3 line as obtained with a confocal microscope

after delaminating the top PET substrate. We note that even though throughout the ablation of

the TCA the laser beam may penetrate and thus dissipate part of the photovoltaic blend layer

this does not affect device performance as the ablation occurs within the dead area, which

does not contribute to the photocurrent of the device. Instead, the main concern for the dead

area when scribing the P3 line is to preserve the electrical pathway by not destroying the

bottom electrode.

Based on the above results, it is clear that the combination of lamination and depth-

resolved laser patterning is particularly attractive because it alleviates both typical constraints

in fabricating thin-film devices from solution as well as active area losses. Furthermore, the

top substrate may simultaneously act as a barrier. In a large-scale fabrication line this

procedure would allow environmental protection of the sensitive active layer at an early

fabrication stage. Simultaneously, this method reduces fabrication complexity as the

adjustment of the P3 line with respect to the P2 and P1 lines can be well controlled with the

laser and obviates mechanical alignment of the substrates. Importantly, the introduction of

depth selective laser scribing constitutes the first step towards a post-patterning technology in

which all patterning lines (P1, P2 and P3) can be scribed by depth-resolved laser patterning.

The later would be applied as the very last device fabrication step upon uniform full-area



Figure 5-14: a) (Left) Ablation depth upon laser patterning of adhesive top electrode versus the laser fluence applied. (Right) Representative ablation depth profiles for different laser fluences as determined from confocal optical microscopy images. b) Schematic representation of post-laser ablation of a P3 line through a PET foil after lamination and corresponding 3D depth profile.

coating of all functional layers. Finally, integration of such a fabrication route in a future solar

factory would allow not only for a continuous production line of inexpensive, semi-

transparent and flexible solar modules but also, more generally, for post-connection of two

parallel production lines into a tandem or multilayer concept.

0 100 200 300 µm

µm

0

50

100

150

200

250

300

µm

0

1

2

3

4

5

6

0 100 200 300 µm

µm

0

50

100

150

200

250

300

µm

0

1

2

3

4

5

6

PET

TCA/AgNWs

a

b



Remarkably, we demonstrate OPV modules (two in series connected solar cells) with

laminated top electrode that exhibit a champion efficiency of 5.3% and thus only minor losses

in PCE (≈10%) as compared to reference solar cells (Figure 5-15a). These losses are

predominantly determined by the Jsc value, which is only ≈10% lower than the maximum

limit for in series connected modules (current corresponding to half of the reference cell), and

can be attributed to the GFF. The GFF was found to be ~91 % (considering the sum of active

and dead area and neglecting possible bus bar losses)317

. The Voc losses are restrained to ~3%

with respect to the maximum Voc of a two cell module. With 62% the FF is even slightly

higher than in the reference cell, confirming the quality of the active layer and interfaces in

the module. The reported mean values and standard deviation population values of the

laminated devices confirm good reproducibility (Table 5-2). The J-V characteristics in the

dark are shown in Figure 5-15b.

Figure 5-15: a) Current – Voltage (J-V) characteristics of organic solar cells and modules with laminated top electrode. b) J-V characteristics under dark conditions for flexible OPV devices with laminated and evaporated top electrode.

a

b



Table 5-2: Key metrics for organic and perovskites solar devices with evaporated and laminated top electrode under AM 1.5G illumination (100 mW cm−2). Best performance and mean values with standard deviation population (shown in parenthesis) were extracted from 10 organic devices and 5 perovskite devices.

Top Electrode V

oc (V) J

sc (mA/cm

2) FF (%) PCE (%)

Device area

(mm2)

PBTZT-stat-BDTT-8: PCBM cell

Evaporated Ag

0.81 (0.80±0.01)

12.70 (12.40±0.18)

60.05 (59.10±0.84)

6.19 (5.87±0.18)

15

PBTZT-stat-BDTT-8: PCBM cell

Laminated

0.81

(0.81± 0.01)

12.10

(11.78±0.28)

59.24

(59.16±0.57)

5.88 (5.65±0.13)

15

PBTZT-stat-BDTT-8: PCBM module

Laminated

1.57 (1.57± 0.01)

5.42 (5.38 ±0.06)

62.31

(60.56±1.28)

5.33 (5.15±0.15)

30

CH3NH

3PbI

3 cell Evaporated Ag

1.04 (1.06±0.02)

18.53 (18.05±0.38)

70.35 (68.09±1.72)

13.56 (13.10±0.32)

15

CH3NH

3PbI

3 cell Laminated

1.01 (1.03±0.02)

16.44 (16.05±0.56)

59.54 (56.61±1.95)

9.80 (9.36±0.40)

15

CH3NH

3PbI

3 module Laminated

2.00 (2.00±0.01)

8.12 (7.90±0.22)

60.00 (57.54±1.60)

9.75 (9.10±0.34)

30

To demonstrate the universality of the proposed method we fabricated perovskite solar

modules with a laminated top electrode featuring the device geometry

ITO/NiO/CH3NH3PbI3/[60]PCBM/ZnO/PEI/Laminated-top-electrode. Methylammonium-

halide based perovskite semiconductors are a particularly challenging example as their

intrinsic sensitivity towards water prohibits top coating of water based solutions. The

photoactive perovskite layer was based on methylammonium lead iodide (see 3.2.4 for

details). A thin film of commercial NiO nanoparticles (Nanograde) was employed as hole

transporting layer while a thin film of ZnO nanoparticles acted as electron-selective interfacial

layer (Figure 5-16a,b). The low-temperature processed NiO nanoparticles were prepared by

flame spray synthesis (see Chapter 3). This process produces crystalline nanoparticles with a

size distribution at the range of 5-8 nm, as determined by transmission electron microscopy

(see Chapter 3). This device architecture was primarily selected because hole and electron

transporting layers along with the intermediate buffer layers can be processed at relatively low

temperatures (140 °C). Moreover, this device geometry has been shown to effectively



suppress hysteresis318, 319

. During this work, we observed that ≈10 nm of PEI on top of ZnO

improved the electrical contact with the TCA. A Cross-section SEM image in Figure 5-16b

shows well-defined layers and a TCA of ≈3μm.

Figure 5-16: a) Device architecture of laminated perovskite solar cell/module. b) Cross-section scanning electron microscopy image of laminated perovskite solar device on glass substrate.

The J-V performance of perovskite solar cells with evaporated

(ITO/NiO/CH3NH3PbI3/[60]PCBM/ZnO/PEI/Ag, evap) and laminated contact as well as the

a

b



corresponding perovskite module is presented in Figure 5-17a. Reference solar cells with a

thermally evaporated silver electrode demonstrate an average efficiency of 13.56%. When

transitioning to a laminated electrode the average efficiency is reduced mainly due to losses in

Jsc and FF, while Voc remains almost unchanged (Table 5-2). The 11% loss in Jsc is in

agreement with previous reports and can be attributed to optical interference effects as is

apparent from the EQE spectra for wavelengths >550 nm.155, 320

Additional losses could

potentially derive from enhanced transport resistance at the TCA interface as noticed in the

case of laminated OPV devices (Figure 5-7b). This interface is most likely not fully

optimized, even upon improvements after adding PEI on top of ZnO.

Figure 5-17: a) J-V characteristics of perovskite solar cells and modules with laminated top electrode. b) J-V

characteristics under dark conditions for perovskite devices with laminated and evaporated top electrode. c) EQE spectra of reference perovskite solar cell with 100 nm evaporated Ag top electrode (blue dashed line), laminated perovskite solar cell measured with reflecting mirror in the back (black line), laminated OPV cell measured without reflecting mirror (green line).

Perovskite modules show minor losses in efficiency (<1%) as compared to the

corresponding laminated solar cells and deliver a PCE of 9.75%. These losses are governed

by Jsc losses while we observe a full Voc (double Voc with respect to the reference device) and

a

c

b



similar FF values. The mean values and the standard deviation of population for all perovskite

devices are shown in Table 5-2. The J-V characteristics in the dark are shown in Figure

5-17b. Importantly, roll lamination of the top electrode initially developed for organic solar

cells and modules can be easily adapted to work with other thin-film photovoltaic

technologies, requiring only an adjustment of the laser fluence for the ablation of the P2 line.

This opens up an entirely new avenue for a cost effective fabrication route of organic and

hybrid electronic appliances.

5.4 Innovating tandem solar cells

As outlined in the previous chapter the tandem concept is a promising approach to

overcome absorption spectrum limitations and thermalization losses, however it presents

several challenges when comes to actual realization. The monolithic development of hetero-

tandem solar cell requires careful planning of active materials, solvent systems, and electrode

materials which they additionally have compatible fabrication methods (e.g. some materials

cannot survive increased temperatures, instead others require temperature annealing). On top

of that, solar cells based on different technologies that follow fundamentally different

production routes with incompatible processes and/or materials are hard if not impossible to

combine. These facts create many times unsolvable puzzles for scientists and engineers. Yet,

the lamination process is a promising fabrication technique to mitigate all these problems and

bring novel solutions.

On this section we present for first time a proof of concept by post-assembling through

lamination process two individually made solar cells in series connection with an

interconnection layer based on solution-processed materials that can be applied in every

photovoltaic technology. The advantages of our concept are tremendous and open new doors

for creation of hybrid technologies tandem devices that is only limited by the imagination of

scientists and engineers.

Our proposed generic fabrication route for realizing laminated tandem solar cell is shown

in Figure 5-18 and is separated to two parallel routes for each sub cell. We utilized our

composite adhesive electrode (PEDOT:PSS:D-Sorbitor/AgNWs) to form the heart of an

efficient adhesive interconnection layer and post-connect in series the two sub-cells.

To prove the generality of our concept we designed and fabricated a hybrid laminated

tandem solar cell based on organic and perovskite sub-cells. The production lines of these

solar cells are diametrically opposed with non-compatible solvents, which constitutes



monolithic development very difficult. To form the organic sub-cell (Solar Cell 1, Figure

5-18), we successively coated on IMI substrate to end up with the following geometry;

IMI/ZnO/PBTZT-stat-BDTT-8:[60]PCBM/ PEDOT:PSS / AgNWs/ TCA//. In parallel, a

perovskite sub-cell (Solar Cell 2, Figure 5-18) was constructed with

ITO/PEDOT:PSS/CH3NH3PbI3/[60]PCBM/ZnO/PEI// structure.

Figure 5-18: Step-wise generic fabrication route of laminated tandem solar cell. ETL (bright yellow), Active layers (bright blue, red), HTL (dark blue), AgNWs (yellow grey), TCA (purple).

Finally, after annealing (120 C/10’) of the organic sub-cell, the two sub cells were laminated

together (pressure ~2bar) to form a hybrid laminated tandem solar cell. The architecture of

the resulting laminated tandem solar cell is illustrated in Figure 5-19.

Solar Cell 1

1. Substrate (I) coated

with transparent

electrode

2. Succesive coating of

ETL/Active

Layer(I)/HTL

1. Substrate (II) coated

with transparent

electrode


HTL/Active

Layer(II)/ETL

Solar Cell 2

Lamination of

Solar Cell 1 // Solar Cell 2

Tandem Solar Cell


adhesive intermediate layer

AgNWs/TCA



Figure 5-19: Architecture of laminated hybrid tandem solar cell.

Figure 5-20 shows the J-V characteristics of the hybrid laminated tandem solar cell and

the corresponding sub-cells under 1000 Wm-2

solar simulator light and in the dark. The

laminated tandem cells demonstrate a Voc of 1.65 V, a short circuit current density of 10.38

mA/cm² and FF of 68%. This corresponds to a power conversion efficiency of 11.65%. and.

Voc values show no losses from the complete summary of the Voc values derived from the

two sub-cells which gives a first indication about the high quality of the adhesive

interconnection layer. The Jsc of the tandem cell is attributed to the efficient current matching

between the current delivered by the two sub-cells and shows relatively low values because of

the strong overlapping of their corresponding EQE spectra (Figure 5-3b, Figure 5-17c).

Higher Jsc values can be obtained in the future with more complimentary materials that cover

a broader spectrum. The unprecedented FF values additionally highlight the quality of our

adhesive intermediate layer. All key metrics with mean values and standard deviation

population values confirm good reproducibility (Table 5-3).

PBTZT-stat-BDTT-8:PCBM

PET/IMI

ZnO

PEDOT:PSS

AgNWs/TCA

ZnO/PEI

PCBM

CH3NH3PbI3

Glass/ITO

PEDOT:PSS



Figure 5-20: a) Current – Voltage (J-V) characteristics of hybrid laminated tandem solar cell and the corresponding single cells with laminated top electrode. b) J-V characteristics under dark conditions for the same devices.

Table 5-3: Key metrics for hybrid laminated tandem solar cell and the corresponding single cells with laminated top electrode under AM 1.5G illumination (100 mW cm−2). Best performance and mean values with standard deviation population (shown in parenthesis) were extracted from 5 devices.

V

oc (V) J

sc (mA/cm

2) FF (%) PCE (%)

Laminated Tandem PBTZT-stat-BDTT-8: PCBM/ CH

3NH

3PbI

3

1.65 (1.64±0.01)

10.38 (10.30±0.20)

67.97 (66.70±1.04)

11.65 (11.43±0.20)

Laminated OPV PBTZT-stat-BDTT-8: PCBM

0.80

(0.80± 0.01)

10.52

(10.25±0.25)

59.64

(59.16±0.57)

5.02 (5.00±0.15)

Laminated Perovskite CH

3NH

3PbI

3

0.85 (0.84±0.02)

17.52 (16.97±0.51)

68.74 (67.03±1.60)

10.24 (10.01±0.35)

5.5 Conclusion

The widespread technological adoption of organic and hybrid electronics hinges on the

availability of suitable materials combined with cost-effective fabrication methods for large

area deployment. The material approaches and device engineering methods based on roll

lamination presented here address both of these concerns. A composite electrode based on

ubiquitous PEDOT:PSS and D-sorbitol is combined with Ag NWs to form a highly functional

conductive adhesive that can be coated on plastic substrates. This enables a mechanically

bendable, potentially ultra-low cost electrode with micrometer thickness that could be printed

on a roll-to-roll coater and combined with various technologies, including semi-transparent

a b



devices. The composite electrode exhibits transport and recombination resistances comparable

with evaporated silver. A key technological advancement in the fabrication of thin film

modules is attained through the demonstration of depth-resolved, layer selective laser

patterning through the laminated device, which simultaneously grants high geometrical fill

factors, a simplified fabrication protocol and a route towards inherent packaging at the earliest

possible state. As a result, we demonstrated organic and perovskite solar cells and modules

without major sacrifices in efficiency. We further extended this lamination technology to

realize a universal fabrication process based on post connection in series of multilayer

devices. By these means we proved that our composite electrode can also form a high quality

interconnection adhesive layer.

We are confident that this concept will be of relevance to other technologies such as

CIGS, a-Si:H, CZTS ( copper zinc tin sulfide) or HIT (heterostructure with intrinsic thin

layer) solar cells facilitating the production line and providing numerous possibilities for

novel architectures in thin film electronics. We further anticipate that the implementation of

high speed, high precision laser post-lamination patterning enhanced with 3D scribing

capability will further advance the production lines.


Chapter 6 Summary and Outlook

6.1 Summary

A core aspect of my thesis studies envisioned the development of materials and device

fabrication strategies for solution processed solar cells that could be easily translated from the

lab scale to large area, roll-to-roll, compatible processes without sacrificing power conversion

efficiency, stability and conformability. I studied the physics of materials casted from solution

to make high quality, stable thin films with specific functionality inside solar cell architecture;

that includes research on hole/electron transport layers, photoactive layers and electrodes. I

investigated smart processing techniques to fend off the fabrication route of solar cells from

slow and vacuum deposition methods. I combined materials and fabrication methods with

novel laser patterning techniques to demonstrate up-scalable prototypes that would exceed the

scientific experimental boundaries and add applicable values to my research. The

aforementioned can be clearly reviewed on my main projects, in which I, together with fellow

colleagues,: i) demonstrated highly efficient, flexible, solution-processed organic tandem

solar modules adopting roll-to-roll compatible processing (Energy Environmental Science,

first author) and ii) proved the long operating lifetime of the tandem structure (Energy

Environmental Science, shared first authorship), iii) designed an adhesive PEDOT:PSS-Ag

nanowire composite electrode and innovate solution-processing fabrication route for efficient

organic and perovskite solar modules via depth-selective laser patterning of this adhesive top

electrode (Energy Environmental Science, first author, PCT application), iv) innovated multi-

junction devices by post-assembling through lamination process two individually made solar

cells in series connection (manuscript under preparation, PCT appliacation). My findings

inform concrete steps towards efficient, stable and conformable photovoltaics.

6.2 Outlook

-Adhesive electrode

The composite electrode presented in this thesis led to high efficient single-junction and

multi-junction devices. However several improvements can bring the concept of adhesive

electrodes even to higher standards. As I described in Chapter 5 an efficient adhesive

electrode essentially requires two main characteristics; adhesion and conductivity. In an ideal



case for solar cells, the adhesion medium will also protect the sensitive part of the device from

the ambient. Thus, an ideal TCA will include a UV-curable epoxy usually used for device

packaging. Nevertheless, epoxies are insulators and a medium that gives conductive

properties should be added. Metallic NPs dispersed over the epoxy matrix and form vertical

conduction pathways could be a possible option. Alongside, interesting characteristics such as

anisotropic conductivity can be promoted.

Another aspect that has space for improvement in adhesive electrodes is transparency. In

our proposed adhesive electrode reduction of TCA and AgNWs thickness certainly had an

impact on transparency, but as was noticed in sub chapter 2.4 HC PEDOT:PSS yields to

much lower σDC,B/ σop ratio (FOM for transparent electrodes) than AgNWs. As a result, the

total transmittance of our adhesive electrode is limited by HC PEDOT:PSS, an important

feature when it comes to multi-junction devices. Ideally, since AgNWs demonstrate the

highest σDC,B/ σop ratio for solution processable transparent electrodes, a solution that

combines AgNWs and adhesive properties should be engineered. For the time I invest on this

study, dilution of D-sorbitol in AgNWs water solution and introduction of AgNWs inside an

epoxy matrix did not lead to desirable results.

-Laser patterning

We hypothesize that our depth resolved laser patterning can be extended in the future,

resulting in a complete post-fabrication depth resolved laser patterning for all P1, P2 and P3

lines required for a solar module. In the proposed web design I have added the laser patterning

steps in the end of the process highlighting the potential outcome of this hypothesis. That

would be complete freedom of structuring the final product in the desired form.

Every coating step shown in the proposed web design can be solution process coating

step compatible to roll-to-roll processes (such as; slot die coating, spray coating, inkjet

coating tampo-printing, flexoprinting e.t.c) or even a vacuum deposition method. Drying can

occur with heating units (such as; hot air ovens, infra-red ovens e.t.c). The realization of a

complete post-fabrication depth resolve laser pattering for all required patterning lines of a

solar module is a complicated research problem that would need further investigation and

possibly more advanced laser optics. Alternatively, laser patterning steps can be added in Line

1 and Line 2.



Figure 6-1: Proposed route for post-fabrication laser patterning of single junction solar device with laminated top electrode.

In Figure 6-1 we proposed a possible route to post-fabrication laser pattern single

junction solar devices with laminated top electrode. In detail, P1 and P3 lines can be patterned

in a similar manner to our proposed depth-resolved laser patterning (sub-chapter 5.3 ). The

contact between two successive cells could be obtained by coating on top substrate a material

that would melt in different laser energy and diffuse through the whole stuck to ensure

electrical connection.

-Tandems

Our laminated tandem solar cells, comprising organic and perovskite sub-cells, clearly

demonstrate the advantages of our novel fabrication route. Employing sensitive active layers

we proved that our materials and processing can be applied to several technologies. In the

boundaries of this thesis we investigated the connection of organics and perovskite solar cells,

however investigation on realizing hybrid tandems by connecting different technologies (such

as CIGS, a-Si:H, CZTS, perovskites and organics) continue.

-Roll-to-roll processing

To help the reader visualize the potential outcome of this thesis, I illustrate in Figure 6-2

a proposed, simplified roll-to-roll set-up that post-connects two parallel production lines (Line

1, Line 2) in a Combined Line. The two lines may produce different solar cells that can be

post-assembled in a tandem concept or solar cell and adhesive top electrode to form a single

junction device. In detail, in Line 1 a first substrate unrolls to meet successively coating and

drying steps (only one pair is shown) that will form all the functional layers. In parallel, and in

Substrate

Bottom Electrode

ETL

Active Layer

TCA

AgNWs

Substrate

Melting contact

Substrate

Bottom Electrode

ETL

Active Layer

TCA

AgNWs

Substrate

pP1

Melting contact

P3



similar manner, in Line 2 a second substrate undergoes coating and drying steps which may

completely vary from the one followed in Line 1. The two substrates are post-connected in a

Combined Line through a lamination process and additional drying processes may be applied.

Figure 6-2: Laminated roll-to-roll web design with post laser patterning.

The novel architectures and smart device fabrication strategies presented in this thesis

extended the knowledge on the main aspects of every photovoltaic technology; efficiency,

cost and stability. I proved every concept with demonstrators that hold great promises to be

transferred in roll-to-roll production lines of the future market.

Line 1

Line 2

LaminationCoating Drying Drying

CoatingDrying

Combined Line

Laser Patterning


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Curriculum Vitae PESONAL INFORMATION

Family name, First name: Spyropoulos, George

Date of birth: 18/09/1986

Nationality: Greek

Marital status: Single

EDUCATION

2012-2016:

Ph.D. Institute of Materials for Electronics and Energy

Technology (i-MEET), Department of Materials Science and

Engineering, Friedrich-Alexander University Erlangen-

Nuremberg (FAU), Germany

Research Supervisor: Prof. Christoph J. Brabec

2010-2012:

M.Sc. Department of Materials Science and Technology, University of Crete, Greece

2004-2010:

B.Sc. Department of Materials Science and Technology, University of Crete, Greece

LUNGAGES

Greek: Native speaker

English: Highly proficient in spoken and written English

German: Good command

LABORATORY EXPERIENCE

2012-current:

Institute of Materials for Electronics and Energy Technology (i-MEET), Full-time PhD student

2011-2012:

Laboratory of Hard Matter, Teaching assistant

2010-2012:

Laboratory of Nanomaterials and Organic Electronics, Full-time post-graduate student

2009-2010:

Laboratory of Nanomaterials and Organic Electronics, Diploma work as under-graduate student

CONFERENCE AND MEETING ATTENDANCE

2014: SPIE Optics + Photonics, USA

2014: International Workshop on Flexible Bio and Organic Printed Electronics, Turkey

2013: 2nd Congress Next Generation Solar Energy (Bayern-Innovativ), Germany

2012: International Conference Organic Phovoltaics (Bayern-Innovativ), Germany

2011: 4th International Symposium on Flexible Organic Electronics, Greece

Curriculum Vitae


2011: 3rd International Conference from Nanoparticles and Nanomaterials to Nanodevices

and Nanosystems (IC4N), Greece

2011: 1st International Conference on Bioinspired Materials for Solar Energy Utilization,

Greece

SUMMER SCHOOL ATTENDANCE 2011: 1st Cost Coinapo Summer School. Characterization Work Group: from functional

nanomaterials to composites, Oxford, UK

2010: “An Introduction to Organic Electronics & Applications” Or.E.A, Greece

PATENTS

1. PCT Patent Application: George D. Spyropoulos, Michael Salvador, Tayebeh Ameri, Hans-

Joachim Egelhaaf, Christoph J. Brabec

“METHOD AND SUB-LAMINATE FOR FABRICATING A SOLAR MODULE AND

SOLAR MODULE”

2. PCT Patent Application: George D. Spyropoulos, Michael Salvador, Tayebeh Ameri, Hans-

Joachim Egelhaaf, Christoph J. Brabec

“METHOD FOT FABRICATING A TANDEM SOLAR CELL BY LAMINATION OF THE

CONSTITUENT SUB-CELLS AND TANDEM SOLAR CELL”

PUBLICATIONS IN INTERNATIONAL PEER-REVIEWED JOURNALS

(Overview: Papers = 20, h-index= 14, citations= 594, as of October 2016; source Google Scholar)

1. G. D. Spyropoulos, C. O. Ramirez Quiroz, M. Salvador, Y. Hou, N. Gasparini, P. Schweizer,

J. Adams, P. Kubis, N. Li, E. Spiecker, T. Ameri, H.-J. Egelhaaf and C. J. Brabec, Energy Environ. Sci., 2016, DOI: 10.1039/C1036EE01555G. citations: 1

2. J. Adams, M. Salvador, L. Lucera, S. Langner, G. D. Spyropoulos, F. W. Fecher, M. M.

Voigt, S. A. Dowland, A. Osvet, H. J. Egelhaaf and C. J. Brabec, Adv. Energy Mater., 2015, 5,

DOI: 10.1002/aenm.201501065. citations: 6

3. E. Kymakis, G. D. Spyropoulos, R. Fernandes, G. Kakavelakis, A. G. Kanaras and E.

Stratakis, ACS Photonics, 2015, 2, 714-723. citations: 11

4. F. Livi, R. R. Søndergaard, T. R. Andersen, B. Roth, S. Gevorgyan, H. F. Dam, J. E. Carlé, M.

Helgesen, G. D. Spyropoulos, J. Adams, T. Ameri, C. J. Brabec, M. Legros, N. Lemaitre, S.

Berny, O. R. Lozman, S. Schumann, A. Scheel, P. Apilo, M. Vilkman, E. Bundgaard and F. C.

Krebs, Energy Technology, 2015, 3, 423-427. citations: 4

5. J. Adams*, G. D. Spyropoulos*, M. Salvador, N. Li, S. Strohm, L. Lucera, S. Langner, F.

Machui, H. Zhang, T. Ameri, M. M. Voigt, F. C. Krebs and C. J. Brabec, Energy Environ. Sci, 2015, 8, 169-176. citations: 29

6. P. Kubis, L. Lucera, F. Machui, G. Spyropoulos, J. Cordero, A. Frey, J. Kaschta, M. M.

Voigt, G. J. Matt, E. Zeira and C. J. Brabec, Organic Electronics: physics, materials, applications, 2014, 15, 2256-2263. citations: 26

7. G. D. Spyropoulos, P. Kubis, N. Li, L. Lucera, M. Salvador, D. Baran, F. Machui, T.

Ameri, M. M. Voigt and C. J. Brabec, Proc. of SPIE, 2014, 9184,91841A. citations: 1

8. F. Machui, L. Lucera, G. D. Spyropoulos, J. Cordero, A. S. Ali, P. Kubis, T. Ameri, M. M.

Voigt and C. J. Brabec, Sol. Energy Mater. Sol. Cells, 2014, 128, 441-446. citations: 16

9. N. Li, D. Baran, G. D. Spyropoulos, H. Zhang, S. Berny, M. Turbiez, T. Ameri, F. C. Krebs

and C. J. Brabec, Adv. Energy Mater., 2014, 4. citations: 56

Curriculum Vitae


10. G. D. Spyropoulos, P. Kubis, N. Li, D. Baran, L. Lucera, M. Salvador, T. Ameri, M. M.

Voigt, F. C. Krebs and C. J. Brabec, Energy Environ. Sci, 2014, 7, 3284-3290. citations: 31

11. F. Machui, M. Hösel, N. Li, G. D. Spyropoulos, T. Ameri, R. R. Søndergaard, M. Jørgensen,

A. Scheel, D. Gaiser, K. Kreul, D. Lenssen, M. Legros, N. Lemaitre, M. Vilkman, M.

Välimäki, S. Nordman, C. J. Brabec and F. C. Krebs, Energy Environ. Sci, 2014, 7, 2792-

2802. citations: 52

12. T. R. Andersen, H. F. Dam, M. Hösel, M. Helgesen, J. E. Carlé, T. T. Larsen-Olsen, S. A.

Gevorgyan, J. W. Andreasen, J. Adams, N. Li, F. Machui, G. D. Spyropoulos, T. Ameri, N.

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REVIEWER IN PEER REVIEW SCIENTIFIC JOURNALS

Advanced Energy Materials (Wiley)

Advanced Functional Materials (Wiley)

Organic Electronics (Elsevier)

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