The effect of the alkylammonium cation on the optical and physical

properties of organic-inorganic perovskite nanoparticles

Sigalit Aharon and Lioz Etgar*

The Hebrew University of Jerusalem, The Institute of Chemistry, Casali Center for Applied

Chemistry, Jerusalem, Israel

E-mail: lioz.etgar@mail.huji.ac.il

Organic-inorganic perovskites (OIPs) function efficiently as active materials in

optoelectronic applications. Confined OIP nanostructures are a promising substance for

efficient optoelectronic devices.

Here we present a facile, low temperature synthesis of OIP nanoparticles (NPs) of well-

defined size and shape. To the best of our knowledge, this is the first time that OIP NPs are

synthesized under ambient atmosphere having defined cubic shape.

As opposed to their inorganic counterparts, the synthesis of OIP NPs is quite challenging, and

major efforts should be invested in finding the right ligand for the stabilization of the surface.

Three alkylammonium cations (C8H17NH3+/ C12H25NH3

+/ C18H37NH3

+) that stabilize the NPs’

surface were studied. The size and the shape, as well as the optical properties of the NPs,

were affected by the length of the alkylammonium cation. The OIP NPs showed a shift in the

absorbance and the photoluminescence to higher energies than the OIP bulk. This shift is an

evidence for their two-dimensional (2D) nature, which was controlled by the length of the

alkylammonium cations. In order to elucidate the effect of the ligands on the optical

properties of the NPs, layers of 2D perovskite of the formula (RNH3)2(MA)n-1PbnX3n+1 (R is

an alkylic residue of the lengths C8, C12, or C18) were synthesized. It can be concluded that

the length of the alkylammonium cations affects the assembly of the OIP NPs and the 2D

perovskite layers. In addition, it also influences on the optical and physical properties of the

NPs, thus enabling the acquisition of many desired colors from the UV to the visible.

Wet Chemistry of mixed cation for high efficiency and stable perovskites using combinatorial material science

Shalom Avadyayev*, Shay Tirosh , Laxman Gouda , Adi Kama , David A. Keller ,David Cahen, and Arie Zaban

Department of Chemistry and Center for Nanotechnology and Advanced Materials, Bar Ilan University, Ramat Gan, 5290002

shalom.avadyayev@gmail.com

As an emerging photovoltaic technology, hybrid perovskite solar cells have attracted enormous

research attention because of their ease of fabrication and high power conversion efficiencies. A

typical formula for the hybrid perovskite is ABX3, where A stands for cation such as MA, FA, Rb,

and Cs, B is a metal, and X represents a halogen anion). However, MAPbI3 and FAPbI3 have

different properties such as thermal stability, moisture-related degradation, and hysteretic I−V

behavior. The incorporation of MA, Cs, and Rb stabilizes the perovskite structure, resulting in

better PV performance. These facts indicate that using mixed cation perovskites combined with

high-throughput experimentation can lead us to improved perovskites. As such, development of

new mixed-cation perovskites, with lower bandgaps as absorbers and better stability for solar cells

is important.

In this work, combinatorial material science was used, to form FAxMA1-xPbI3, as an opening shot

for further development and research capabilities. The FAxMA1-xPbI3 was synthesized in a

combinatorial library by dip-coating of a MAPbI3 film in a solution of FAI dissolved in 2-propanol

(Cation Exchange). Optical characterizations reveal that the obtained bandgaps for the change

from MAPbI3 to FAxMA1-xPbI3 is 0.19 eV, allowing fine tuning of the bandgap. The cation

exchange process was characterized by x-ray diffraction (XRD) and PL measurements The

capability of tunable bandgaps by different ratio of the mixed-cation in a perovskite indicate its

potential to be used as a new technique to get better absorber materials in photovoltaic devices.

Correlative Scanning Electron and Probe Microscopy Characterization of Inorganic Halide-perovskite-type Materials

Sebastián Caicedo-Dávila1, Carolin Rehermann1, Eva Unger1, Christian Müller2, Michael Sendner3, Robert Lovrincic2, Daniel Abou-Ras1

1 Helmholtz Zentrum Berlin for Materials and Energy, Berlin, Germany 2 Institute for High-Frequency Technology, InnovationLab, TU Braunschweig, Heidelberg, Germany 3 Kirchhoff Institute of Physics, Heidelberg University, Heidelberg, Germany

Understanding what limits the performance in wide-gap, halide-perovskite-based solar cells is a critical issue for the design of high-efficiency devices. Therefore, correlating microscopic electrical and optoelectronic properties with macroscopic material and device characteristics is a fundamental task in research and development. One of the wide-gap, halide-perovskite-based materials of interest is CsPbBr3 (band-gap energy of 2.3 eV). We report preliminary results obtained by using scanning electron microscopy (SEM) imaging, energy-dispersive X-ray spectrometry, and cathodoluminescence at room temperature, which allowed for correlating local composition and optoelectronic properties of CsPbBr3 thin films synthesized by spin-coating and evaporation. Additionally, local transport measurements by scanning probe microscopy were performed on the same areas as the SEM analyses. We will discuss the challenges concerning specimen preparation and the issue of damaging by the electron beam.

Dis-covering Self-Healing in Halide Perovskites

D. R. Ceratti1, Y. Rakita1, L. Cremonesi2, R.Tenne1, V. Kalchenko1, G. Hodes1, Dan

Oron1, M. A. C. Potenza2, D. Cahen1 1Weizmann Institute of Science, 234 Herzl Street, 7610001, Rehovot, Israel.

2Department of Physics and CIMAINA, University of Milan, via Celoria, 16 20133, Milan, Italy.

davide.ceratti.cdf@gmail.com / davide-raffaele.ceratti@weizmann.ac.il

There is considerable evidence that Halide Perovskites have, if prepared well, surprisingly

low densities of optically and electronically active defects 1–3.

One explanation that was proposed for this, is that these are self-healing materials, i.e., if

some damage that degrades their optoelectronic properties is induced by light or particle

beams, the materials can return to the status quo ante 4, in a way that may be similar to

what is known about another solar cell absorber, CuInSe2 (and Cu(In,Ga)Se2, CIGS) 4.

Here we report the results of our experiments following the dynamics of defects and of

degradation / curing processes in situ. We do so by studying the main currently used

halide perovskites, varying the monovalent cations (Methylammonium, Formamidinium

and Cesium) and anions (Iodide and Bromide) without interference from other materials,

interfaces, surfaces or any electric/electronic effect typical of many configurations,

including that of a solar cell. The time scales of the inspected phenomena vary over several

orders of magnitude, are specific to each halide perovskite material and are affected by

doping and composition, in addition to temperature and illumination.

All these results will be summarised, compared to and evaluated against what is known

(from the literature) in order to analyse the relations between the different timescales and

the proposed damaging and restoration mechanisms 5–7. We will emphasize light-induced

damage as function of light intensity, time and temperature, and explore, analyze and

conclude on the possible roles of ion migration.

References

1. Brenner, T. M., Egger, D. A., Kronik, L., Hodes, G. & Cahen, D. Hybrid organic—inorganic perovskites: low-cost

semiconductors with intriguing charge-transport properties. Nat. Publ. Gr. (2016). doi:10.1038/natrevmats.2015.7

2. Stoumpos, C. C. & Kanatzidis, M. G. Halide Perovskites: Poor Man’s High-Performance Semiconductors. Adv. Mater. 28,

5778–5793 (2016).

3. Berry, J. et al. Hybrid Organic-Inorganic Perovskites (HOIPs): Opportunities and Challenges. Adv. Mater. 27, 5102–5112

(2015).

4. Nie, W. et al. ARTICLE Light-activated photocurrent degradation and self-healing in perovskite solar cells. Nat.

Commun. 7, (2016).

5. Sang-Won Lee, Seongtak Kim, Soohyun Bae, KyungjinCho, T., Laura E. Mundt, Seunghun Lee, Sungeun Park1,2,

Hyomin Park1, M. S., StefanW.Glunz2,3, Yohan Ko4, Yongseok Jun, Yoonmook Kang, H.-S. L. & & Donghwan Kim.

UV Degradation and Recovery of Perovskite Solar Cells. Sci. Rep. (2016). doi:DOI: 10.1038/srep38150

6. Klein-Kedem, N., Cahen, D. & Hodes, G. Effects of Light and Electron Beam Irradiation on Halide Perovskites and Their

Solar Cells. Acc. Chem. Res. 49, 347–354 (2016).

7. Yoon, S. J. et al. Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation.

ACS Energy Lett. 1, 290–296 (2016).

Quantifying hysteresis: developing tools to characterize the transient response of metal halide perovskite devices on different time scales.

A. Czudek, L. Kegelmann, M. Jost, K. Hirselandt, P. Tockhorn, E. Unger

Hysteresis is among the most debated phenomena in metal halide perovskite solar cell research1. Apart from being an obstacle and source of inaccuracy in reported device efficiency metrics, the underlying causes for hysteresis can be related to reversible and irreversible changes in devices. If evaluated as a quantitative metric, hysteresis can be an indicator of transient processes in devices that might affect long term stability. We employed two different approaches to quantify hysteresis phenomena in different device types prepared in our laboratory. The first is a detailed analysis of the hysteresis index as function of delay time, inspired by the work of Cahen et al.2. The second is the analysis of current transients upon voltage steps to extract time constants of the transient response3.

Hysteresis indices can be a valid method of characterization of perovskite solar cells, as long as they are not used at one single scan condition, but analyzed as a function of delay time. These plots often exhibit "peaks" that indicate the temporal regimes of maximum discrepancy (hysteresis), which provides direct insight into time constants of capacitive effects causing current-voltage hysteresis. We here present a quantitative comparison between different architecture types and discuss how the nature of the interface between selective contact and and perovskite critically influence the magnitude and temporal response domain of metal halide perovskite solar cells. In comparison, we analyzed and transient photocurrent response time constants. We discuss data for different device types using these two different analysis approaches to compare the practical value of these characterization methods forthe quantification of hysteresis phenomena in metal halide perovskite devices. Both can be used as quantitative tools to determine the magnitude and timescale of processes responsible for hysteresis and provide reliable comparison between different cell architectures.

1. Unger, E. L. et al. Hysteresis and transient behavior in current–voltage measurements of hybrid-perovskite absorber solar cells. Energy Environ. Sci. 7, 3690–3698 (2014).

2. Levine, I. et al. Interface-dependent ion migration/accumulation controls hysteresis in MAPbI3 solar cells. J. Phys. Chem. C 120, 16399–16411 (2016).

3. Christoforo, M., Hoke, E., McGehee, M. & Unger, E. L. Transient Response of Organo-Metal-Halide Solar Cells Analyzed by Time-Resolved Current-Voltage Measurements. Photonics 2, 1101–1115 (2015).

Preparation and in-system study of SnCl2 precursor

layers: First step towards the synthesis of Pb-free

perovskites at EMIL

Roberto Félix,1 Núria Llobera-Vila,1 Claudia Hartmann,1 Carola Klimm,1 Dan

R. Wargulski,1 Manuel Hartig,1,2 Regan G. Wilks,1,3 and Marcus Bär1,3

1Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-

Meitner-Platz 1, D-14109 Berlin, Germany 2 Technologie für Dünnschicht-Bauelemente, Technische Universität Berlin - Fak. IV, HFT 5-

2, Einsteinufer 25, D-10587 Berlin, Germany 3Energy Materials In-Situ Laboratory Berlin (EMIL), Helmholtz-Zentrum Berlin für

Materialien und Energie GmbH, Albert-Einstein-Straße 15, D-12489 Berlin, Germany

Hybrid organic-inorganic metal halide perovskite-based solar cells – particularly those using

APbX3 (A = CH3NH3+, HC(NH2)2

+, Cs+ and X = I-, Cl-, Br-) as the absorber layer – have

demonstrated rapid improvement in power conversion efficiencies in recent years, reaching

values in excess of 21% for lab-scale devices. A major concern related to this type of absorber

is the toxicity of the Pb. Ongoing efforts to replace Pb by Sn have so far yielded relatively low-

performing solar cells, likely limited by defect formation in the absorber material due to the

tendency of Sn to oxidize from Sn+2 to Sn+4. Preparing Sn-based perovskite films under vacuum

conditions may be the key to inhibiting Sn oxidation and improving cell performance.

We present the first experimental results towards synthesizing Pb-free perovskite thin-films at

the Energy Materials In-Situ Laboratory Berlin (EMIL). A detailed x-ray photoelectron and

Auger electron spectroscopy study of SnCl2 precursor layers of different thicknesses vacuum-

deposited on Mo/glass substrates reveals significant changes in the chemical environment of

Sn and Cl along the layer profile. These findings show the effect that substrate conditions can

exert on the deposited material.

Structural and Compositional Analyses of Perovskite-like Cesium Lead Halides Hannah Funk, Carolin Rehermann, Eva Unger, Robert Lovrincic, Christian Müller, Michael Sendner, Frederike Lehmann, René Gunder, Alexandra Franz, Daniel Abou-Ras Inorganic CsPbX3 (X=I, Cl, Br) thin films are investigated as supposedly more stable alternative to methyl-ammonium-containing, perovskite-like CH3NH3PbI3 layers for thin-film solar cells. For the present contribution, wide-gap CsPbBr3 thin films were synthesized by spin-coating and evaporation. In addition, also corresponding powder samples were produced as reference. Structural properties were obtained by X-ray diffraction as well as by high-resolution imaging and electron diffraction in transmission electron microscopy, while elemental distributions were analyzed by means of energy-dispersive X-ray spectrometry. Evaluation of these results has so far not resulted in unambiguous phase identification of the Cs-Pb-Br thin films, in contrast to the powder sample. The present contribution will report also about experiences with beam damage in the electron beam and how to avoid it.

High Resolution Study of TiO2 Contact Layer Thickness on the

Performance of Over 800 Perovskite Solar Cells

Laxman Gouda,† Kevin J. Rietwyk,+ Jiangang Hu, Adi Kama, Adam Ginsburg, Maayan Priel, David A.

Keller, Shay Tirosh, Simcha Meir, Ronen Gottesman and Arie Zaban*

Department of Chemistry, Institute for Nanotechnology & Advanced Materials, Bar-Ilan University,

Ramat Gan 5290002, Israel.

Abstract:

In this research, we systematically explore the influence of the TiO2 thickness with nanometric

variations over a range of 20–600 nm on the photovoltaic parameters (open-circuit voltage, short circuit

current, fill-factor and power conversion efficiency) of CH3NH3PbI3 based solar cells. We fabricate

several sample libraries of 13 × 13 solar cells on large substrates with spatial variations in the thickness

of TiO2 layers while maintaining similar properties for the other layers. We show that the optimal

thickness is ~ 50 nm for maximum performance; thinner layers typically resulted in short circuited cells

while increasing the thickness led to a monotonic decrease in performance. Furthermore, by assuming

a fixed bulk resistivity of TiO2 we were able to correlate the TiO2 thickness to the series and shunt

resistances of the devices and model the variation in the photovoltaic parameters with thickness using

the diode equation to gain quantitative insights.

S. Gupta, 10-2017

The Critical Roles of SnF2 in the Optoelectronic Properties of Lead-

Free Tin Halide Perovskites

Satyajit Gupta, Tatyana Bendikov‡, Gary Hodes

* and David Cahen

*

Department of Materials & Interfaces Weizmann Institute of Science, Rehovot, 76100,

Israel. ‡Chemical

Research Support Unit, Weizmann Institute of Science, Rehovot, 76100, Israel.

Lead-based halide perovskites (APbX3) have shown a dramatic improvement in

efficiencies in the past few years. Due to toxicity issues of lead, tin-based halide

perovskites are being studied. It is observed that tin fluoride (SnF2) addition can generally

improve the device properties of tin-based halide perovskites. This effect is thought to be

due to suppression of Sn(II) Sn(IV) oxidation, Sn(II)Sn (0) reductions, and /or a

decrease in Sn vacancy concentration. All these effects will change the doping, but in

opposite directions. Here we report on the role of SnF2 concentration on various

properties of cesium tin bromide (CsSnBr3)-based halide perovskite, such as energetics

(work function - WF and ionization energy, IE, X-ray beam damage and device properties.

We find that the SnF2 concentration strongly influences the device properties (open

circuit voltage-VOC, short circuit current-ISC, fill factor-FF and photoconversion

efficiency-η) but not the work function (WF) or ionization energy By monitoring the XPS

signals over time as a function of SnF2 concentration, we find that pristine CsSnBr3

(without SnF2 added) is highly susceptible to X-ray beam damage if deposited on titania

(TiO2), gold (Au) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate

(PEDOT:PSS) substrates, but is stable if deposited on ZnO substrates. About 5 mol% of

SnF2 was sufficient to protect the perovskite from beam damage on all the substrates,

possibly because of a SnF2-induced change in the Sn2+

/Sn0 electrochemical potential that

makes that process energetically less probable. For solar cells adding ~20 mol% of SnF2

was found to be optimum for device performance. Time-resolved surface photovoltage

(SPV) results are consistent with SnF2 addition reducing trap state concentration. Apart

from that, we found that SnF2 (1) improves conduction band-alignment of the perovskite

with TiO2, at their interface in a solar-cell (2) making the perovskite somewhat less ‘n-

type’, compared to highly n-doped pristine CsSnBr3.

Abstract 24/10/17

Utilizing combinatorial material science and high-throughput

analysis as a new method to promote and improve the field of

halide perovskites Adi Kama, Laxman Gouda, Hannah Noa Barad, Shay Tirosh, Arie Zaban*

Department of Chemistry, Center for Nanotechnology & Advanced Materials, Bar-Ilan

University

In order to accelerate the process of developing and improving perovskite materials and

device structures, a combinatorial synthesis approach with high-throughput analysis

techniques is applied. In the combinatorial method we define a library as a single

substrate with 169 individual experiments, which are individual solar cells, making it

possible to study more materials and gain more knowledge about them. In our lab we

adapted all measurements to suit and automatically scan the libraries.

This work focuses on the interface between the electron selective contact, TiO2, and the

perovskite absorbing material, MAPbI3. Throughout this interface, many charge transfer

processes occur. Forward movement, injection of electrons in to conduction band of TiO2,

contribute to solar cell photocurrent, as well as some recombination processes that reduce

solar cell photocurrent. In order to affect this interface, increase injection of electrons,

and reduce recombination rate, different thin insulating oxide layers were applied in

between the TiO2 and the perovskite. By preventing the electrons from going back to the

perovskite layer, the recombination rates at this interface were reduced, with the intension

of improving solar cell performance. Characterization was performed by high-throughput

analysis and combinatorial material science methods.

Abstract

Submitted by: Hadar Kaslasi (MSc.)

Supervisor: Prof David Cahen

Synthesis and Characterization of Mixed Cesium-Methylammonium Lead

Bromide Single Crystals

Hadar Kaslasi, Y. Rakita, G. Hodes, D. Cahen.

Weizmann institute of Science, Department of Materials and Interfaces, Rehovot, Israel, 7610001;

Halide perovskites (APbX3) are an intriguing group of optoelectronic materials, due, in part, to

their remarkably high open circuit voltages. These voltages make them relevant for use in

photovoltaic solar cells as, for instance, a high photo energy-absorbing component in a tandem

solar cell configuration. In this context, two types of high bandgap X=Br perovskites are of

special interest: The Cubic hybrid organic−inorganic Methylammonium lead bromide

perovskite (MAPbBr3) and its orthorhombic, all inorganic analog, CsPbBr3. MAPbBr3 Shows

somewhat better open circuit voltages than CsPbBr3 which is, in turn, more stable. To

understand the role of the A group (i.e. organic vs. in-organic) and its influence on these

desirable properties, fundamental studies of mixed single crystals with both the organic MA

and inorganic Cs cations are needed. We present first results of the growth of such crystals

with varying Cs/methylammonium (Cs/MA) ratios, and their characterization using structural

and compositional techniques.

Mixtures of PEDOT and dopant-free Spiro-OMeTAD

as hole selective contact in regular perovskite solar cells

Lukas Kegelmann1, Christian M. Wolff2, Philipp Tockhorn1, Lars Korte1, Dieter Neher2,

Bernd Rech1, Steve Albrecht3 1 Helmholtz-Zentrum Berlin, Institute Silicon Photovoltaics, Berlin, 12489, Germany.

2 University of Potsdam, Institute of Physics and Astronomy, Potsdam, 14476, Germany. 3 Helmholtz-Zentrum Berlin, Young Investigator Group Perovskite Tandem Solar Cells,

Berlin, 12489, Germany.

Email: lukas.kegelmann@helmholtz-berlin.de

The highest efficiencies of regular perovskite solar cells reported to date are obtained

with doped hole selective contacts (HSC).[1,2] Although these dopants are necessary to

provide sufficient conductivity in the HSC, they were shown to impose the dominating

source for non-radiative recombination in high efficiency solar cells.[3] It is therefore of

key importance to suppress recombination losses at the perovskite/HSC interface in

order to increase the open circuit voltage (VOC) and consequently the efficiency of the

solar cells.

In this study, a poly(3,4-ethylenedioxythiophene) (PEDOT) layer doped with sulfonated

copolymers is coated from a water-free dispersion and used as the HSC in regular

perovskite solar cells. By adding undoped 2,2′,7,7′-tetrakis(N,N-di-p-

methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) into the PEDOT

dispersion, the VOC of the resulting devices increases to above 1.15 V. This even exceeds

the VOC for reference solar cells prepared here with doped Spiro OMeTAD as HSC.

Furthermore, the stabilized efficiency is boosted beyond 15 % which more than doubles

the highest value of around 7.5 % reported so far for regular perovskite solar cells

comprising PEDOT as the hole contact.[4,5]

Transient photoluminescence and electroluminescence measurements imply enhanced

hole extraction at the perovskite interface for solar cells with Spiro-OMeTAD enriched

PEDOT layers. Photoelectron spectroscopy reveals a strong shift in ionisation energies at

the surfaces of the mixed PEDOT layers and conductivity measurements shows a linear

dependence of lateral conductivity of the resulting mixed films with the amount of added

Spiro-OMeTAD. Both results indicate a homogeneus distribution of Spiro-OMeTAD

throughout the PEDOT layer and the shift in ionization energies correlates with the

increase in VOC observed for the resulting solar cells.

In conclusion, a mixture of PEDOT and Spiro-OMeTAD as HSC is introduced in this

study. The high VOC values achieved for solar cells with this mixed HSC indicate

suppressed recombination losses at the perovskite/HSC interface by the dopant-free

Spiro-OMeTAD while the significantly more conductive PEDOT facilitates charge

transport to the metal electrode. This mixed material approach and its capability to omit

dopant-induced recombination losses might therefore provide potential to further

enhance the overall efficiencies in regular perovskite solar cells.

References

[1] M. Saliba, T. Matsui, K. Domanski et al., Science (2016), vol. 354, pp. 206–209.

[2] W.S. Yang, B.-W. Park, S.I. Seok et al., Science (2017), vol. 356, pp. 1376–1379.

[3] J.-P. Correa-Baena, W. Tress, A. Hagfeldt et al., Energy Environ. Sci. (2017),

vol. 10, pp. 1207–1212.

[4] J. Liu, S. Pathak, H.J. Snaith et al., J. Phys. Chem. Lett. (2015), 6 (9), 1666–1673.

[5] Y. Hou, H. Zhang, C. Brabec et al., Adv. Energy Mater. (2015), 5 (15), 1500543.

Control over self-doping in high-band gap perovskite films

Michael Kulbak1, Igal Levine

1, Einav Barak-Kulbak

1, Gary Hodes

1, Doron Azulay

2, Oded Milo

2,

Isaac Balberg2*

and David Cahen1

1Dept. of Materials & Interfaces, Weizmann Inst. of Science, Rehovot 76100, Israel

2The Racah Institute, The Hebrew University, Jerusalem 91904, Israel

The effects of different cations and anions on carrier diffusion lengths and formation of a

junction in high bandgap halide perovskite (HaP) film-based solar cells is studied in

detail. HaP cells are of interest as high bandgap ones in solar spectrum splitting for

boosting solar to electrical energy conversion efficiency/area by adding them to c-Si

photovoltaic cells and driving photo-electrochemical reactions for chemical energy

storage. Resolving how the addition of cations and anions change the (unintentional)

doping of the HaP is of great importance for understanding the film and device physics as

well as for performance improvement. We study Pb-based, APbX3, HaP films, where A

can be a mixture of formamidinium, methylammonium and cesium and X a mixture of

bromine and chlorine, using a combination of Dark-conductivity, Photoconductivity and

Steady-State Photocarrier-Grating (SSPG) techniques [1]. This way we measure the

effect of the different cations and anions compositions on the majority and minority

carrier diffusion lengths. We also use Electron Beam Induced Current (EBIC), [2] to

identify the formation of the junction and built-in voltage and to track the position and

size of the space charge region width following the changes in the HaP composition. In

some HaP structures, EBIC is needed to measure diffusion length when it is

unmeasurable in SSPG. We find mixed-cation HaP form a p-i-n junction with relatively

long and ambipolar carrier diffusion lengths, in contrast to the single cation based

bromide HaPs, who form a p-n junction and shorter diffusion lengths.

References

[1] I. Levine, et al., J. Phys. Chem. Lett. 2016, 7, 5219.

[2] N. Kedem, M. Kulbak, et al., Phys. Chem. Chem. Phys. 2017, 19, 5753.

Polyoxometalate as novel materials for Perovskite Solar Cells

Yasemin Topal1,2

, Esma Yenel2, Mahmut Kuş

2

1Pamukkale Üniversitesi Çal Meslek Yüksek Okulu, Çal, Denizli, Turkey

2Selcuk University Advanced Technology Research and Application Center, Konya, Turkey,

Perovskite solar cells (PSCs) have been gaining great attention due to their low cost and high

efficiency. However, reporoducibility and stability problems are main disadvantages must be

solved for large area production and commezilation.

Here we report interface engineering for improving the satbility and reporducibility of PSCs.

Polyoxometalates (POMs) whihc are interesting clusters and exhibit excellent optical and

electrochemical behaviours used as surface modification agent at c-TiO2 and perovskite

interface. Modification of c-TiO2 surface with POM derivatives leads decrease in pin holes on

c-TiO2 surface and thus efficiency increases. The average increase in efficiency is around

20% in comparison with reference PSC. Beside using POMs as surface modification material,

they also used as electron transport layer in inverted type of PSCs. The results show that

POMs are extremely interesting material as electron transport layer as well as a material for

interface engineering.

Measurement of Mobility-Lifetime products in MAPbI3 films

Igal Levine1, Doron Azulay2 , Satyajit Gupta1, Gary Hodes1, David Cahen1, Oded Millo2 and Isaac

Balberg2

1Dept. of Materials & Interfaces, Weizmann Inst. of Science, Rehovot 76100, Israel

2The Racah Institute, The Hebrew University, Jerusalem 91904, Israel

Photovoltaic solar cells operate under steady-state conditions that are established during charge

carrier excitation and recombination. However, hitherto no model of the steady-state

recombination scenario in the Halide Perovskites has been proposed. Here we present such a

model that is based on a single type of recombination center, which is deduced from our

measurements of the illumination intensity-dependence of the photoconductivity and the

ambipolar diffusion length in those materials. We find that the dominant recombination

mechanism is trap-mediated via a recombination level lying close to mid-gap. We also find that

under steady-state illumination conditions MAPbI3 is ambipolar, with hole and electron diffusion

lengths of ~800 and 350 nm, respectively.

Is it possible to identify solar illumination-induced phase segregation in perovskite thin-film solar cells

absorber by hard x-ray photoelectron spectroscopy?

D. Liu1, C. Hartmann1, E. Handick1, R. Félix1, R.G. Wilks1,2, E.L. Unger3,4, M. Bär1,2,5

1 Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB), Hahn-Meitner Platz 1,

14109 Berlin, Germany

2 Energy Materials In-Situ Laboratory Berlin, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH,

Albert-Einstein-Straße 15, 12489 Berlin, Germany

3 Institut für Silizium-Photovoltaic, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Kekuléstrasse 5,

12489 Berlin, Germany

4 Chemical Physica and Nano Lund, Lund University, Box 124, SE-22100 Lund, Sweden

5 Institut für Physik und Chemie, Brandenbrgische Technische Universität Cottbus-Senftenberg, Platz der Deutschen

Einheit 1, 03046 Cottbus, Germany

It has been observed that in methylammonium lead and cesium lead bromide-iodide perovskite solar cells the VOC of

the device increases with the band gap of the absorber, which increases with bromine concentration, up to a certain

saturation point. The VOC then cannot be increased further until a pure bromide perovskite absorber is used, at which

point the VOC attains the expected value. It is believed that in certain cases this behavior is related to illumination-

induced phase segregation - the so-called “Hoke Effect.” Aiming at studying this effect, we have measured

CH3NH3Pb(Br0.75I0.25)3, CsPb(Br0.75I0.25)3, and CsPb(Br0.5I0.5)3 films on FTO using hard x-ray photoelectron

spectroscopy (HAXPES) before, during, and after illumination. Focusing on the spectral changes in the core level

and valence band HAXPES spectra, we hope to gain insight into related changes in chemical and electronic

structure. In our contribution, we compare the composition dependence of our results to published observations of

the effects and likelihood of illumination-induced phase segregation.

Photoluminescence Excitation Microsocpy of Halide Perovskite Material

Aboma Merdasaa, Alexander Kiligaridisb, Jonas Stöberb, Caroline Rehermanna, Katrin Hirselandta, Boris

Louisb, Marina Gerhardc, Eva L. Ungera,b, Ivan Scheblykina

aHelmholtz-Zentrum Berlin für Materialen und Energie GmbH, Kekulestraße 5, 12489 Berlin, Germany dDepartment of Chemical Physics, Lund University, P.O. Box 118, 22100 Lund, Sweden

E-mail: aboma.merdasa@helmholtz-berlin.de

While the recent development of perovskite based devices has seen an unprecedented progress, there are

still many questions that remain unanswered with respect to how the material behaves under the influence

of light. Some of the more specific questions relate to the stability and efficiency where prolonged exposure

to light in different ambient environments has demonstrated to significantly alter these. When reporting on

the quality of a material, typical spectroscopic characteristics such as absorption and PL quantum yield are

used, but how complete of a picture do these measurements alone? Further, it has been quite well

demonstrated that optical properties do not necessarily distribute homogeneously within a film, which calls

for a spatially resolved optical method to study these.

Here we demonstrate a method where we employ spatially resolved photoluminescence excitation (PLE)

spectroscopy with simultaneous measurement of absorption over prolonged periods of light soaking in

different ambient conditions. Comparing the PLE and absorption spectra at each excitation wavelength

provides more information on where charges may be lost after being absorbed by the material. If PL

quantum yield is constant for all excitation wavelengths, the shapes of PLE and absorption spectra should

compare. If the two spectra deviate, we are provided a clue of where (in terms of energy) losses within the

material occurs. The spatial resolution helps us understand where (in terms of space) they occur.

We will show recent results relating to MAPbI3 where we determine the rate of photoinduced formation of

PbI2 together with its spatial distribution and temporal evolution. We will also present preliminary results

of MAPb(BrxI1-x)3 where spatially resolved PLE spectra may give an insight into the formation of either

Iodide or Bromide rich regions within the film. We also manipulate the excitation scan and see some

intriguing phenomena to which we currently lack an explanation.

Fully functional semi-transparent perovskite solar cell

fabricated at ambient air.

Stav Rahmany, Michael Layani, Sholomo Magdassi, Lioz Etgar.

Abstract

Organic-inorganic perovskite functions as an efficient light harvester in solar cells. The

possibility to tune its optical properties, as well as the option to use it as thin film absorber

(few hundreds nanometers thickness) make this material highly attractive in the

photovoltaic research. These properties also enable the utilization of perovskite for the

fabrication of semitransparent solar cells. In this work, we demonstrate the fabrication of

perovskite solar cells of controlled transparency, by a mesh assisted deposition process.

Sequential fabrication of perovskite was performed under ambient atmosphere, where in

the first step a PbI2 grid is formed, and following the grid reacts with methylammonium

iodide, resulting in a perovskite grid pattern. The most efficient solar-cells included a

photoanode that is composed of mesoporous TiO2 with Al2O3 nanoparticles. The resulting

semi-transparent perovskite solar cells, including a semi-transparent contact composed of

MoO3/ Au/ MoO3 yielded a power conversion efficiency of 5.5% with an average

transparency of 26% and efficiency of 8% for cells fabricated with a gold contact.

The innovative fabrication methods that were used in this work, as well as the novel

architecture of fully semitransparent solar cell, illustrates the potential of perovskite in

future PV technologies.

1. Rakita et al. ; MRS Comunications, 5, 623 (2015)

2. Lubomirsky, I. & Stafsudd, O. ; Rev. Sci. Instrum. 83, 051101 (2012).

3. Rakita, Y, et al.; PNAS , E5504-E5512, (2017), ; doi: 10.1073/pnas.1702429114

Bypassing the Soft Nature of Photovoltaic Halide-Perovskites to Prove its

Ferroelectricity

Gary ,(1)Yagel Peleg ,(1)Hadar Kaslasi, (1)Elena Meirzadeh, )(2lilE-Omri Bar, *(1)Yevgeny Rakita

Hodes(1), Igor Lubomirsky(1), Dan Oron(2), David Ehre(1)*, David Cahen(1) Weizmann institute of Science,

(1)Department of Materials and Interfaces and

(2)Physics of Complex Systems,

Rehovot, Israel, 7610001; *yevgeny.rakita@weizmann.ac.il ; david.ehre@weizmann.ac.il

Perovskites are a wide-ranging family of materials that often serve in switches, solid-state

memory devices and other electronic components, and owe their properties to a characteristic

crystalline structure. Until recently, the most commonly studied and used materials in this family

were the hard and stable oxide perovskites. Ferroelectricity (the ability to change the

spontaneous polarization in a material by an external electric field), which is well known for

oxide perovskites, has been suggested as a possible reason for the outstanding solar-to-electrical

energy conversion of halide perovskites - especially methylammonium lead iodide and bromide

(abbr. MAPbI3 and MAPbBr3). Low carrier recombination rate, high voltage efficiencies and an

efficient exciton separation are some of the possible benefits of ferroelectric domains to

photovoltaic performance.

The relatively weaker interatomic bond nature of these halide perovskites1 (in contrast to

common ferroelectric materials) made the simple task of proving ferroelectricity by the straight-

forward method of polarization measurement as a function of an applied DC electric field not so

simple. The challenge for proving the existence of ferroelectricity occupied the scientific

community for a couple of years with contradicting reports.

As polarity is a prerequisite condition for ferroelectricity, using the periodic temperature

change (Chynoweth) method2 under different surrounding temperature conditions, we show that

the cubic phases of MAPbI3 (>330K) and MAPbBr3 (>236K) phase are clearly non-polar, which

exclude any possible ferroelectric activity at these phases. However, the room-temperature

(tetragonal MAPbI3; 330K) phase shows clear pyroelectricity, which proves the polar nature of

MAPbI3 at room-temperature. Second harmonic generation - a proof for non-centrosymmetry

that is a prerequisite condition for polarity - has been found to be consistent with the polarity

results. When trying to find a final proof for ferroelectricity - a direct polarization measurement –

it was found that leakage currents are very dominant and, therefore, must be taken into account

when analyzing the polarization response, unlike the commonly analyzed capacitive currents.

Together with cooling down to -70oC (still at the tetragonal phase), to avoid decomposition of

1. Rakita et al. ; MRS Comunications, 5, 623 (2015)

2. Lubomirsky, I. & Stafsudd, O. ; Rev. Sci. Instrum. 83, 051101 (2012).

3. Rakita, Y, et al.; PNAS , E5504-E5512, (2017), ; doi: 10.1073/pnas.1702429114

the material under the applied electric field, we could clearly detect ferroelectric polarization

response.3

Direct Metal to Halide Perovskite (HaP) Transformation

an Alternative Route to HaP films

Yevgeny Rakita, Satyajit Gupta, David Cahen, Gary Hodes Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, 76100, Israel

* yevgeny.rakita@weizmann.ac.il

We will present a simple process to convert a metallic film of Pb(0)

, Sn(0)

or a mixture

of those to an ABX3 halide perovskite by introducing to AX [e.g.: A - methylammonium

iodide (MA), formamidinium (FA) or Cs; X - I, Br] salts dissolved in simple alcoholic

solvents.[1] The novel approach allows a high-quality continuous films of various

(including mixed) halide perovskites, that can be easily up-scaled to large areas using

a low toxicity process. The much diminished toxicity of this fabrication method is

achieved by avoiding the use of commonly used polar aprotic solvents, such as

dimethylformamide or dimethylsulfoxide, which become very toxic when containing

Pb salts (such as PbX2 or Pb-acetate).

Figure: Pb film (~ 100 nm) evaporated on d-TiO2 /FTO/glass substrate glass before and after

treatment with MAI dissolved in IPA

We will describe of our findings, including examples of the direct transformation from

Pb or Sn to, for example, MAPbI3, MAPbBr3, MAPb(Br,I)3, MASnI3 and the pseudo-

perovskite Cs2SnI6. We will show the broad morphological tunability allowed by this

process and present how electrochemistry can further assist in optimization of the

process. Apart from I-V characterizations of full devices, morphological, optical and

(opto-)electronic characterizations will be presented.

References

[1] Y. Rakita, N. Kedem, D. Cahen, G. Hodes, Pat.Appl. # IL 245536 2016-024 ‘Process

for the preparation of halide perovskites and perovskite-related materials’

Insights into Kinetics and Reaction Pathway of 2-Step conversion of MAPb(IxBr1-x)3

series via in-situ UV-vis measurements

C. Rehermann, K. Hirselandt, A. Merdasa, E. Unger

Metal halide perovskites are an interesting material for tandem solar cells according to their

band gap tunability. Therefore, perovskite semiconductors with high band gaps up to 2.3 eV

can be prepared by exchanging halides.1 Burschka et al.2 introduced the 2-step method also

called sequential deposition as a preparation method for perovskite solar cells. Reaction

parameters influence the formation of the perovskite thin film. Ummadisingu et al.3 showed

that illumination during the conversion step influences the transformation into perovskite by

activating the nucleation. The mechanism might be an intercalation or dissolution-

reconstruction process.4 Understanding the formation process will help to form homogenous

films with high band gaps.

PbI2 + MABr MAPbBrI2 (dicrect conversion product)

PbI2 + MABr (excess) Pb2+ + MA+ + 3 Br- MAPbBr3 (dissolution/ reformation product)

In this work, the second step of this sequential deposition is monitored by in-situ UV-vis

measurements. The second step is the conversion from a lead iodide (PbI2) film into a mixed

halide perovskite film MAPb(IxBr1-x)3 by immersion of MABr. Using MABr we can distinguish

different reaction products and hence reaction pathways and track the different conversion

kinetics between a direct conversion and a dissolution-reformation process. The influence of

following parameters is investigated: film thickness and morphology, reaction time and

concentration of the MABr dipping solution. Additional UV-vis, SEM and XRD measurements

after annealing are carried out to complete the study.

In-situ UV-vis measurements show a change of the evolution of the absorption onset of the

direct conversion product, MAPbBrI2, that disappears over time in favor ofa pure dissolution-

reformation product of MAPbBr3. Due to the two competing reaction pathways, the PbI2 films

are converted into a perovskite phase with varying halide composition in MAPb(IxBr1-x)3

depending on dipping time and film thickness. XRD shows residual PbI2 not converted in the

dipping step. Dependencies on reaction parameters are giving insights into the reaction

pathway. Our experiments allow us to distinguish the two different reaction pathways and under

which conditions one or the other product is favored. Understanding transformation is necessary

to control degree of material conversion and morphology.

1. Unger, E. L.; Kegelmann, L.; Suchan, K.; Sorell, D.; Korte, L.; Albrecht, S., Roadmap and roadblocks for the band gap tunability of metal halide perovskites. Journal of Materials Chemistry A 2017, 5 (23), 11401-11409. 2. Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M., Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499 (7458), 316-319.

3. Ummadisingu, A.; Steier, L.; Seo, J.-Y.; Matsui, T.; Abate, A.; Tress, W.; Grätzel, M., The effect of illumination on the formation of metal halide perovskite films. Nature 2017, advance online publication. 4. Ko, H.; Sin, D. H.; Kim, M.; Cho, K., Predicting the Morphology of Perovskite Thin Films Produced by Sequential Deposition Method: A Crystal Growth Dynamics Study. Chemistry of Materials 2017, 29 (3), 1165-1174.

Structural investigation of organic-inorganic perovskite

based solar cells with carbon cathodes

Avi Schneider, Lioz Etgar

The Institute of Chemistry at the Hebrew University of Jerusalem

Organic-Inorganic Perovskite photovoltaics have gained much scientific attention over the

past decade as a potential competitor or complimentary strategy for the currently prevalent

and commercialized solar cell technologies. In recent years a number of groups have focused

their research on replacing the expensive metal counter electrodes of the cells with carbon

based electrodes, with the general goal of lowering production costs. In 2013 Han et. al.

reported a novel cell structure based on the use of a screen printed conductive porous

carbon film as the counter electrode of the cell, through which the photoactive Perovskite

solution is filtrated, reaching and penetrating the functional layers beneath it. Benefits of

this structure include a large area of interface between light absorber and counter

electrode, increased stability of the cell due to the hydrophobicity of the carbon, and a

better compatibility for large scale production, owing to the simple and cheap methods and

materials. Other cell structures and methods, implementing Carbon films as their counter

electrodes, have been reported by various groups over the past few years, making carbon

based PSC's a quite popular approach to overcoming some of the present limitations of this

type of cell. Though the various Carbon based PSC's show much promise, they are still

lacking in performance when compared with some of the leading Perovskite cell structures.

Our main goal in this research is to explore ways to improve the photovoltaic performance

of carbon based PSC's, while maintaining the low cost and simple fabrication methods of

these cells. Our strategies involve thorough investigation of the nano-scale structure of

these cells while intelligently tailoring new methods and materials aimed at maximizing the

performance of this specific cell type. A significant improvement could label these cells as

the most suitable Perovskite based cells for commercial mass production.

Au - Decorated ZnO Nanorod Arrays for Mixed-cation Lead Mixed-halide Perovskite

Photovoltaics

Tulusa,b, Magdalena Marszaleka, Andreas Peukerta, Martin Slamana, Yulia Galaganc, Simon

Christian Böhmea and Elizabeth von Hauff a

a. Physics of Energy, Faculty of Sciences, Vrije Universiteit Amsterdam, The Netherlands.

b. Center of the Polymer Technology, Agency for the Assessment and Application of Technology

(BPPT), Indonesia.

c. Solliance / TNO Eindhoven, Eindhoven, The Netherlands

Abstract

We fabricated ZnO nanorods decorated with Au nanoparticles for electron transport layers in

perovskite solar cells. We compared the performance of double : Cs/ FA and triple : Cs/FA/MA cation

with mixed double halide : I/ Br perovskite layers in this configuration . The current-voltage

characteristics and impedance spectra indicate that Au improves charge extraction in the devices,

confirmed by changes in the photoluminescence spectra. The use of Au particles in the solar cells

resulted in an increase in power conversion efficiency from 11.52 % to 12.62 % (double cation) and

from 11.9 % to 12.66 % (triple cation).

How does I2 Dope MAPbI3 layers p-type?

Insights from optoelectronic properties

Arava Zohar, Igal levine, Satyagit Gupta, Omri Davidson, Doron Azulay, Oded Millo,

Isaac Balberg, Gary Hodes* and David Cahen*

Department of Materials and Interfaces,

Weizmann Institute of Science, Rehovot 76100, Israel

Abstract

Getting insight in, and ultimately control over electronic doping of halide perovskites

may improve tuning of their remarkable optoelectronic properties, reflected in what

appear to be low defect densities, and as expressed in various charge transport and

optical parameters. These properties determine the remarkable performances of

photovoltaic and light-emitting devices. Doping is important for charge transport

because it determines the electrical field within the photo-absorber layer, which

strongly affects charge collection efficiency. Here we report on intrinsic doping of

Methyl-ammonium lead tri-iodide, MAPbI3, thin films of the types used for solar cells

and LEDs, by I2 vapor at a level that does not affect the optical absorption, and leads to

a small (< 20 meV, ~ 9 nm) red-shift in the photoluminescence peak. Doping MAPI layers

makes the films ten times more conductive electronically, in the dark, which we show is

due to p-type doping because we find their work function to increase by 150 mV. The

majority carrier (hole) diffusion length increases upon doping, making the material less

ambipolar. Our results are well-explained by I2-exposure decreasing the density of

donor defects, likely iodide vacancies (VI) or defect complexes, containing VI. Invoking

Iodide interstitials, which are acceptor defects, seems less likely based on calculations of

the formation energies of such defects in agreement with a recent report on pressed

pellets, rather than polycrystalline thin ( 0.35 um) films used here.