i

Nanoscale Interfaces in Colloidal Quantum Dot Solar

Cells: Physical Insights and Materials Engineering

Strategies

by

Kyle Wayne Kemp

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Department of Electrical and Computer Engineering

University of Toronto

© Copyrighted by Kyle Wayne Kemp 2014

ii

Nanoscale Interfaces in Colloidal Quantum Dot Solar

Cells: Physical Insights and Materials Engineering

Strategies

Kyle Kemp

Doctor of Philosophy

Department of Electrical and Computer Engineering,

University of Toronto

2014

Abstract

With growing global energy demand there will be an increased need for sources of renewable

energy such as solar cells. To make these photovoltaic technologies more competitive with

conventional energy sources such as coal and natural gas requires further reduction in

manufacturing costs that can be realized by solution processing and roll-to-roll printing.

Colloidal quantum dots are a bandgap tunable, solution processible, semiconductor material

which may offer a path forward to efficient, inexpensive photovoltaics. Despite impressive

progress in performance with these materials, there remain limitations in photocarrier collection

that must be overcome.

This dissertation focuses on the characterization of charge recombination and transport in

colloidal quantum dot photovoltaics, and the application of this knowledge to the development of

new and better materials.

iii

Core-shell, PbS-CdS, quantum dots were investigated in an attempt to achieve better surface

passivation and reduce electronic defects which can limit performance. Optimization of this

material led to improved open circuit voltage, exceeding 0.6 V for the first time, and record

published performance of 6% efficiency.

Using temperature-dependent and transient photovoltage measurements we explored the

significance of interface recombination on the operation of these devices. Careful engineering of

the electrode using atomic layer deposition of ZnO helped lead to better TiO2 substrate materials

and allowed us to realize a nearly two-fold reduction in recombination rate and an enhancement

upwards of 50 mV in open circuit voltage.

Carrier extraction efficiency was studied in these devices using intensity dependent current-

voltage data of an operational solar cell. By developing an analytical model to describe

recombination loss within the active layer of the device we were able to accurately determine

transport lengths ranging up to 90 nm.

Transient absorption and photoconductivity techniques were used to study charge dynamics by

identifying states in these quantum dot materials which facilitate carrier transport. Thermal

activation energies for transport of 60 meV or lower were measured for different PbS quantum

dot bandgaps, representing a relatively small barrier for carrier transport. From these

measurements a dark, quantum confined energy level was attributed to the electronic bandedge

of these materials which serves to govern their optoelectronic behavior.

iv

Dedication

To Brian and Lori Kemp

Thanks for all the love and support throughout the years.

v

Acknowledgements

I would first like to express my sincere gratitude and admiration for my supervisor, and mentor,

Professor Edward H. Sargent. I greatly appreciate his dedication to research, his endless drive for

success, and his continued support of my professional career. The past 4 years in the Sargent

group have been the most impactful of my life. I will take all the lessons accrued here with me in

my future endeavors

I would like to thank all members of the Sargent group, my coworkers and friends, both past and

present for both their support in and out of the lab: Andre Labelle, Alex Ip, Susanna Thon, Lisa

Rollny, David Zhitomirsky, Jeffrey McDowall, Daniel, Armin Fischer, Illan Kramer, Brandon

Sutherland, Chris Wong, Michael Adachi, Zhijung Ning, Damir Jamakosmanovic, Leyla

Soleymani, Ratan Debnath, Xihua Wang, Andras Pattantyus-Abraham, Aaron Barkhouse, Ghada

Koleilat, Vlad Sukhovatkin, Jin Young Kim, Mingjian Yuan, Silvia Masala, Fengjia Fan, Osman

Ahmed, Andrei Buin, Haopeng Dong, Gabriel Moreno-Bautista, and Huan Liu. I would

specifically like to thank Larissa Levina for her advice and synthesis of quantum dots; Elenita

Palmiano, Remigiusz Wolowiec, and Damir Kopilovic for keeping the lab from falling into

complete disarray ; Sjoerd Hoogland for this advice, enlightening discussions, and help with

experimental design; Oleksandry Voznyy for his valued insight and DFT modelling; and

Lukasz Brzozowksi for his great leadership and continued guidance throughout the years.

Additionally to all my other friends who have made life in Toronto so enjoyable even in the most

stressful of days, I would like to thank: Julie Ciurria, Davis Holmes, Jeff Grant, Stephanie

Fisher, Malgosia Ip, Jason Godfrey, Edmund Lee, Elise Andrey, Andrew Achkar, Yonni

Friedlander, and Karen Yu.

I want to extend my appreciation to all my collaborators over the years; Professor John Asbury

and his group for our work pump-probe spectroscopy, Professor Aram Amassian and his group

for their help with TEM and GISAXS; and Dr. Neil Coombs and Ilya Gourevich for SEM and

TEM assistance.

vi

I would like to thank my brothers Devin and Travis and my sister Ashlan. Even though we didn't

always get along, I cannot imagine growing up without them. I know that I will always have

their love and support, as they will always have mine.

Finally, I would like to thank my parents Brian and Lori Kemp for always supporting me in my

endeavors and for always believing that I could succeed in anything I put my mind to, no matter

how difficult.

vii

Statement of Personal Contributions and

Collaborations

Dr. Larissa Levina synthesized the PbS quantum dots used in the studies reported herein. The

Cd-oleate treatments were performed by either Dr. Levina or myself. Purification and isolation

of the quantum dots were performed by our research group technical specialist Elenita Palmiano

or myself. I performed all ligand exchanges along with all FTIR and photoluminescence

characterization. TEM imaging of the ligand exchanged samples was carried out in the Centre

for Nanostructure Imaging at the University of Toronto with assistance from Dr. Neal Coombs

and Ilya Gourevich.

I prepared all GISAXS samples which were then measured by Kang Wei Chou at the Advanced

Light Source at Lawrence Berkeley National Laboratories. Device fabrication for independent

certification by Newport was conducted by Dr. Jiang Tang and myself. The best samples

between the two of us were shipped for certification measurement. Energy filtered TEM

elemental mapping were conducted by microscopy specialists at the King Abdullah University of

Science and Technology on TEM samples prepared by me. I fabricated the photodiodes and

measured the frequency response and EQE data.

Unless otherwise specified I carried out fabrication of all devices reported herein and completed

all device I-V testing under AM1.5G conditions. Monochromatic I-V measurements and voltage

biased EQE measurements were carried out by me. I also conducted all fitting of the

experimental data and derived the final model.

Atomic layer deposition and all substrate post treatments were performed by me. TiO2 substrates

were fabricated by the entire group on a rotation schedule. Transient photovoltage measurements

were performed by me based on procedures developed by Dr. Sjoerd Hoogland.

The TRIR and ultrafast measurements were conducted by members of the Asbury group at Penn

State University. At the onset of this study I visited the Penn State to demonstrate thin film

deposition techniques and assist in initial measurements. I was responsible for interpretation of

all experimental data and developed the main conclusions based on these measurements. I also

viii

measured the temperature dependent photoconductivity and Stokes Shift for all quantum dot

samples. Density functional calculations were performed by Dr. Oleksandr Voznyy based on

discussions with myself and Professor John Asbury.

ix

Table of Contents Abstract ........................................................................................................................................... ii

Dedication ...................................................................................................................................... iv

Acknowledgements ......................................................................................................................... v

Statement of Personal Contributions and Collaborations ............................................................. vii

List of Tables ................................................................................................................................. xi

List of Figures ................................................................................................................................ xi

Chapter 1 Introduction and Motivation........................................................................................... 1

1.1 A Brief History of Photovoltaics ........................................................................................................ 1

1.2 Current Outlook for Solar Energy ....................................................................................................... 2

1.3 Solution Processed Photovoltaics ....................................................................................................... 3

1.3 Thesis Objectives ................................................................................................................................ 5

1.4 Thesis Outline ..................................................................................................................................... 6

Chapter 2 Photovoltaics and Colloidal Quantum Dots ................................................................... 8

2.1 Photovoltaics Background .................................................................................................................. 8

2.1.1 Solar Spectrum ............................................................................................................................. 8

2.1.2 Theory and Operation of Photovoltaics ....................................................................................... 9

2.2 Colloidal Quantum dots .................................................................................................................... 13

2.2.1 Synthesis and Optical Properties ................................................................................................ 13

2.2.2 PbS CQD Optoelectronics ......................................................................................................... 17

2.3 Conclusions ....................................................................................................................................... 23

Chapter 3 Early Studies: Core-Shell Quantum Dots ..................................................................... 24

3.1 Introduction ....................................................................................................................................... 24

3.2 Core-Shell Heterojunction Strategies ................................................................................................ 25

3.3 PbS-CdS Core-Shell Synthesis and Characterization ....................................................................... 27

3.4 Pyridine Ligand Exchange ................................................................................................................ 30

3.5 Mercaptopropionic Acid Exchange .................................................................................................. 32

3.6 Conclusion ........................................................................................................................................ 35

Chapter 4 The Advent of Atomic Ligand Passivation .................................................................. 36

4.1 Introduction ....................................................................................................................................... 36

4.2 Motivation ......................................................................................................................................... 36

4.3 Cation Passivation ............................................................................................................................. 37

x

4.4 Anion Passivation ............................................................................................................................. 42

4.5 Conclusion ........................................................................................................................................ 47

Chapter 5 Interface Recombination in Depleted Heterojunction CQD Solar Cells ...................... 48

5.1 Introduction ....................................................................................................................................... 48

5.2 Interface Recombination ................................................................................................................... 48

5.4 Interface Modification....................................................................................................................... 54

5.6 Conclusions ....................................................................................................................................... 61

Chapter 6 Direct investigations of photocurrent extraction efficiency inside an operating device

....................................................................................................................................................... 62

6.1 Introduction ....................................................................................................................................... 62

6.2 Device J-V Characteristics ................................................................................................................ 62

6.3 Photocurrent Loss Mechanisms ........................................................................................................ 63

6.4 Collection Extraction Models ........................................................................................................... 66

6.5 Intensity Dependence of Diffusion Length ....................................................................................... 71

6.6 Conclusions ....................................................................................................................................... 72

Chapter 7 The fate of photocarriers in CQD solids ...................................................................... 73

7.1 Introduction ....................................................................................................................................... 73

7.2 Ultrafast Photoinduced Absorption ................................................................................................... 73

7.3 Time Resolved Infrared Spectroscopy ............................................................................................. 76

7.4 Theoretical Origins of Stokes Shift ................................................................................................... 81

7.5 Conclusions ....................................................................................................................................... 84

Chapter 8 Conclusions and Future Work ...................................................................................... 85

8.1 Thesis Findings and Contributions ................................................................................................... 85

8.2 Future Work ...................................................................................................................................... 86

References ..................................................................................................................................... 88

Appendix ....................................................................................................................................... 95

Appendix A NMR Spectra ...................................................................................................................... 95

Appendix B Temperature Dependent JSC ................................................................................................ 97

Appendix C SCAPS Modelling Parameters ............................................................................................ 98

Appendix D Publications ........................................................................................................................ 99

xi

List of Tables

Table 4-1 Exciton shift and enhancement for Cd-TDPA-OLA treatment of PbS Dots (N=11). Exciton

enhancement is determined from the ratio of the peak-to-valley ratios defined in chapter 2.. ................... 39

Table 5-1 Device performance summary for modified TiO2 electrodes. ................................................... 57

Table 7-1 Comparison of activation energies (Ea) for TRIR and Photoconductivity Decay. .................... 80

Table A1 Basic Material Properties for SCAPS Model . ........................................................................... 98

Table A2 PbS Trap States for SCAPS Model ............................................................................................ 98

Table A3 TiO2 Trap States for SCAPS Model ........................................................................................... 98

Table A4 Interface Trap States for SCAPS Model. ................................................................................... 98

List of Figures

Figure 1-1: Predicted global energy consumption in less developed (LDC) and more developed (MDC)

countries [10]. ............................................................................................................................................... 2

Figure 1-2: Total installed global photovoltaic capacity. [Adapted and reprinted with permission] [11]. .. 3

Figure 1-3: Timeline for best research solar cell efficiencies for different architectures. [Reprinted with

permission] [13]. ........................................................................................................................................... 4

Figure 1-4: Timeline for emerging PV technologies from Figure 1-3. ......................................................... 5

Figure 2-2: I-V characteristics of a solar cell in the dark and under illumination. Intersection of the shaded

rectangle and the I-V curve indicates the maximum power point. [Reprinted with Permission][25]. ........ 10

Figure 2-3: Integrated photocurrent (right) as a function of bandgap using the AM1.5 spectrum. ............ 11

Figure 2-4: Thermodynamic limit for diode saturation current density (right). Theoretical limit for VOC

with (red) and without (blue) consideration of thermodynamic losses. ...................................................... 12

Figure 2-5: Ideal solar cell efficiencies as a function of bandgap (Eg) at 300 K for 1 sun and 1000 sun

concentrations. [Reprinted with Permission][25]. ...................................................................................... 13

Figure 2-6: Schematics showing the nucleation and growth of colloidal nanocrystals in which various

sizes of crystals can be isolated at different times. (b) Simple synthetic apparatus showing the preparation

of CQDs. [Reprinted with Permission][27]. ............................................................................................... 15

Figure 2-7: Size dependent absorption of PbS quantum dots. [Reprinted with Permission][31]. .............. 16

Figure 2-9: a) A schematic representation of a PbS photodiode device. b) Frequency dependence of

photocurrent at zero bias. [Adapted and reprinted with Permission][40]. .................................................. 18

Figure 2-10 a) Schematic of the Schottky device architecture consisted of PbS CQDs as active material

and Al as metal contact. The inset shows the electron micrograph of PbS film after n-butylamine

xii

exchange. b) Energy band diagram showing the band bending takes place at Al/PbS interface. Under

illumination, electrons and holes are swept away by the built-in electric field in the depletion layer. c)

Current–voltage data of the device influenced by the variation of the simulated solar illumination source.

The device shows AM1.5 PCE of 1.8%. d) EQE spectra for devices using PbS ........................................ 20

Figure 2-11: a) The depleted heterojunction architecture showing the various components of the device

(FTO/porous TiO2/PbS QD/Au) along with the band diagram close to maximum VOC. EF,n and EF,p are the

electron and hole quasi-Fermi levels; Ec and Ev are the conduction and valence band edges; Jp, PV and Jn,PV

are the hole and electron photocurrents (and are equal at steady-state); Jp,fwd is the hole current in the

forward bias direction. The Fermi level is shown as a dashed line. b) Apertured dark and illuminated

current density-Voltage (J-V) response of the photovoltaic device c) EQE and absorption spectra of the

photovoltaic device. [Adapted and reprinted with permission][46]. ........................................................... 22

Figure 3-1: Schematic representation of the energy-level alignment in different core/shell systems realized

in semiconductor NCs to date. [Reprinted with Permission][47]. .............................................................. 25

Figure 3-2 TEM imaging of PbSe-CdSe core-shell nanocrystals. Inset: Individual core-shell quantum dot.

[Reprinted with Permission][49]. ................................................................................................................ 26

Figure 3-3: Synthesis of core-shell PbS-CdS quantum dots by cation exchange. The PbS cores are

synthesized and then purified. Formation of the core-shell dots is accomplished by reaction with the Cd-

oleate complex. ........................................................................................................................................... 28

Figure 3-4: a) Absorption spectra for Cd-Oleate treated PbS quantum dots. b) Photoluminescence spectra

for Cd-Oleate treated PbS quantum dots. .................................................................................................... 29

Figure 3-5: Ligand exchange of oleic acid with pyridine. .......................................................................... 30

Figure 3-6: a) Absorption measurements of PbS and pyridine exchanged PbS-CdS. b) Fourier Transform

Infrared (FTIR) spectroscopy of pyridine exchanged PbS-CdS The feature at 2360 cm-1 is CO2 absorption

in the baseline subtraction . ......................................................................................................................... 31

Figure 3-7: Ligand Exchange of Oleic Acid with Mercaptopropionic acid (MPA) ................................... 33

Figure 3-8: Absorption spectra for PbS-CdS and MPA exchanged PbS-CdS. ........................................... 33

Figure 3-9 TEM imaging of PbS-CdS and MPA exchanged PbS-CdS nanocrystals (Scale Bar=100 nm). 34

Figure 4-1: Absorption spectra of PbS nanocrystals before and after Cd-TDPA-OLA treatment. ............. 39

Figure 4-2: Initial device data for cation passivated (CD-TDPA-OLA) PbS quantum dots. ...................... 40

Figure 4-3: Absorption spectra for PbS nanocrystals before and after TDPA-OLA treatment. ................. 41

Figure 4-4: a) Device performance data for PbS, Cd-TDPA-OLA treated PbS, and TDPA-OLA treated

PbS. b) Photoluminescence spectra for devices in a). ................................................................................. 42

Figure 4-5: a) Device data for certified device fabricated with atomic ligand passivation. b) External

Quantum Efficiency (EQE) for certified device in a). [Reprinted with permission][61]. ........................... 43

Figure 4-6: A) TEM image of atomic ligand passivated PbS nanocrystals used for EF-TEM elemental

mapping. B) Bromine elemental mapping. C) Lead elemental mapping. D) Sulfur elemental mapping. E)

Combined elemental mapping. [Reprinted with permission][61]. .............................................................. 44

Figure 4-7: GISAXS spectra for a) MPA treated PbS nanocrystals and b) CTAB treated PbS

nanocrystals. [Reprinted with permission][61]. .......................................................................................... 45

Figure 5-1: a) Band diagram for PbS-TiO2 depleted quantum dot heterojunction solar cells at open circuit

conditions. Interface recombination is depicted as being assisted by interface trap levels. b) Temperature

dependent VOC measurements of devices for 1.33 eV and 1.51 eV bandgap PbS nanocrystals. ................ 49

Figure 5-2: Size dependence of conduction and valence band edges of PbS nanocrystals (Red).

[Reprinted with Permission][69]. ................................................................................................................ 51

xiii

Figure 5-3: Simulation results for VOC as a function of window layer (TiO2) electron affinity and interface

trap density. Trap densities are evenly spaced on a logarithmic scale. [Reprinted with permission][78]. . 53

Figure 5-4: a) Device architecture for CQD depleted heterojunction solar cells with interfacial buffer

layer. b) Device J-V data for modified TiO2 electrodes with different buffer layer materials. [Reprinted

with permission][78]. .................................................................................................................................. 55

Figure 5-5: Performance comparisons of TiO2 and modified ZnO-TiO2 electrodes: a) J-V curves; b) Static

VOC; c) Static JSC and d) Static efficiency values at the maximum power point. [Reprinted with

permission][78]. .......................................................................................................................................... 56

Figure 5-6: Dark J-V comparison between TiO2 and ZnO Modified TiO2. [Reprinted with

permission][78]. .......................................................................................................................................... 58

Figure 5-7: Summary of recombination analysis for TiO2 and ZnO-TiO2 substrates. a) VOC as a function

of incident photo flux. b) Recombination rates. c) Injected carrier densities. d) Recombination lifetimes

(τ). [Reprinted with permission][78]. .......................................................................................................... 59

Figure 6-1: Current-voltage characteristics for a CQD solar cell under simulated AM1.5 and in the dark.

.................................................................................................................................................................... 63

Figure 6-2: Intensity dependent photocurrent density for a hybrid passivated CQD solar cell. Lines of best

fit are included to demonstrate unity power law relationship. [Reprinted with permission][88]. .............. 64

Figure 6-3: a) Spectrally resolved voltage biased EQE of a CQD solar cell. b) Fractional loss (compared

to 0V JSC conditions) in spectral EQE with forward bias. ........................................................................... 66

Figure 6-4: Collection efficiency and analytical fit [Equation (6.14)] for: a) Hybrid-passivated CQD solar

cell and b) Organically cross-linked organic CQD solar cell. [Reprinted with permission][88]. ............... 67

Figure 6-5: Transport and Recombination processes for a) Gärtner p-n junction model [Equation (6.7)]; b)

Hecht drift transport model [Equation (6.8)] and c) Proposed drift-limited p-n junction model [Equation

(6.13)]. [Reprinted with permission][88]. .................................................................................................. 68

Figure 6-6: Intensity dependence of diffusion length for hybrid passivated CQD solar cells [Equation

(6.12)] [Reprinted with permission][88]. .................................................................................................... 72

Figure 7-1: Schematic for a semiconductor system characterized by photoinduced spectroscopy.

Photocarriers are pumped high into the band. They then relax quickly to lower lying states (1Se) where

they can recombine back to the 1Sh ground state or undergo photoinduced absorption from a probe

excitation. .................................................................................................................................................... 74

Figure 7-2: Ultrafast transient absorption kinetic traces for PbS quantum dot materials treated with

different ligands. A pump energy of 1.55 eV and a probe energy of 0.24 eV were used to excite the

samples. Normalized kinetic traces are included in the primary panel with the actual transient data in the

inset. ............................................................................................................................................................ 75

Figure 7-3: Normalized ultrafast transient absorption kinetic traces for hybrid passivated PbS films as a

function of pump intensity. A pump energy of 1.55 eV and a probe energy of 0.33 eV were used to excite

the samples. At 321 μJ cm-2 it is estimated that 1.84 excitons are generated per quantum dot. ................. 76

Figure 7-4: a) Time Resolved Infrared Spectroscopy (TRIR) spectra for different quantum dots of

different bandgap (1Sh-1Se). b) Comparison of TRIR peak positions with calculated 1Se-1Pe energy

splitting and Stokes Shift for each bandgap material. Error bars for the ΔStokes + 1Se-1Pe energies are

indicated to the right of each data point. ..................................................................................................... 77

Figure 7-5: New model of PbS quantum dot band structure which includes the presence of a sub-gap dark

state. The TRIR signal is represented as the sum of both the 1Se-1Pe energy splitting and the Stokes Shift.

.................................................................................................................................................................... 78

xiv

Figure 7-6: a) Arrhenius plots and activation energies for photoconductivity decay rates as for different

bandgap quantum dots (Inset: Interdigitized electrode test structure.). b) Arrhenius plots and activation

energies for TRIR decay rates for different bandgap quantum dots. .......................................................... 79

Figure A-2: :NMR 1H spectra for MPA capped PbS-CdS core-shell dots in deuterated dimethylsulfoxide.

Spectra was taken at 300.06 MHz on a Varian Mercury-300 Spectrometer ............................................... 96

Figure B-1: Temperature dependence JSC measurements of devices for 1.33 eV and 1.51 eV bandgap

PbS nanocrystals. ........................................................................................................................................ 97

xv

List of Abbreviations

AM1.5 Air Mass 1.5 Global

CQD Colloidal Quantum Dot

DFT Density Functional Theory

D* Specific Detectivity

Ea Activation Energy

EC Conduction Band Edge

EV Valence Band Edge

EQE External Quantum Efficiency

FF Fill Factor

FTIR Fourier Transform Infrared Spectoscopy

IQE Internal Quantum Efficiency

I-V Current-Voltage

JSC Short Circuit Density

MPA 3-Mercaptopropionic Acid

NMR Nuclear Magnetic Resonance Spectroscopy

Nt Trap State Density

PCE Percent Conversion Efficiency

TEM Transmission Electron Microscopy

TRIR Time Resolved Infrared Spectroscopy

VOC Open Circuit Voltage

1

Chapter 1 Introduction and Motivation

1.1 A Brief History of Photovoltaics

In 1883 an American inventor by the name of Charles Fritts coated a purified sample of selenium

with a thin layer of gold [1]. The result was the first demonstration of a working photovoltaic

device. Although the estimated power conversion efficiencies were less than 1% it served as a

significant milestone in the development of a technology that is now poised to make significant

contributions to our energy production in the near future.

The progression of photovoltaic technology proceeded relatively slowly over the next 60 years.

Some notable advancements over this period include the development of single crystal silicon

ingots by Jan Czochralski in 1918 [2], and the demonstration of the photovoltaic response in

cadmium selenide in 1932 [3]. Perhaps most importantly of all was a seminal paper of Albert

Einstein in 1905 on the photoelectric effect [4] which helped established the quantum nature of

light.

Despite these major advancements to the field it wasn't until the early 1940's in Bell Labs that the

technology of photovoltaics really started to make significant gains. It was, after all, a Bell Labs

engineer by the name of Russell Ohl who, in 1941, discovered the p-n junction [5] in a silicon

crystal. The discovery, some may argue, was the birth of the modern solar cell. Bell Labs

invested heavily in research in the fields of materials science, physics, chemistry, and electrical

engineering in order to use basic scientific research to develop technologies [6]. It was this

scientific foundation which helped lead to discoveries in semiconductor physics and crystal

growth which would be responsible for further advancement in photovoltaics.

In 1954, soon after Russell Ohl's first discovery of a p-n junction Bell researchers Gerald

Pearson, Daryl Chapin and Calvin Fuller used artificially doped crystalline silicon to develop a

6% efficiency solar cell device [7]. Since then continued research in semiconductor physics and

materials science has helped further photovoltaic technology to the present day where

performance has advanced to 28.8% for single junctions and for 37.9% for multijunction devices

2

[8]. Along the way commercialization of different photovoltaic technologies has led to the

founding of numerous companies including Suntech and First Solar with 2012 revenues for the

photovoltaic industry reaching $25.5 billion [9].

1.2 Current Outlook for Solar Energy

Currently the threat of climate change, uncertainties in fossil fuel markets, and the continued

growth in energy demand have made renewable energy sources, specifically solar cells,

incredibly important. By some projections global energy demand may increase by over 50% by

2040 from present day, much of this increase occurring in less developed countries (LDC) [10]

(Figure 1-1). This is expected to put more strain on the planet's already strained resources and

will make our efforts to curtail climate change even more challenging.

Figure 1-1: Predicted global energy consumption in less developed (LDC) and more developed (MDC) countries [10].

Fortunately the outlook for solar energy remains quite positive. In 2012 installed global capacity

of solar energy surpassed 100 GW of capacity for the first time with 31 GW installed in 2012

alone [11]. This has been part of an overall trend of exponential growth over the past decade

(Figure 1-2).

3

Figure 1-2: Total installed global photovoltaic capacity. [Adapted and reprinted with permission] [11].

As part of its Sunshot Initiative [12] the United States Department of Energy has set a goal for

reduction in solar installation costs of 75% by 2020. This reduction would ensure a price of solar

electricity of approximately $0.05 per kWh. At this level, it will become cost-competitive with

conventional energy sources, most notably, fossil fuels such as coal and natural gas. Continued

research and development of inexpensive photovoltaic technologies will be necessary to achieve

these milestones and establish a more sustainable energy future.

1.3 Solution Processed Photovoltaics

A look at the state of current research indicates that all of the emerging technologies (Figure 1-3,

Figure 1-4) are compatible with solution processing fabrication techniques. This list includes

Dye-Sensitized Solar Cells, CZTSSe, various Organic based architectures, and Colloidal

Quantum Dot (CQD) photovoltaics.

4

Drop casting, spray coating, and ink-jet printing are all scalable fabrication techniques allowing

for the possibility of roll-to-roll processing on light weight, flexible substrates, at low

temperature. As economic viability is so crucial for further growth of solar energy generation,

these low cost manufacturing methods offer advantages that do not exist for other established

technologies.

Figure 1-3: Timeline for best research solar cell efficiencies for different architectures. [Reprinted with permission] [13].

5

Figure 1-4: Timeline for emerging PV technologies from Figure 1-3.

CQD photovoltaics, in particular, have seen significant growth in just a short period of time. In

the past 5 years performances have increased from 2.1% [14] to 7% [15]. Not only do CQDs

readily form ink-like dispersions in organic solvents, their quantum size effects offer bandgap

tuning from the visible throughout the near infrared spectrum, making them a very attractive

option for photovoltaic development.

1.3 Thesis Objectives

At the outset of this study colloidal quantum dot photovoltaic performances were approaching

4% efficiencies, up from sub-percent levels just a few years before. Despite this progress,

performances were still well below the 31% predicted by the Shockley-Queisser limit [16] for a

single junction solar cell. As the field matures, further enhancements to performance will rely

more heavily on deeper insights of basic material properties.

6

This dissertation focuses on the development of new materials for CQD photovoltaics by

leveraging insights obtained through characterization of recombination and transport

processes.

This work focuses on the following research questions:

1) How can we use surface passivation to improve upon device performance?

We know from previous reports how changes in surface passivation through different ligand

treatments can have a significant effect on the electronic and optical properties of CQD thin

films[17][18]. Despite these accomplishments very little work has found its way into improving

photovoltaic device performance. We sought to develop new surface passivation strategies to

improve device performance.

2) What are the primary loss mechanisms in PbS CQD films?

Our initial work on these materials was based on the premise that photocarrier recombination

occurs through poor surface passivation. While this has allowed us to optimize device

performance a deeper understanding is still required to pinpoint the main contributors to loss in

these CQD photovoltaics. How does interface recombination at the PbS-TiO2 heterojunction

affect device performance? How do these electronic defects in the CQD absorber influence

carrier transport and the overall band structure of the material? Can we understand the origins of

recombination on a molecular level?

1.4 Thesis Outline

Chapter 2 provides necessary background for both photovoltaics and, more specifically, CQD

solar cells. We start by introducing the solar energy spectrum and introduce fundamental

photovoltaic concepts to explain the origins of Shockley-Queisser analysis. We next introduce

the concept of the CQD, its origins, and basic properties. A brief review of the work done on

CQD optoelectronics by this group follows to establish the existing prior art.

7

Chapter 3 explores our initial attempts to use core-shell (PbS-CdS) quantum dots to passivate

surface defects and reduce carrier recombination. Initial characterization of the synthesis is

reported. Attempts to complete solution-phase ligand exchanges with were conducted to make

easily processible CQD materials with high nanocrystal packing densities.

Chapter 4 presents the development of atomic ligand passivation for PbS quantum dots. In this

chapter we use a post-synthesis cadmium cation treatment to passivate exposed sulfur sites on

the PbS nanocrystal surface. Our devices show a significant enhancement in open circuit voltage.

We combined this technique with a halide anion treatment that passivates lead surface atoms

achieving an inorganic atomic ligand passivation strategy which passivates both types of

nanocrystal surface atoms. Using this technique we were able to obtain record high performance

with certified results of 5.1% and 6% in the lab.

Chapter 5 establishes the importance of interface recombination on device performance. In this

chapter we use advanced characterization techniques such as temperature dependent current-

voltage and transient photovoltage measurements to investigate recombination kinetics in these

materials. Using atomic layer deposition we designed an interfacial buffer layer which led to

improved open circuit voltage and enhanced photocurrent collection.

Chapter 6 details our investigation into photocarrier collection efficiencies in our devices. Using

device J-V data we examine recombination loss mechanisms as a function of intensity. An

analytical model is developed to fit the experimental data. We use this model to accurately

predict a diffusion length of 90 nm for our best CQD materials.

Chapter 7 builds on the insights on recombination and transport developed in Chapter 6.

Transient photoinduced absorption and photoconductivity measurements are used to study

photocarrier kinetics. With this information we establish time scales for carrier relaxation,

trapping, and transport. From Time-Resolved Infrared Spectroscopy we identify sub-bandgap

states, which serve as an effective electronic bandgap in this material.

Chapter 8 summarizes the major findings made throughout this dissertation. We conclude with

recommendations for future work to further advance the field of PbS CQD photovoltaics.

8

Chapter 2 Photovoltaics and Colloidal Quantum Dots

2.1 Photovoltaics Background

2.1.1 Solar Spectrum

Radiation from the sun is found to closely resemble that of a black body emitter with a

temperature of 5800 K (Figure 2-1). Much of this power is concentrated in the visible spectrum

with a long tail extending into the infrared spectrum. At the top of the Earth's atmosphere the

composition of the solar radiation consists of approximately 50% infrared, 40% visible, and 10%

in the ultraviolet portions of the electromagnetic spectrum [19] and is designated AM0 [20]. The

total power over all wavelengths is 1366 W m-2

. After entering the Earth's atmosphere much of

the ultraviolet radiation is absorbed by ozone (O3). Additional losses occur at distinct bands due

to absorption from O2, H2O and CO2 in the atmosphere. Finally taking into account the zenith

angle of sunlight at mid-latitudes we can derive the solar AM1.5 spectrum [21] with a total

integrated intensity of 1000.4 W m-2

. The AM1.5 spectrum is considered the standard for

characterizing and comparing photovoltaic technologies.

9

Figure 2-1: Spectral irradiance for a 5800 K black body emitter, AM0 [20] and AM1.5 [21] conditions. Absorption lines in

the AM1.5 for O3, O2, H2O and CO2 are labeled.

2.1.2 Theory and Operation of Photovoltaics

In their 1961 seminal paper, Shockley and Queisser were the first to calculate the limiting

efficiencies of p-n junction solar cells through thermodynamic analysis [22]. The basis of their

analysis relied on the assumption that photovoltaics rely on the Principle of Superposition [23].

According to this principle the total current from the solar cell under illumination was a linear

supposition of the dark diode current of the p-n junction, IDiode (V) and generated photocurrent,

IL:

(V) (2.1)

The diode current of a p-n is the sum of diffusion currents on each side of the junction [24]:

(2.2)

where I0 is the dark saturation current, n is the ideality factor of the device, V is the applied bias,

k is Boltzmann's constant, and T is the temperature. Under illumination the I-V characteristics

are shifted vertically by the illumination current IL (Figure 2-2). The result is that power is being

10

generated in the 4th I-V quadrant. The boundaries of the I-V curve are marked by the short-

circuit current ISC and the open circuit voltage VOC.

Figure 2-2: I-V characteristics of a solar cell in the dark and under illumination. Intersection of the shaded rectangle and

the I-V curve indicates the maximum power point. [Reprinted with Permission][25].

The maximum efficiency of the solar cell is defined by:

(2.3)

where Vm and Im are the voltage and current at the maximum power point and Pinc is the total

incident power. A more conventional form of the equation is to relate efficiency to ISC and VOC

11

(2.4)

where FF is the device Fill Factor which is used to describe the shape of the I-V curve.

Using these I-V relationships Shockley-Queisser were able to theoretically determine the output

power of a solar cell and the limits to photovoltaic efficiency. In their analysis they assumed that

every photon with hυ>Eg generates a photocarrier which quickly relaxes to the bandedges of the

semiconductor. The total photocurrent is given simply by the integral of all absorbed photons

from the incident solar spectrum [25]. Figure 2-3 shows the integrated photocurrent as a

function of wavelength overlapped with the AM1.5 spectrum. For convenience we normalize by

the device area and calculate photocurrent density.

Figure 2-3: Integrated photocurrent (right) as a function of bandgap using the AM1.5 spectrum.

If we assume an ideality factor of n = 1 for an ideal solar cell in Equation (2.2) and if I0 is

known the efficiency of the solar cell can be determined. By treating the photodiode as a black

body, in thermal equilibrium with its surroundings, a lower limit could be placed on the

saturation current density and loss factors in the device. At equilibrium, and with no other loss

12

mechanisms to consider, photons are still absorbed by the diode from the black body emission

from the surroundings (T = 300 K). For this condition of equilibrium to be true the number of

absorbed photons from black body radiation must be equal to the number of photons emitted

from radiative emission loss. It is these radiative losses which make up the dark saturation

current I0, which can now be calculated much like was done for IL by integrating over all photons

absorbed from a 300 K black body spectrum (Figure 2-4). Again for convenience we normalize

by the device area to yield the saturation current density J0. The addition of radiative losses

decreases the theoretical work that can be done by an absorbed photon, reducing the quasi-Fermi

splitting, and open circuit voltage VOC. By re-arranging Equation (2.1) we can look at the

theoretical limit of VOC as a function of bandgap.

(2.5)

Without losses VOC could be equal to Eg/q as indicated by the blue curve in Figure 2-4. The

additional losses imposed by J0 (black curve) decreases the quasi-Fermi level splitting and VOC

(red curve in Figure 2-4).

Figure 2-4: Thermodynamic limit for diode saturation current density (right). Theoretical limit for VOC with (red) and

without (blue) consideration of thermodynamic losses.

Taking all these considerations into account the theoretical efficiency of a p-n junction solar cell

operating under the AM1.5 spectrum has been determined as a function of device bandgap

(Figure 2-5).

13

Figure 2-5: Ideal solar cell efficiencies as a function of bandgap (Eg) at 300 K for 1 sun and 1000 sun concentrations.

[Reprinted with Permission][25].

At AM1.5G operating conditions a maximum theoretical efficiency of 31% is predicted [25].

With higher intensities this efficiency can be even higher due largely to the intensity dependence

of VOC as predicted from Equation (2.5). Overlapped with these efficiency predictions are the

bandgaps of many common semiconductor materials. Two notable examples are silicon and

gallium arsenide whose bandgaps lie near to the regime of maximum efficiency.

2.2 Colloidal Quantum dots

2.2.1 Synthesis and Optical Properties

A quantum dot is a semiconductor nanocrystal where the electronic and optical properties

demonstrate quantum size effects. Generally speaking if the dimensions of a nanocrystal are

smaller than the Exciton-Bohr radius of the bulk semiconductor material, or electron-hole pairs,

photocarriers will experience quantum confinement. In addition to its bulk bandgap, the bandgap

of the quantum dots (E*) will have an additional size-dependent quantum confinement and

Coloumbic interaction energy according to:

14

(2.6)

where ħ is Plank's constant, R is the nanocrystal radius, me and mh are the masses of the electron

and hole respectively, and ε is the dielectric constant of the material. Here the second term in

Equation 2.6 represents the quantum confinement energy and the last term represents the

Coloumbic interaction energy.

This equation was originally developed by Louis Brus [26] in 1986, after being the first to

observe quantum dots in colloidal solutions. Since their first discovery CQDs have become an

important topic of research due to both their interesting chemistry and their unique physical

properties which can be leveraged for use in optoelectronic devices.

Much of the synthesis of CQDs are based on a model developed by LaMer [27][28] (Figure 2-

6). In a coordinating solvent reagents are added at temperatures up to 300 °C resulting in the

formation monomers of the nanocrystal compounds [29]. If the concentration of theses

monomers is high enough to breach a nucleation threshold crystallization will occur and the

initial nuclei of the nanoparticles will form. As these nuclei form the concentration of the initial

monomers begin to drop significantly. Under ideal conditions concentrations will fall below the

nucleation threshold. Once the nucleation phase of the reaction is complete the remaining

monomers react with the existing nuclei resulting in growth of the nanoparticles. Proper control

of the synthesis conditions can result in a monodisperse distribution of particles with standard

deviations of less than 5% without subsequent processing [30].

15

Figure 2-6: Schematics showing the nucleation and growth of colloidal nanocrystals in which various sizes of crystals can

be isolated at different times. (b) Simple synthetic apparatus showing the preparation of CQDs. [Reprinted with

Permission][27].

Variation of the temperature, precursor concentrations, and duration of the reaction can be used

to synthesis quantum dots of different sizes and thus different bandgaps. In Figure 2-7 we see

absorption spectra for different sized PbS quantum dots with diameters ranging from 3 to 10 nm

[31]. Using PbS nanocrystals gives access to bandgaps extending from the visible to the infrared

portions of the electromagnetic spectrum.

16

Figure 2-7: Size dependent absorption of PbS quantum dots. [Reprinted with Permission][31].

The quality of the quantum dot sample can be gauged by comparing absorption at both the

exciton peak and local minimum at slightly shorter wavelengths (Figure 2-8). This figure of

merit, hence forth called the peak-to-valley ratio, is a function of both homogeneous and

inhomogeneous broadening [32][33][34] where the inhomogeneous broadening is influenced by

polydispersity in both quantum dot size and shape. An important optical characteristic of

quantum dots is the common presence of a Stokes Shift where emission occurs at longer

wavelengths than absorption [27][35]. The origin of Stokes Shift in quantum dots is often

attributed to the presence of a dark exciton state [36][37], although experimental values for

Stokes Shift energies, such as those reported in Table 7-1, are significantly larger than those

predicted by theory from Coulombic and exchange interactions [38][39]. This matter will be

covered in more detail in Chapter 7 of this work.

17

Figure 2-8: Absorption and ohotoluminescence spectra for PbS quantum dots with a 950 nm exciton. The quality of the

quantum dot sample can be quantified by comparing absorption at the exciton peak and the local minimum. The energy

difference between absorption and photoluminesence peaks is the Stokes Shift.

2.2.2 PbS CQD Optoelectronics

Over the past several years the Sargent group has focused much of its research efforts on the design and

characterization of PbS quantum dot based optoelectronics. This started out in the development of PbS

photoconductors and photodiodes. Eventually this research was expanded into the design and fabrication

of photovoltaics.

a. Photodetectors

The bandgap tunability of PbS quantum dots allowed for the engineering of solution processed

photodetectors sensitive to specific regions of the electromagnetic spectrum, particularly the near infrared.

By using solid state treatments with dithiol passivating ligands, photodiodes were fabricated with

quantum dots with bandgaps of ~1500 nm [40].

ΔStokes

18

Overall a specific detectivity (D* ) of 1011 Jones was obtained with a bandwidth of 20 kHz (Figure 2-9).

Specific detectivity is defined by:

2-7)

where Δf is the bandwidth of the detector, A is the device area, and NEP is the Noise Equivalent Power.

The NEP is the optical power where the photodetector signal is equivalent to the noise floor.

Figure 2-9: a) A schematic representation of a PbS photodiode device. b) Frequency dependence of photocurrent at zero

bias. [Adapted and reprinted with Permission][40].

19

b. Schottky CQD Photovoltaics

The first demonstrated CQD photovoltaics were fabricated using Schottky architectures

[41][42][43]. The advantage of a Schottky architecture is its relative simplicity. The

semiconductor absorber material is sandwiched between two asymmetric contacts. On one side

an Ohmic contact collects the majority carrier whereas on the other side a Schottky junction

creates a built-in field which drives charge separation and collection of the minority carrier. The

early Schottky architectures provided an excellent proving ground for optimization and

demonstrating the photovoltaic properties of CQD solids. By treating the quantum dot films with

ethanedithiol or butylamine it was possible to passivate surface defects and bring the quantum

dots closer together into a much more conductive film. With these advances AM1.5 photovoltaic

performance eventually achieved 1.8% [44] (Figure 2-10).

20

Figure 2-10 a) Schematic of the Schottky device architecture consisted of PbS CQDs as active material and Al as metal

contact. The inset shows the electron micrograph of PbS film after n-butylamine exchange. b) Energy band diagram

showing the band bending takes place at Al/PbS interface. Under illumination, electrons and holes are swept away by the

built-in electric field in the depletion layer. c) Current–voltage data of the device influenced by the variation of the

simulated solar illumination source. The device shows AM1.5 PCE of 1.8%. d) EQE spectra for devices using PbS

quantum dots having different excitonic peak. [Adapted and reprinted with Permission][44].

c. Heterojunction PbS CQD Photovoltaics

Although the initial Schottky architectures demonstrated very promising results, new

architectures were also being explored in an attempt to greatly enhance performance. One of the

primary limitations of Schottky solar cells is that the built in field is limited to half the bandgap

[45]. This places a limit both on open circuit voltage, VOC and the depletion region thickness for

charge collection. Another limiting factor was that for these p-type materials the transparent

conductive oxide was used to form the Ohmic contact at the front of the cell. Low work function

21

metals would then form both a Schottky and reflecting contact at the back of the cell. This

proved to be limiting as most of the photocarriers are generated furthest from the Schottky

junction where the built-in field is strongest and charge extraction is most efficient. Inspired by

the potential of work conducted on Dye-Sensitized Solar Cells in recent years work began on

using PbS quantum dots as the absorber in this architecture. Contrary to the initial objectives the

best devices were obtained without reliance on infiltration of the n-type TiO2 window layer with

PbS quantum dots. Instead a depleted N-p heterojunction architecture was designed with PbS

CQD films deposited directly on TiO2 electrodes. The highly n-doped TiO2 window layers

allowed for depletion regions several hundred nanometers thick extending throughout the entire

thickness of the devices at zero bias conditions. Unlike the Schottky architecture illumination

occurred on the side of the minority collecting TiO2-PbS interface allowing for more efficient

current extraction. In the move towards the depleted heterojunction design the solid state

treatment was changed from short dithiols such as ethanedithiol to mercaptopropionic acid

(MPA) a bidendate ligand containing both thiol and carboxylic acid functional groups. The new

MPA treatment proved to greatly improve film conductivity and led to significant enhancement

in device performance. Overall AM1.5 performance efficiencies of 5.1% [46] were obtained, a

near three fold improvement to the Schottky devices developed 2 years earlier (Figure 2-11).

22

Figure 2-11: a) The depleted heterojunction architecture showing the various components of the device (FTO/porous

TiO2/PbS QD/Au) along with the band diagram close to maximum VOC. EF,n and EF,p are the electron and hole quasi-

Fermi levels; Ec and Ev are the conduction and valence band edges; Jp, PV and Jn,PV are the hole and electron

photocurrents (and are equal at steady-state); Jp,fwd is the hole current in the forward bias direction. The Fermi level is

shown as a dashed line. b) Apertured dark and illuminated current density-Voltage (J-V) response of the photovoltaic

device c) EQE and absorption spectra of the photovoltaic device. [Adapted and reprinted with permission][46].

23

2.3 Conclusions

Device performance efficiencies of 5.1% proved to be a major advancement over the sub-percent

values obtained just several years earlier. These advancements have mainly come from

improvements to the device architecture and better surface passivation of the quantum dot

absorber. Despite this progress, performance is still well below the theoretical maximum of 31%

predicted by the Shockley-Queisser limit. Further improvements must be made through reduced

recombination by improved defect passivation at the heterojunction interface and on the quantum

dot surface in the active absorber.

24

Chapter 3 Early Studies: Core-Shell Quantum Dots

3.1 Introduction

In the previous chapter we established that CQDs were particularly interesting for use in

optoelectronics due to their combination of bandgap tunability and solution processibility. Each

of these features of CQDs is a direct consequence of the nanoscopic dimensions of these crystals.

This produces a high surface-to-volume ratio, increasing the potential for recombination losses in

solar cells based on these materials: unpassivated surface states or dangling bonds may serve as

recombination centers for photogenerated carriers, limiting carrier lifetimes and restricting the

efficiency of charge transport and the extent of Fermi-Level splitting.

Mitigating these challenges requires a strategy to engineer surface passivation, removing surface

defects while maintaining the desirable properties that make CQDs so appealing.

Core-shell nanostructures may serve as a possible remedy to this problem. Growing

semiconductor shells on nanocrystal cores builds off the field of epitaxial growth of

semiconductors [47]. Previous reports on CQDs have shown that implementing a core-shell

structure can significantly improve photoluminescent quantum yield by reduction of the non-

radiative recombination. Additionally, a shell may serve to protect against photo-oxidation

effects by acting as a barrier between photogenerated carriers and the external environment.

In this chapter, the early studies of the application of the core-shell quantum dot concept to CQD

solar cells are reported. Initially an analysis of the various options for core-shell materials, based

on heterojunction type selection is described. We then report the development of the synthesis of

these nanoparticles. We build devices and discover some of the important practical challenges

that limit the use of established core-shell synthesis methods in CQD photovoltaics. These

studies pave the way for the new materials processing avenues explored in Chapter 4 and

beyond, approaches that overcome the limitations of the traditional core-shell approach.

25

3.2 Core-Shell Heterojunction Strategies

There exist four different classes of core-shell structures, which are differentiated by the band-

alignment between the core and shell materials (Figure 3-1). In type-I, both the electron and the

hole are confined to the core. In reverse type-I, both the electron and hole are confined to the

shell. In type-II heterostructures, the electron and hole are localized to different regions (one in

the shell, the other in the core).

Figure 3-1: Schematic representation of the energy-level alignment in different core/shell systems realized in

semiconductor NCs to date. [Reprinted with Permission][47].

After photogeneration electrons and holes will relax to the lowest lying bandedge. In the case of

both Type-II heterostructures, along with the Reverse Type-I heterostructure, the total energy of

the electron-hole pair system will be reduced. Only in the case of the Type-I heterostructure is

the energy of the absorbed photocarriers preserved after initial relaxation. If shell thicknesses can

be made thin enough charge transport can be still be efficient and the improved surface

passivation of the nanocrystals from the shell may reduce recombination losses from surface

defects. Thus we desire a Type-I heterostructure design to reduce carrier losses from surface

recombination, protects the core from photo-oxidation, and maintains energy of the energy of the

photogenerated electron-hole pair.

Type-I Core-shell quantum dots may be synthesized by injecting the precursors for the shell

material after initial synthesis of the core nanocrystals. This technique has been used to synthesis

core-shell structures such as CdSe-ZnS [48]. One of the challenges with this technique is the

26

potential that a heterogeneous solution may be produced consisting of two different nanocrystal

materials instead of the desired core-shell structure.

Another possibility is to make use of a cation exchange [49]. After the initial synthesis of the

core nanocrystals the desired cation is added at elevated temperatures. Under the right conditions

a cation exchange may occur on the surface of the quantum dots where the original cation of the

core is replaced, resulting in a core-shell structure. This technique has been utilized to produce

PbSe-CdSe core shell structures (Figure 3-2). Here the elevated temperatures and heavy excess

of Cd-oleate lead to favourable conditions for replacement of Pb with Cd atoms.

Figure 3-2 TEM imaging of PbSe-CdSe core-shell nanocrystals. Inset: Individual core-shell quantum dot. [Reprinted with

Permission][49].

Here we utilize this approach for our PbS quantum dots. Based on the varying reports of the

electron affinity for CdS [47][50] we predict that PbS-CdS may form either a type-II or Type-I

heterostructure.

27

3.3 PbS-CdS Core-Shell Synthesis and Characterization

The synthesis of PbS nanocrystals was performed using the standard air-free Schlenk-line

technique based on the variation of a previously reported method (Figure 3-3) [35]. A stock

solution of lead oleate was prepared by pumping a mixture of 4.0 mmol of PbO (0.45g), 4.8

mmol of oleic acid (1.34g ), and 56.2 mmol of 1-octadecene (14.2g) at 95 °C under vacuum for

16 h. The sulfur precursor was made by mixing 1.0 mmol bis(trimethylsilyl)sulfide (0.18g) with

10 mL of 1-octadecene in a nitrogen-filled glovebox. The stock solution was stirred vigorously

while being heated to 120 °C under argon in a three-neck flask equipped with a thermocouple.

The sulfur precursor was swiftly injected into the flask. The solution turned brown immediately

after injection and the reaction was quenched to 36 °C. To isolate the nanocrystals, 50 mL of

anhydrous acetone was injected into the flask and the dispersion was centrifuged. After removing

the supernatant, the mixture was re-dispersed in toluene and then re-precipitated again with the

addition of 20 mL acetone. The final PbS nanocrystals, with an average diameter of ~3.5 nm

was redispersed in toluene (~200 mg mL-1

).

The Cd-oleate solution was synthesized by adding 2.0g of CdO, 12.0 mL of oleic acid, and 32.0

mL of diphenyl ether in a three neck flask and then heated under N2 to 255 °C (Figure 3-3). As

the boiling point of the diphenyl ether is 258 °C we require the use of a water chilled condenser

for the recondensing diphenyl ether vapor. As the reaction proceeds the cadmium oxide reacts

with the oleic acid forming the Cd-oleate complex and water.

To complete the Cd-oleate exchange occurs by adding 10 mL of PbS quantum dots in toluene

(15 mg mL-1

) to a three neck flask under N2 (Figure 3-3). The solution was heated to 90°C

before injection of 22 mL of Cd-oleate solution. Isolation of the sample was completed by

adding 5 mL of the CQD solution to a mixture of 3 mL chloroform, 3 mL methanol, and 3 mL

acetone and then centrifugation at 1500 rpm for several minutes. The addition of chloroform is

specifically important for proper removal of diphenyl ether from the Cd-oleate complex. Finally

28

the resulting precipitate was dried under N2 and the final product was redispersed in 2 mL

toluene.

Figure 3-3: Synthesis of core-shell PbS-CdS quantum dots by cation exchange. The PbS cores are synthesized and then

purified. Formation of the core-shell dots is accomplished by reaction with the Cd-oleate complex.

To gauge the success of the exchange we measured absorption on the samples throughout the

exchange (Figure 3-4a). As expected, implementation of the core-shell structure leads to a blue

shift in the absorption spectra as the core size is reduced from cation exchange at the surface.

Over the course of an hour the exciton position shifts 160 nm from 1250 nm to 1090 nm with

much of the shift occurring over the first 10 minutes. During this phase of the reaction excess

cadmium atoms have easy access to lead atoms on the surface of the dot. As the reaction

proceeds it becomes more difficult for cadmium and lead atoms to diffuse between the core and

the outer environment. As a result, after 30 minutes there is very little change in the absorption

spectra and the reaction appears to be self-limiting. It is also clear that as this blue shift proceeds,

the exciton becomes less well-defined. This is evident if we compare the relative absorption at

the local minimum and maximum near the exciton as indicated in Figure 3-4a. If the cation

exchange occurs at different rates for different quantum dots due to local fluctuations in reagent

29

concentrations initial polydispersity in size of surface faceting the polydispersity of the final

product may be made worse by the cation exchange treatment.

Photoluminescence (PL) measurements were conducted on these samples (Figure 3-4b). A 633

nm HeNe laser was used to excite the sample and photoluminescent spectrum was recorded

using an InGaAs (Ocean Optics NIR-512) detector. Absorption of the solutions was carefully

matched at the excitation wavelength by varying the concentration of the sample to ensure

deviations of no more than 10-20% in optical density. It is clear that the Cd cation treatment has

a profound effect on the photoluminescent spectra. With initial treatment the photoluminescent

intensity and therefore PLQY is greatly enhanced as the outer surface becomes passivated with

Cd atoms and a PbS-CdS core-shell structure is formed. Over time there is a loss in PL intensity

which coincides with a loss in exciton definition. It is likely that for these thicker shells the

greater degree of Cd penetration into the PbS core creates surface defects which may begin to act

as recombination centers that quench PL.

Figure 3-4: a) Absorption spectra for Cd-Oleate treated PbS quantum dots. b) Photoluminescence spectra for Cd-Oleate

treated PbS quantum dots.

30

3.4 Pyridine Ligand Exchange

Development of core-shell quantum dots is only the first step in fabrication of high efficiency

CQD photovoltaics. A pathway to densely packed, highly conductive quantum dot films is also

necessary. To accomplish this we set out to introduce a solution phase ligand exchange on the

core-shell nanocrystals from the long chain oleic acid to short capping layers such as pyridine

(Figure 3-5). Pyridine is a much shorter capping molecule with a proven binding affinity with

cadmium and has been used previously for fabrication of CdSe nanocrystal films [51]. The short

pyridine capping ligand allows for fabrication of densely packed quantum dot solids. An

additional benefit is that due to the weak binding energy of the pyridine nitrogen bond to the

quantum dot surface there is a relatively low activation barrier for pyridine removal. Through

additional annealing strategies it may be possible to completely remove the organic pyridine and

yield an all inorganic CQD solid which maintains quantum confinement due to the protective

CdS shell.

Figure 3-5: Ligand exchange of oleic acid with pyridine.

To conduct the pyridine exchange 1 mL of core-shell quantum dots in toluene (150 mg mL-1

)

was isolated by adding 2 mL of the methanol and centrifugation at 1500 rpm for several minutes.

The precipitate was dried for several minutes under nitrogen before redispersal in 10 mL of

pyridine. The solution was injected into a three neck flask under nitrogen and stirred for 3 hours

at 90 °C. The quantum dots were then isolated by addition of hexane (4:1 hexane:pyridine) and

centrifugation. Finally the sample was then dried and redispersed in chloroform to achieve a final

concentration of ~70 mg mL-1

.

31

We measured the absorption of the exchanged dots with the original material (Figure 3-6a). The

effect of the pyridine treatment was a complete loss of the characteristic exciton. This was

interpreted as a consequence of the corrosive nature of pyridine, a base, which may etch or even

destroy the quantum dots.

We evaluated the success of the ligand exchange by using Fourier Transform Infrared (FTIR)

spectroscopy (Figure 3-6b) to look at absorption of vibrational modes for specific functional

groups found in the ligands featured in Figure 3-5. Thin films were prepared by spin casting the

pyridine exchanged quantum dots directly on KBr substrates. Results show that there is still a

significant amount of C-O and C=O vibrational features in the new material. This is highly

suggestive that significant oleic acid is still present and has not been completely exchanged by

pyridine.

Figure 3-6: a) Absorption measurements of PbS and pyridine exchanged PbS-CdS. b) Fourier Transform Infrared

(FTIR) spectroscopy of pyridine exchanged PbS-CdS The feature at 2360 cm-1

is CO2 absorption in the baseline

subtraction .

Pyridine is a neutral-donor (L-type) ligand [52] whereas oleic acid, when deprotonated, is a

charged, lattice terminating (X-type) ligand [52] . Displacement of oleic acid with pyridine thus

requires additional charge balancing to maintain overall charge neutrality. Charge neutrality can

be acheived by removal of surface atoms from the nanocrystal surface [52] or through charged

species, such as displaced oleic acid, in the colloidal solution. This need for charge neutrality

places a barrier preventing the desired ligand substitution from reaching completion.

32

We found that both the loss in the exciton and the incomplete success of the ligand exchange

would not help us reach our goals of a densely packed, monodispersed, conductive quantum dot

solid. Instead we explored a solution based ligand exchange with mercaptopropionic acid

(MPA) (Figure 3-7). Mercaptopropionic acid has consistently proved to be the best ligand for

fabrication of quantum dot photovoltaics. Since 2009 all device record efficiencies using PbS

quantum dots have incorporated MPA [46]. Additionally the thiol functional group is highly

reactive with metal atoms like lead and thus would greatly facilitate an exchange and removal of

oleic acid.

3.5 Mercaptopropionic Acid Exchange

To complete the exchange core-shell quantum dots diluted in toluene (50 mg mL-1

) were mixed

with equal amounts of MPA for ~2 hours. Afterwards hexane, a non-solvent, was added to the

solution and a precipitate immediately began to form. After the supernatant was removed,

tetrahydrofuran was added to redissolve any excess MPA in the remaining precipitate. The

suspension was centrifuged and the clear supernatant was removed. The final precipitate was

dried under vacuum and then finally redispersed in dimethylsulfoxide (DMSO). To check the

colloidal stability the resulting solution was passed through a 0.2 μm filter. All the solution was

able to pass through the filter without clogging indicating a good colloidal dispersion had been

formed. Due to the long alkyl chains oleic acid PbS quantum dots have only demonstrated

colloidal stability in non-polar solvents such as octane or toluene [35][53]. The fact that colloidal

solutions could be prepared in DMSO, a highly polar solvent, was an initial indication of a

successful exchange to MPA.

33

Figure 3-7: Ligand Exchange of Oleic Acid with Mercaptopropionic acid (MPA)

We examined the absorption spectra before and after the MPA exchange (Figure 3-8). Unlike

the previous pyridine exchange there is still a clear exciton feature present after the exchange. A

slight blue shift of the exciton appears to have occurred and is likely a result of minor etching

from the acidic environment of the MPA.

Figure 3-8: Absorption spectra for PbS-CdS and MPA exchanged PbS-CdS.

To further test the extent of the exchange solution Nuclear Magnetic Resonance (NMR)

spectroscopy was used to measure the ratio of oleic acid to MPA (Appendix A). Oleic acid has a

specific NMR feature due to the carbon double bond [54] (5.25-5.35 ppm) that is not present in

MPA. Similairly MPA which binds to the nanocrystal surface through the thiol functional group

shows a NMR signature of the carboxylic acid [54] (12.15 ppm) functional group. This feature is

absent for bound, and therefore deprotonated, oleic acid. This allows us to directly compare the

34

presence of MPA and oleic acid in the final product. Results indicate that the exchange is almost

entirely complete with ~1% of total organic species being oleic acid.

We performed TEM imaging of the MPA exchanged quantum dots and compared them with

some of the original oleic acid capped nanocrystals (Figure 3-9a). The TEM imaging showed

that MPA capped nanocrystals (Figure 3-9b) are much closer packed than those functionalized

with oleic acid thus indicating a high degree of success for this exchange.

Figure 3-9 TEM imaging of PbS-CdS and MPA exchanged PbS-CdS nanocrystals (Scale Bar=100 nm).

We attempted to make films for photovoltaic devices but found that the boiling point of the

DMSO was too high, and the evaporation rate too low, to make the optically dense films

necessary to make photovoltaic devices. To test the electrical properties of the core-shell

quantum dots we instead used a solid state treatment with MPA similar to those previously

reported [46]. Device data showed very little photovoltaic response with JSC values ~0.1 mA cm-

2. It was believed that the thick CdS shell was too thick for efficient charge carrier extraction.

35

3.6 Conclusion

In summary, we demonstrated the synthesis of PbS-CdS core-shell quantum dots. These

exhibited the enhanced photoluminescence characteristic of well-passivated quantum dots. We

attempted device fabrication, but found very low performance below 0.1%.

These early studies emphasized both the desired passivation-enhancing, but also the undesired

transport-blocking, role of shells. They motivated investigations of innovative strategies that

would overcome this compromise. We will see in the ensuing chapter that – fortunately – new

concepts inspired by the core-shell preliminary findings enabled a breakthrough in no-

compromise CQD passivation.

36

Chapter 4 The Advent of Atomic Ligand Passivation

4.1 Introduction

In the previous chapter we found that the thick CdS shell on the PbS dots was too insulating and

inhibited photocarrier transport and photovoltaic performance. Yet we also found that inorganic

strategies have much to offer in improving passivation.

This inspired a new concept: the idea that – perhaps – one of the constituents (the cations or

anions) employed in shell growth might be the prime agent benefiting passivation. If one of these

played a predominant role in improving trap state passivation, then perhaps a shell analogue –

yet less than a lattice constant thick – could confer the benefits of improved passivation with

little or no cost to transport.

In this chapter, we begin with a brief review of early studies along these lines. We then present a

detailed hypothesis as to how an atomic ligand passivation strategy could proceed. We develop

and optimize a new process that shows a dramatic improvement in photoluminescent quantum

yield. We deployed these new nanoparticles and demonstrated a dramatic improvement in solar

cell current and voltage. We conclude with an articulation of the path forward for further

improvements in CQD photovoltaic efficiency.

4.2 Motivation

Previous work exploring the air stability of PbS quantum dots demonstrated the importance of

surface composition on device performance [55]. Quantum dots were synthesized with a high

lead to sulfur composition (1.8:1) indicative of a stoichiometric core and highly lead rich surface.

The highly lead rich surface significantly reduced the formation of PbSO3 and PbSO4. Both these

species are believed to introduce recombination centers which are likely detrimental to

performance [56]. Having a cation rich surface is thus beneficial as it passivates exposed surface

states and reduces the probability of defect formation. Based on our work in the previous chapter

37

we attempted to develop a Cd cation treatment to passivate these exposed sulfur sites. The higher

binding energy of cadmium vs lead, which was responsible for the success of the cation

exchange process, should also lead to more complete cation coverage of the surface and better

protection from defect formation.

For this passivation strategy we wanted to form a cadmium protection layer on the outside

surface of the quantum dot and avoid the cation exchange process, which in the previous chapter

we demonstrated to be detrimental to performance. By using a different coordinating ligand for

the Cd precursor we hoped to develop a less reactive Cd reagent to prevent Pb removal through

cation exchange. For inspiration we looked to synthesis work on CdS nanocrystals [57]. Here

phosphonic acids are considered a staple coordinating ligand due to their strong binding energy

to Cd atoms and their ability to yield colloidally stable quantum dots in organic solvents. The

stronger binding energy of phosphonic acids compared to carboxylic acids [58] should help

reduce the reactivity of the Cd precursor.

4.3 Cation Passivation

The PbS quantum dots were synthesized according to procedures discussed earlier. Parameters

were established to yield excitons of ~950 nm [46]. After initial synthesis the quantum dots are

isolated in a nitrogen glovebox and redispersed in octane. Approximately 1 mL of PbS quantum

dots in toluene are added to a test tube with ~12 mL of methanol. Not only is methanol a non-

solvent for these quantum dots, it also has a strong affinity for oleic acid and thus allows for

removal of any excess capping ligand. The solution is centrifuged at 1500 rpm for 10-15 minutes

resulting in a dark PbS nanocrystal precipitate and a clear supernatant. The supernatant is

removed, and the quantum dots are dried under vacuum then redispersed in toluene. Another

isolation is performed and the dots are dried again for >1 hour to ensure full removal of any

excess solvent. The dry precipitate is finally redispersed in octane to a final concentration of 50

mg mL-1

.

A new cadmium complex was synthesized by mixing 2 mmol of CdCl2, with 0.4 mmol of

tetradecylphosphonic acid (TDPA) and 10 mL oleylamine (OLA) in a three neck flask attached

38

to a Schenk line. The solution is heated under N2 to 100°C and magnetically stirred for 30

minutes. Here CdCl2 acts at the Cd cation source, tetradecylphosphonic acid acts as the

coordinating ligand, and oleylamine as the coordinating solvent. In a typical treatment 12 mL of

the pre-synthesized PbS quantum dot solution is added to 24 mL of toluene in a three neck flask

and heated to 60 °C under N2. Finally ~4 mL of the Cd-TDPA-OLA complex is added and the

reaction is allowed to proceed for 5 minutes before quenching with ~40 mL of acetone at room

temperature. The quantum dot material is isolated by centrifugation and redispersed in toluene

to a concentration of 150 mg mL-1

. This isolation procedure is repeated an additional two times

before final dispersal in octane at a concentration of 50 mg mL-1

.

Absorption measurements before and after the Cd-TDPA-OLA treatment show a slight red shift

in the exciton position (Figure 4-1). This is in contrast to the blue shifts witnessed for the Cd-

oleate treatments described earlier. This suggests that this new Cd source is not displacing Pb

atoms on the surface. Rather it appears that the Cd is being added onto the surface of the

quantum dots, effectively increasing their size, resulting in the slight red shift of the exciton. As

indicated in T