CH3NH3PbBr3 Perovskite Solar Cells for
Tandem Application – Demonstrations and
Characterizations
Rui Sheng
A THESIS IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
School of Photovoltaic and Renewable Energy Engineering
Faculty of Engineering
The University of New South Wales
March 2017
1 Supervisor
Dr. Anita Ho-Baillie*, Prof. Martin Green*
Co-supervisor
Dr. Shujuan Huang*, Dr. Xiaojing Hao*
* School of Photovoltaic and Renewable Energy Engineering, University of
New South Wales, Sydney, Australi
2 THE UNIVERSITY OF NEW SOUTH WALES
Thesis/Dissertation Sheet
Surname or Family name: SHENG
First name: RUI Other name/s:
Abbreviation fordegree as given in the University calendar: Ph.D
School: SCHOOL OF PHOTOVOLTAIC AND Faculty: ENG [NEERING RENEWABLE ENERGY ENGINEERING
Title: CH3NH3Pb8r3 PEROVSKITE SOLAR CELLS FOR TANDEM APPLICATION - DEMONSTRATIONS AND CHARACTERZA TIONS
Abstract 350 words maximum:
Perovskite solar cells based on organometal halides have experienced an unprecedentedly rapid development since 2012, when the first efficient perovskite-based solar device with solid-state structure was reported. Increasing amount of research interests are focusing on characterizing material properties, developing deposition methods, resolving the instability and toxicity, as well as designing tandem structure.
This thesis will present a comprehensive study of CH3NH3PbBr3 perovskite-based solar cell, including a novel deposition method for high efficiency
CH3NH3Pb8r3 solar cells, which shows substantial improvement over conventional one-step solution process, not only in the power conversion efficiency
(1.7% vs 9.1 %), but also in hysteresis. In the material characterization section, a study using one-photon and two-photon microscopy was conducted to reveal additional insight in the understanding of grain formation and carrier extraction. Then an investigation on fluorescence blinking in vapour assisted deposited film was performed to study charge accumulation and migration. In the last part of this section, a dynamic aging study of vapour assisted deposited film was carried out to demonstrate the spontaneously grain growing and defect generation. In this study, FLIM (Fluoresc�nce lifetime imaging microscopy) was used to reveal the carrier lifetime of deposited filmin a larger scale. The final aim of this thesis is to demonstrate the potential of using CH3NH3Pb8r3 for tandem application. Therefore, a four-terminal tandem structure was demonstrated using spectrum-splitting approach. In this work, a great potential was shown when CH3NH3Pb8r3 cell is coupled with a CH3NH3Pbh cell; when a CH3NH3PbBr3 cell is coupled
with a high efficiency PERL (passivated emitter rear locally diffused) silicon solar cell; and when a CH3NH3PbBr3 cell is coupled with a commercial screen printed silicon solar cell demonstrating power conversion efficiencies at 13.4%, 23.4% and 18.8% respectively. The last section of this thesis demonstrated a 2-terminal monolithic ITO/compact TiOJmesoporous TiO,/CH3NH3PbI,/Spiro-OMeTAD/PEDOT: PSS/C60/CH3NH3PbBr,/Spiro
OMeTAD/Au solar cell by developing a novel composite carrier recombination stack which protects the underlying sub-cell and provides an
interconnection with matching working functions. Remarkable voltage output at 1.96 V was obtained with the designed structure. In addition,
simulation work shows the potential of this structure with further performanceimprovement to be expected.
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I
ACKNOWLEDGEMENTS
My most humble and sincere thanks to:
First and foremost, my supervisors Prof. Martin Green, Dr. Anita Ho-Baillie, Dr.
Shujuan Huang and Dr. Xiaojing Hao, for their great inspiration, valuable guidance and insightful suggestions. Also A/Prof. Ashraf Uddin, for his help that led me to do the
PhD. Additionally, Prof. Henry Snaith, for his talent and endless good humor.
Dr. Mark Keevers, for his knowledge on spectrum splitting systems. Dr. Xiaoming
Wen, for his expertise on fluorescence characterization and photophysics. Especially for the initial discussion on one-photon and two-photon work, and the optical measurements he did in Chapter 4.
My irreplaceable friend Dr. Jessica Jiang, for her great help on optical simulation and her pitch-perfect instincts on all matters, large and small, as well as her friendship.
The entire perovskite group in UNSW, for their enthusiasm, creativity, support and friendly ambiance in the group. With special thanks to Qingshan Ma, Arman Soufiani and Dr. Sanghun Woo.
The awesome Snaithlings at Oxford University, for their brilliant ideas, lovely lunch and coffee breaks and scientific discussions.
My great friends in Monash University, Dr. Manda Xiao, Dr, Feng Li, Dr. Jiangjing
He, Dr. Wenchao Huang, Prof. Yong Peng and Prof. Fuzhi Huang, for their help during my visit in Monash-Perovskite group.
My dear friends in China, Australia and UK, Boqi Wu, Xue Han, Geng Wang, Kan
Gao; Ran Chen, Alex Li, Alex Chan, Li Wang, Lu Wang, Boon Ng, Jeanne Han,
Johnny Fan; Rebecca Sutton, Maximilian Hoerantner, Martina Congiu, Nakita Noel,
1 Zhiping Wang, Sai Bai and Mingze Gao, for their occasional hangout and wonderful trips and extraordinary cooking skill.
My exceptional housemates Lin Yuan and Zijue Xu, for their five years of company, patience and tolerance.
Last but not least, my loving parents and all my family, for their support and encouragement in my life.
2 ABSTRACT
Perovskite solar cells based on organometal halides have experienced an unprecedentedly rapid development since 2012, when the first efficient perovskite- based solar device with solid-state structure was reported. Increasing amount of research interests are focusing on characterizing material properties, developing deposition methods, resolving the instability and toxicity, as well as designing tandem structure.
This thesis will present a comprehensive study of CH3NH3PbBr3 perovskite-based solar cell, including a novel deposition method for high efficiency CH3NH3PbBr3 solar cells, which shows substantial improvement over conventional one-step solution process, not only in the power conversion efficiency (1.7% vs 9.1%), but also in hysteresis. In the material characterization section, a study using one-photon and two- photon microscopy was conducted to reveal additional insight in the understanding of grain formation and carrier extraction. Then an investigation on fluorescence blinking in vapour-assisted deposited film was performed to study charge accumulation and migration. In the last part of this section, a dynamic aging study of vapour-assisted deposited film was carried out to demonstrate the spontaneously grain growing and defect generation. In this study, FLIM (Fluorescence lifetime imaging microscopy) was used to reveal the carrier lifetime of deposited film in a larger scale. The final aim of this thesis is to demonstrate the potential of using CH3NH3PbBr3 for tandem application.
Therefore, a four-terminal tandem structure was demonstrated using spectrum-splitting approach. In this work, a great potential was shown when CH3NH3PbBr3 cell is coupled with a CH3NH3PbI3 cell; when a CH3NH3PbBr3 cell is coupled with a high efficiency
PERL (passivated emitter rear locally diffused) silicon solar cell; and when a
CH3NH3PbBr3 cell is coupled with a commercial screen printed silicon solar cell demonstrating power conversion efficiencies at 13.4%, 23.4% and 18.8% respectively.
3 The last section of this thesis demonstrated a 2-terminal monolithic FTO/compact
TiO2/mesoporous TiO2/CH3NH3PbI3/Spiro-OMeTAD/PEDOT: PSS/C60/CH3NH3PbBr3/
Spiro-OMeTAD/Au solar cell by developing a novel composite carrier recombination stack which protects the underlying sub-cell and provides an interconnection with matching working functions. Remarkable voltage output at 1.96 V was obtained with the designed structure. In addition, simulation work shows the potential of this structure with further performance improvement to be expected.
4 TABLE OF CONTENTS
ACKNOWLEDGEMENTS ...... 1
ABSTRACT ...... 3
TABLE OF CONTENTS ...... 5
LIST OF FIGURES ...... 9
LIST OF TABLES ...... 14
1. INTRODUCTION ...... 15
1.1. Motivation ...... 16
1.2. Aims and Significance ...... 17
1.3. Thesis Outline ...... 18
2. LITERATURE REVIEW ...... 23
2.1. History and Development ...... 23
2.2. Material Properties ...... 26
2.3. Deposition Method ...... 28
2.3.1 One-step solution process ...... 29
2.3.2 Sequential process ...... 30
2.3.3 Dual-source evaporation method ...... 31
2.3.4 Vapour-assisted solution process (VASP) ...... 31
2.3.5 Other deposition methods ...... 32
5 2.4. Photovoltaic Device Structures ...... 33
2.4.1 Mesoporous structure ...... 33
2.4.2 Inverted structure ...... 35
2.5. Tandem Application ...... 36
2.5.1 Four-terminal tandem structure ...... 37
2.5.2 Two-terminal monolithic tandem structure ...... 38
2.6. Challenges and Perspectives ...... 40
2.6.1 Hysteresis ...... 40
2.6.2 Toxicity ...... 41
2.6.3 Stability ...... 41
3. METHYLAMMONIUM LEAD BROMIDE PEROVSKITE- BASED SOLAR CELLS BY VAPOUR-ASSISTED DEPOSITION 56
3.1. Introduction ...... 56
3.2. Experimental details ...... 57
3.3. Results and Discussion...... 60
3.3.1 Film properties of VASP and one-step solution process ...... 60
3.3.2 Photovoltaic devices fabricated by VASP and one-step solution
process 69
3.4. Conclusion ...... 74
4. MATERIAL CHARACTERIZATION FOR CH3NH3PBIXCL3-X AND CH3NH3PBBR3 PEROVSKITE ...... 80
6 4.1. MORPHOLOGY AND CARRIER EXTRACTION STUDY OF
CH3NH3PbIXCl3-X PEROVSKITE BY ONE- AND TWO-PHOTON
FLUORESCENCE MICROSCOPY ...... 80
4.1.1 Introduction ...... 81
4.1.2 Results and Discussion ...... 83
4.1.3 Conclusion ...... 90
4.2. Grain Formation and Mobile Charge Induced Fluorescence Intermittency
in vapour-assisted CH3NH3PbBr3 Perovskite films ...... 91
4.2.1 Introduction ...... 91
4.2.2 Results and Discussion ...... 92
4.2.3 Conclusion ...... 100
4.3. Photoluminescence Characterizations of Dynamic Aging Process of
Organic-inorganic CH3NH3PbBr3 Perovskite ...... 101
4.3.1 Introduction ...... 101
4.3.2 Experimental Detail ...... 102
4.3.3 Results and Discussion ...... 103
4.3.4 Conclusion ...... 114
5. 4-TERMINAL TANDEM SOLAR CELLS USING CH3NH3PBBR3 BY SPECTRUM SPLITTING ...... 129
5.1. Introduction ...... 129
5.2. Results and Discussion ...... 130
5.3. Conclusion ...... 140
7 6. MONOLITHIC PEROVSKITE/ PEROVSKITE TANDEM SOLAR CELLS WITH ORGANIC RECOMBINATION LAYER 145
6.1. Introduction ...... 145
6.2. Experimental Details ...... 146
6.3. Results and Discussion...... 148
6.4. Conclusion ...... 156
7. CONCLUSION ...... 162
7.1. Discussions...... 162
7.2. Novelty and contribution ...... 164
7.2.1 Novelty of the thesis ...... 164
7.2.2 Contribution of the author ...... 165
7.3. Perspectives of future research ...... 166
APPENDIX ...... 168
8 LIST OF FIGURES
Figure 2.2.1 AMX3 perovskite structure ...... 26
Figure 2.3.1 Schematic diagram of one-step spin-coating deposition method ...... 29
Figure 2.3.2 Schematic diagram of sequential deposition method ...... 30
Figure 2.3.3 Schematic diagram of dual-source evaporation deposition method ...... 31
Figure 2.3.4 Schematic diagram of vapour-assisted solution process ...... 32
Figure 2.4.1 Architecture schematic diagram of mesoporous structure perovskite Planar
structure ...... 34
Figure 2.4.2 Architecture schematic of planar structure perovskite solar cells ...... 34
Figure 2.4.3 Architecture schematic of inverted structured perovskite solar cells ...... 35
Figure 2.5.1 Architecture schematic of four-terminal tandem solar cell ...... 38
Figure 2.5.2 Architecture schematic of four-terminal tandem solar cells using spectrum
splitting ...... 38
Figure 2.5.3 Architecture schematic of monolithic tandem perovskite-based solar cells
...... 39
Figure 3.3.1(a) XRD pattern of CH3NH3PbBr3 film deposited by VASP method
compared with that of CH3NH3Br and PbBr2 films as references. (b) SEM top
view of VASP deposited CH3NH3PbBr3 film ...... 61
Figure 3.3.2(a) SEM top view of CH3NH3PbBr3 film prepared by one-step solution-
based process, (b) XRD patterns of CH3NH3Br (MABr), one-step solution
processed and VASP deposited CH3NH3PbBr3 (MAPbBr3). (c) Absorption
coefficient of the two CH3NH3PbBr3 films ...... 63
9 Figure 3.3.3 (a) Absorption coefficient (black) and PL spectrum (red) of the
CH3NH3PbBr3 film. (b) tr-PL taken at emission wavelength of 536nm±10nm for
samples with and without quenchers: glass/CH3NH3PbBr3 (black), glass/c-
TiO2/CH3NH3PbBr3 (blue) and glass/CH3NH3PbBr3 /spiro-OMeTAD (red). The
solid lines represent the stretched exponential fit for glass/ CH3NH3PbBr3 PL data
and diffusion model fits for the CH3NH3PbBr3 in the presence of quenchers...... 68
Figure 3.3.4(a) SEM cross-sectional image (b) external quantum efficiency of a
complete device...... 70
Figure 3.3.5 Device SEM cross-sectional view of complete device using (a) one-step
solution based method and (b) VASP deposition method...... 71
Figure 3.3.6 Current density-voltage curves measured in opposite sweeping directions
at a rate of (a) 3V/s, (b) 0.15V/s...... 72
Figure 3.3.7 Current density-voltage curve of CH3NH3PbBr3 devices by one-step
solution process at scan rate of (a) 3V/s (b) 0.15V/s ...... 73
Figure 4.1.1 Illustration of one- and two- photon excitation fluorescence imaging.20
Adapted by permission from Macmillan Publishers Ltd., Scientific Reports,
copyright 2013...... 83
Figure 4.1.2(a) The PL spectra of the CH3NH3PbIXCl3-X perovskite measured by a Si-
CCD-spectrometer (one-photon, blue) with 405 nm excitation; and measured by a
Leica TCS SP5 microscope with an excitation of 488 nm (one-photon, black) and
950 nm (two-photon, red). (b) The PL decay traces of CH3NH3PbIXCl3-X
perovskite (blue), perovskite/PCBM (black) and perovskite/spiro-OMeTAD (red)
from a 50 x 50 µm2 region excited by 470 nm 5 MHz pulsed laser...... 84
10 Figure 4.1.3 One-photon (1P; 488 nm excitation) and two-photon (2P; 950 nm
excitation) fluorescence imaging with 750/40 nm band-pass filter, (a) 1P and (b)
2P images of perovskite/glass; (c) 1P and (d) 2P fluorescence images of
PCBM/perovskite/glass. (e) 1P and (f) 2P fluorescence images of spiro-OMeTD
/perovskite/glass...... 87
Figure 4.1.4(a) One- photon and (b) two-photon fluorescence images of epoxy/higher
concentration spiro-OMeTAD/perovskite/glass...... 88
Figure 4.1.5 The PL decay traces of “bright” and “dark” regions in CH3NH3PbIXCl3-X
perovskite covered by PCBM and Spiro-OMeTAD ...... 90
Figure 4.2.1 SEM images of (a) the top surface and (b) the cross section of vapour-
assisted deposited CH3NH3PbBr3 film, (c) XRD pattern and (d) absorption
coefficient and the photoluminescence (PL) spectrum with a peak at 536 nm of
the CH3NH3PbBr3 film...... 93
Figure 4.2.2(a) SEM and (b) fluorescence image of vapour-assisted deposited
CH3NH3PbBr3 film, (c) and (d) SEM images of isolated CH3NH3PbBr3
nanoparticles...... 93
Figure 4.2.3 Top surface SEM images of vapour-assisted CH3NH3PbBr3 films with
different coverage...... 94
Figure 4.2.4 Spectroscopy images with time stamp ...... 95
Figure 4.2.5 Schematic diagram of blinking process in vapour-assisted CH3NH3PbBr3
film...... 95
Figure 4.2.6 Time traces of the fluorescence intensity of (a) an isolated nanoparticle at
different excitation intensities; and (b) a single point of the CH3NH3PbBr3 film.
11 Fluorescence microscopy images of (c) isolated nanoparticles and (d) the
CH3NH3PbBr3 film...... 97
Figure 4.3.1 SEM top-view images of CH3NH3PbBr3 films (a) when it is freshly made
(b) after 2 weeks of storage in N2, (c) after 1 day; (d) 2 days; (e) 1 week; (f) 2
weeks of aging in air where relatively humidity RH=50% to 60% and temperature
T = 25 ºC. (g) Histogram of grain size at different stages...... 104
Figure 4.3.2 XRD patterns of (a) freshly made and N2 stored (2 weeks) samples, and (b)
air stored samples. (c) Steady-state PL of the CH3NH3PbBr3 film at different
aging stages. (d) PL decay traces and (e) Change in PL intensity and PL lifetime
for the films over time...... 107
Figure 4.3.3 Fluorescence image of (a) N2 stored (2 weeks) and (b) air stored (2 weeks)
CH3NH3PbBr3 film...... 108
Figure 4.3.4 FLIM images of (a) N2 stored and (b) aged (2 weeks) perovskites. The
scale bar is in nanoseconds; (c) PL decay traces of the same N2 stored and aged (2
weeks) samples averaged over a 20×20 μm2 area, and (d) the PL decay traces
from a small, bright point and a large, dark point from Figure 4.3.4 (b)...... 111
Figure 4.3.5 PL intensity as a function of excitation intensity for (a) N2 stored and (b)
aged (2 weeks) perovskite samples. Both figures are plotted on logarithmic scales
so that the power factors can be obtained by linear fitting in the low excitation
range...... 113
Figure 5.2.1(a) A schematic of the spectrum splitting system set up and (b) a photo of
the set up inside the enclosure for the measurement of light current-voltage
characteristics. (c) Measured transmittance and reflection and calculated
absorption of the FELH0550 longpass filter at an incident of 45 degrees...... 132
12 Figure 5.2.2(a) Si PERL (b) multi-crystalline SP Si solar cells used in the spectrum
splitting systems...... 132
Figure 5.2.3(a) J-V curves and (b) EQE of the CH3NH3PbBr3 (green); CH3NH3PbI3
(yellow); Si PERL (grey); and SP Si cells (violet) measured before and after
spectral splitting...... 134
Figure 5.2.4 Hysteresis of (a) iodine cell without filter, (b) iodine cell with filter, (c)
bromide cell without filter, (d) bromide cell with filter, ...... 135
Figure 6.3.1(a) Schematic energy band diagram and (b) device architecture of
perovskite/ perovskite tandem structure...... 149
Figure 6.3.2(a) Cross-sectional SEM image of tandem cell and (b) JV curves for
individual cell and monolithic tandem. (c) Stabilized efficiency and current output
of tandem cell...... 152
Figure 6.3.3 Hysteresis of monolithic tandem solar cell ...... 152
Figure 6.3.4 Measured and modelled transmittance of the MAPbBr3 cell and
recombination layer...... 153
Figure 6.3.5(a) measured EQE curve without light bias (b) simulated EQE, and (c)
EQE of optimized cell architecture...... 155
13 LIST OF TABLES
Table 3.3.1 Experimentally determined diffusion coefficient (D) and diffusion lengths
(LD) of CH3NH3PbBr3 film in this work...... 68
Table 3.3.2 Electrical characteristics of the same device as shown in Figure 3.3.6
measured under different scan speeds and sweeping directions...... 72
Table 3.3.3 Electrical characteristics of the same device as shown in Figure 3.3.6 and
3.3.7 measured in opposite sweeping directions...... 73
Table 5.2.1 Output parameters of the spectral splitting systems using CH3NH3PbBr3 132
Table 5.2.2 Hysteresis of CH3NH3PbBr3 cell before and after spectrum splitting...... 135
Table 5.2.3 Hysteresis of CH3NH3PbI3 cell before and after spectrum splitting...... 136
Table 5.2.4 Performance of CH3NH3PbI3/Si cell via various spectral splitting
arrangements ...... 138
Table 6.3.1 Photovoltaic performance of single junction and tandem cell...... 152
14 1. Introduction
Energy is one of essential elements for modernization and urbanization. The increase in energy demands entails the depletion of world fossil fuel resources.
Moreover, the consumption of fossil energy has a detrimental side effect which is the global warming that is caused by the enormous level of greenhouse gas emission.
Meanwhile, the energy resources crisis has also put a lot of pressure on peace and development. Therefore, substantial scientific researchers have been focusing on developing a greener and more sustainable energy to alleviate the environmental impacts. Photovoltaic (PV) is a technology that uses semiconducting materials that exhibit photovoltaic effects and convert sunlight into usable electricity. Of all the alternative energy sources, PV is particularly the most promising as the solar power is pollution-free during daily consumption and the wastes and emissions resulted from the productions have been deemed to be manageable using the existing pollution controls.
End-of-use recycling technologies are however under development1. Furthermore, the recent development of mainstream silicon solar cell technology has successfully reduced its manufacturing cost significantly opening up the feasibility of the sustainable reliance on using the cost-effective photovoltaic devices in our daily activities.
The progressive solar power utilization requires not only low-cost solar cells, but also PV devices to convert sunlight in a more efficient manner than the current-state-of- the art. i.e. silicon solar panels which dominate the market. However, these highly efficient devices often require cost-intensive purification and fabrication processes leading to lower return on investment. A new generation of mixed organic-inorganic halide perovskite emerged to prominence in the past few years2-6. Compare to conventional silicon solar cell, perovskite-based solar cell has higher energy return on investment due to low material utilization and ability to solution process7. Perovskite
15 has been deemed to have advantageous electrical8, structural9,10, optical11,12 and ferroelectric13 properties which makes it to be an excellent photovoltaic material.
Additionally, the ability to capitalize on the know-how on dye-sensitized solar cells
(DSSC) and organic solar cells (OPV) opens up the possibility to produce a cost- effective while highly efficient perovskite solar cell.
1.1. Motivation
Intensive research studies have been reported on the preparation of iodine
14-19 perovskite (CH3NH3PbI3) solar cells which yielded a band gap of 1.5 eV . To enhance the device efficiency, tandem architecture is deemed to be promising due to its more effective spectrum consumption. Perovskite material has low non-radiative recombination rates relative to other thin-film polycrystalline semiconductors as well as relatively higher calculated external radiative efficiencies2 which make it particularly more photovoltaically sound for high gap cell in tandem cell stacks3, in which the high voltage output from a perovskite cell yields a better tandem device performance.
Therefore, a large band gap bromide perovskite (CH3NH3PbBr3) is worth to be developed for this tandem application.
In order to design and fabricate a perovskite-based tandem solar cell, a high efficiency single junction cell with reasonable reproducibility is required, especially for
CH3NH3PbI3 top cell. The conventional one-step spin-coating deposition technique typically results in a non-uniform film surface, incomplete coverage and uncontrollable morphology. Therefore, a novel deposition method is required to develop and fabricate
CH3NH3PbI3 solar cell resulting in higher efficiency.
Meanwhile, optimization of the perovskite film quality requires novel characterization techniques to disclose unknown material properties. Better
16 understanding of those properties is vital for the selection of carrier transport material and the design of cell structure.
Before designing the monolithic tandem structure and proposing the recombination layer composition, a demonstration of the tandem concept using a four-terminal spectrum splitting system is effective in verifying the integration of two materials in consuming the spectrum more efficiently than single junction cell.
Finally, the ultimate motivation of this thesis is to demonstrate and fabricate perovskite/ perovskite monolithic tandem solar cells using absorbing materials with different band gaps
1.2. Aims and Significance
This thesis aims to characterize the perovskite material properties using novel techniques, particularly on CH3NH3PbBr3. Then it proposes a fabrication method to produce efficient large band gap perovskite solar cell. The fabricated cell will be coupled with smaller band gap solar devices to demonstrate the potential of tandem solar cell using spectrum splitting. The ultimate aim of this thesis is to fabricate and characterize a layer-by-layer monolithic perovskite/ perovskite tandem solar cell.
Firstly, this thesis will report a novel deposition method to produce high efficiency
CH3NH3PbBr3 solar cells. It is the first time that the vapour-assisted solution process is used for the preparation of CH3NH3PbBr3 film.
Secondly, a comprehensive investigation on the perovskite material properties will be presented. In the first part, the study was conducted on one- or two-photon fluorescence imaging to study film morphology and carrier extraction, without special specimen preparation or electron bean damage of perovskite structures. The one- or
17 two-photon fluorescence imaging is a promising technology for perovskite solar cell characterization and the findings in this study detail additional insights in the knowledge of carrier extraction. In the second part, fluorescence intermittency (also known as blinking) was investigated. The evident fluorescence blinking was observed in a densely populated CH3NH3PbBr3 perovskite film which was composed of nano- particles in close contact with each other. This finding provides unique insight into the charge accumulation and migration, and thus is of crucial importance for the understanding of device operation. In the third part, various characterization techniques were used to study the dynamic aging process for the vapour-assisted fabricated
CH3NH3PbBr3 film. Methods including advanced photoluminescence techniques, steady state photoluminescence (SS-PL), time resolved PL (TRPL), nano-resolved fluorescence lifetime imaging microscopy (FLIM) and fluorescence imaging microscopy were evaluated comprehensively.
Thirdly, a high bandgap CH3NH3PbBr3 perovskite solar cell was applied in a spectrum splitting system, which demonstrates remarkable energy conversion efficiencies when coupled with a Si solar cell, regardless with whether it is a high performance silicon passivated emitter rear locally diffused (PERL) solar cell; or a commercially relevant multi-crystalline-screen-printed-Si solar cell.
Finally, a perovskite/ perovskite monolithic tandem solar cell with a novel recombination layer was designed and fabricated. This is, so far, the only layer-by-layer deposited perovskite-based tandem cell using different absorber.
1.3. Thesis Outline
This thesis is divided into seven chapters, presented comprehensively in the following sequence, the background of the research, material characterization, single
18 junction solar cell fabrication, spectrum splitting system and monolithic tandem structure, as well as the overall discussion and perspective for future work.
Chapter 2 briefly introduces the concise history of perovskite solar cell, and its development. Additionally, the current-state-of-the-art deposition techniques and device structures were also discussed.
Chapter 3 proposes a novel deposition method for CH3NH3PbBr3 cells to achieves an open circuit voltage (Voc) of 1.45V with a short circuit current density (JSC) of
9.75mA/cm2, a fill factor (FF) of 61.5%, and an efficiency of 8.7%.
Chapter 4 presents the material characterization works that have been carried out on perovskite cells which divided into three main parts. In the first part, one- or two-photon fluorescence imaging was used to study film morphology and carrier extraction. The second part is dedicated to blinking phenomenon which was observed in CH3NH3PbBr3 thin film. The last part is about dynamic aging process in CH3NH3PbBr3 material, which reveals that CH3NH3PbBr3 grains grow bigger spontaneously without material decomposition.
Chapter 5 demonstrates the spectrum splitting system that capitalizes on high bandgap CH3NH3PbBr3 solar cell. The great potential of utilizing bromide perovskite cell to build up a two-terminal tandem cell is discussed in this chapter.
Chapter 6 constructs a two-terminal monolithic tandem solar cell and achieves open circuit voltage of 1.96 V.
Finally, chapter 7 summarizes the key results and conclusion of this thesis and presents perspectives for further research.
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(CH3NH3)PbI3 for solid-state sensitised solar cell applications. Journal of Materials
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22 2. Literature review
In this chapter, literature on history and development of perovskite solar cells will be reviewed, followed by fundamental knowledge of deposition methods and device structure, as well as the introduction of tandem application.
2.1. History and Development
Organometal halide light absorbers based Perovskite solar cells have been considered a promising photovoltaic technology due to its outstanding power conversion efficiency (PCE) and low material costs1.
The Perovskite materials have been widely studied for decades2,3. It is a class of calcium titanium oxide mineral, which was discovered in 1839 by Gustav Rose and is coined after the Russian mineralogist Lev Perovski4. In the early 1990s Mitzi and co- workers carried out extensive investigations to research the optoelectronic properties of organic-inorganic perovskite5. The Initial research interest focused on tin (II)-based halide perovskite because of its superior metallic behavior and charge carrier mobility compared to lead (II)-based perovskite. These properties are significant in the application of organic light-emitting diode (OLED), superconductor and thin film transistor (TFT)6,7. The first incorporation into a solar cell was reported by Miyasaka et al. in 2009.8 This was based on dye-sensitized solar cell (DSSC) architecture, and yielded 3.8% of PCE. A typical DSSC is composed of three main components, mesoscopic n-type metal oxide that is sensitized by a light-absorbing dye followed by filling with a redox-active electrolyte. In Miyasaka’s structure, mesoporous TiO2 was used as a scaffold to adhere perovskite sensitizer, as well as increase the surface area with holes transport material through its penetration to the porous. An improved
23 efficiency (6.5%) of the CH3NH3PbI3 sensitized PV cell with liquid electrolyte was reported by Nam-Gyu Park et al. in 20119. In their study, 1-2 nm size perovskite nanocrystal was synthesized and employed as a sensitizer. Moreover, they proposed that the combination of higher absorption coefficient of a perovskite material and thinner mesoporous TiO2 is likely to result in higher photocurrent density, this unlocks more studies on planer device structures. However, because corrosive liquid electrolyte was utilized to conduct photo-generated holes to the back contact, the cell was only stable for a very short period.
A decisive breakthrough came in 2012 when Snaith et al. implemented solid-state
DSSC (ss-DSSC) architecture where Spiro-OMeTAD was used as a hole transport material to replace the liquid ones to increase the device stability10. They demonstrated that the efficiencies of almost 10% were achievable using this structure10. Interestingly,
10 higher efficiency of 10.9% was attained by replacing TiO2 scaffold with inert Al2O3 .
Further experiments revealed that devices using Al2O3 would yield a higher open-circuit voltage and a mild improvement in efficiency of 3-5% was achievable in comparison to
TiO2. This has led to the hypothesis that a scaffold is not necessary for electron extraction, which was later proved to be true. This important finding was further extended to a series of studies to demonstrate that the planar structure thin-film perovskite solar cells with PCE over 10% were indeed accomplishable 11-13. In the same year, Gratzel and Park et al. published a work which also demonstrates a mesoscopic structure with efficiency approaching 10%14.
A rapid evolutionary development in both planer and meso-structure perovskite photovoltaic devices was evidenced in 2013. Burschka et al. reported a sequential deposition technique for the sensitized architecture yielding a PCE of over 15% on average15. And at a similar time Liu et al. demonstrated that it was possible to fabricate
24 planar solar cell by dual source thermal evaporation to achieve a PCE of 15.4%16.
Inverted configuration was presented by Docampo et al. with a PCE of 10%. It was a typical organic solar cell architecture where the generated holes are conducted through the glass region17 (see Figure 2.4.3). In addition, the feasibility of flexible substrate was found by using a low temperature sintering process. This is an important milestone and has become the current state-of-the-art as it removes most barriers to adoption of the perovskite technology in the organic photovoltaic community, and can thus utilize the extensive existing knowledge of hybrid interfaces for further device improvements and flexible process platforms.
A range of novel deposition techniques and even higher efficiencies were reported in 2014. Yang et al. presented a PCE of 19.3% using a planer structure18 where they suppressed the carrier recombination in the absorber by delicately manipulating the perovskite layer formation and strategically selecting of interlayer materials. In addition,
ITO/ PEIE and modified TiO2 were used as frond contact and electron transport layer, respectively. In 2014, Seok and co-workers made a substantial improvement on their research. Solvent engineering methods were developed to achieve extremely uniform perovskite layers, and a PCE of 16.5% was demonstrated19. An extension work was carried out where a higher efficiency (19%) was obtained by the same group using compositional engineering method20. In order to achieve this high efficiency, they
21 incorporated MAPbBr3 into FAPbI3 to stabilize the perovskite phase of FAPbI3 .
Shortly afterwards, a device fabricated by Seok et al. was certified by NREL with a
PCE of 20.1% being reported22.
In December 2015, a novel certified record PCE of 21% was achieved by the researchers at EPFL22. However, the record was overwritten by the scientists at KRICT and UNIST soon after, a 22.1% PCE cell was certified at NREL, in which this cutting
25 edge technology yielded the highest single junction perovskite solar cell efficiency up- to date.
2.2. Material Properties
A general chemical formula for pure perovskite compounds is AMX3, where “A” is a cation occupying in a cubo-octahedral site, and “M” cation is occupying in a octahedral site, "X” is an anion that binds to both “A” and “M”( Figure 2.2.1).
Figure 2.2.1 AMX3 perovskite structure
Geometric tolerance factor (t) was defined by a group of researchers in 1927. The formability of perovskite was estimated based on t value23,
푡 = (푟퐴 + 푟푀)/√2(푟푀 + 푟푋) Eqn 2.2.1
where rA, rM and rX were the effective ionic radii for A, M and X ions, respectively.
Based on effective ionic radii, the rA in APbX3 (X=Cl, Br, I) perovskite was calculated to be t=0.8 to t=124. Cations with radii between 1.60Å and 2.5Å were found to form perovskite structures. Therefore, methylammonium cation is suitable for lead halide perovskite because its ionic radius is 1.8Å. Since the tolerance factor of CH3NH3PbI3 was calculated as 0.83, mild deviation from an ideal cubic structure was expected24,25.
26 The knowledge of optical properties plays a critical role in better understanding the electronic structures of perovskite materials and optimizing photovoltaic device structures, and therefore further improves the device performances. Many researches have been carried out on both thin films26,27 and single crystals28. The initial interest focused on methylammonium (MA) lead trihalide (CH3NH3PbX3, where X is halide such as iodide, bromide or chloride), with an optical bandgap of 1.5 to 2.3eV, it is important to note that recent developments are shifting attention towards formamidinum
(HC(NH2)2PbI3 or “FA”) lead tri-halide. Most efficient perovskite based solar cells were fabricated using a mixture of FAPbI3 and MAPbBr3.
Perovskite lattice forms three phases (orthorhombic, tetragonal and cubic) at different temperatures. All the perovskites of photovoltaic interest tend to produce less symmetric orthorhombic structure at lower temperature whilst the increase in temperature will shift the phase towards more symmetrical arrangements, until the symmetrical cubic structure is made. It is important to note that two or more phase transitions is likely to happen in the process 29,30. During the geometry transition from tetragonal to cubic, both bandgaps of CH3NH3PbI3 and CH3NH3PbBr3 and binding energies remain unchanged. While these parameters alter when phase transfer happens
26 from orthorhombic to tetragonal . For the case of CH3NH3PbI3, the phase transition from cubic to tetragonal undergoes at circa 330.4 K, while for the case of
CH3NH3PbBr3, it happens at 154.0 K. CH3NH3PbI3 crystallises an orthorhombic structure below approximately 161.4 K, but CH3NH3PbBr3 requires temperature down to 148.8 K to generate an orthorhombic geometry31-34.
27 2.3. Working Mechanism of Photovoltaic Devices
In perovskite solar cells, Figure 2.3.1, the perovskite material operates as a semi- conductor light absorbing layer which allows ambi-polar transport of photo-generated carriers that are selectively conducted by charge transport layers to the corresponding electrodes.
Figure 2.3.1 Schematic diagram of the working mechanism of a perovskite solar cell.
The performance of a perovskite solar cell is limited by a number of factors. In general, they can be divided into two categories: structure and material properties. First of all, planar or mesoporous structure, normal or inverted configuration need to be decided before choosing the material (for ETM and HTM), which is also a key parameter of device performance. Proper ETM and HTM result in ideal band alignment, which lead to good charge transportation. In addition, the film thicknesses also affect the cell output, film that is too thin results in less light harvesting and non-uniform coverage; film that is too thick often leads to increase in series resistance or poorer voltage if the film quality is not ideal. In terms of material properties, the deposition method and post treatments affect the crystallinity, electrical and optical properties of the material, and in turn, affect the device performance. Besides, the amount of additive
28 for both charge transport material and perovskite absorber is critical to the solar cell performance as well.
2.4. Deposition Method
2.4.1 One-step solution process
One-step spin-coating (Figure 2.4.1) is the technique that Miyasaka et al. was using when they introduced perovskite into photovoltaic community for the first time8.
CH3NH3X and PbX2 were mixed in DMF with 1:1 molar ratio at a mild temperature
(e.g. 90⁰C). Precursor solution was spin-coated onto the surface of the substrate followed by annealing. The main advantage of this method is less time consuming and suitable for all perovskite formation by different lead sources. However, because of the uncontrolled precipitation of perovskite, one-step spin-coating produces the films with incomplete coverage resulting in large morphological variations, yielding poor photovoltaic device performances and low reproducibility.
Figure 2.4.1 Schematic diagram of one-step spin-coating deposition method
29 2.4.2 Sequential process
To overcome the disadvantages of one-step spin-coating approach, Gratzel et al. proposed a sequential deposition method15. The schematic diagram shows in Figure
2.4.2. PbI2 was firstly spin-coated on the substrate followed by dipping the annealed sample in a CH3NH3I solution for a certain time; the change of color of the film implied the formation of perovskite phase in the solution. The last step was to anneal the substrate again to remove solvent residual and allow the grains grow larger and denser.
In contrast, the two-step deposition technique was expected to be particularly effective for preparing films of organic-inorganic systems, in which the organic and inorganic component had incompatible solubility characteristics. An evenly distributed perovskite based films could therefore be achieved by this method. However, the fast reaction which happens in CH3NH3I solution resulted in relatively small grains and rough surface5,35. Thus, thicker hole transport material was required to prevent shunting.
This would result in large series resistance, and in turn affected the device performance.
Besides, it was worthwhile noting that the sequential deposition was often reported to achieve high efficiency with nanostructured TiO2 scaffolds, but was seldom reported to be applicable for fabricating planar structure solar cells.
Figure 2.4.2 Schematic diagram of sequential deposition method
30 2.4.3 Dual-source evaporation method
In order to achieve smooth film with complete and even coverage, a novel dual- source evaporation method (Figure 2.4.3) was developed by Snaith and co-workers16.
This vacuum evaporation was considered as a high-performance technique to grow oriented thin films of layer perovskite with a precise control of the film properties.
However, this evaporation deposition method requires high vacuum chamber and long processing time due to low evaporation rate, which without further improvement is energy costly for commercial production.
Figure 2.4.3 Schematic diagram of dual-source evaporation deposition method
2.4.4 Vapour-assisted solution process (VASP)
Another two-step deposition method was proposed by Yang et al36 that is of similar to the first step of sequential process. As expressed in Figure 2.4.4, PbI2 was spin- coated on substrate followed by annealing, instead of dipping substrates into solution, the film was then exposed to hot CH3NH3I vapour, and perovskite was formed in a PbI2
31 film in the closed chamber, which a glassy petri-dish was commonly used. Over time, pre-deposited PbI2 was converted to perovskite completely; isopropanol was employed to sanitize the excessive CH3NH3I residuals on the film surface followed by drying under nitrogen stream.
Figure 2.4.4 Schematic diagram of vapour-assisted solution process
2.4.5 Other deposition methods
Apart from addressing the techniques previously, many other deposition methods were developed to achieve ideal films for characterizations and device fabrications. One of the concepts was to substitute the lead source or halide source. Chloride substitution into the precursor solution was proposed by Snaith et al.10. This approach produced a new crystallization condition to achieve fully covered film. The same group also developed a method using lead acetate instead of lead halide as the lead source to obtain ultra-smooth film37. Researchers from Monash University reported a fast-crystallization concept to obtain perovskite with superior quality, they utilized a one-step solution process, either using an anti-solvent38 or Argon stream39 during the spin-coating step, to remove solvent residuals from the precursor rapidly. This method dictates the dynamics of nucleation and perovskite grain growth to produce a high quality perovskite film in a rapid way with a high reproducibility. Korean researchers also made a great contribution in advancing perovskite deposition methods. Seok et al. developed a compositional engineering approach21 which produced a high efficiency of perovskite-
32 based solar cell. MAPbBr3 was incorporated into FAPbI3 to stabilize the perovskite phase of FAPbI3. Recently, the same group demonstrated an intramolecular exchange
40 concept which involved FAPbI3 crystallization by the direct intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalating in PbI2 with formamidinum iodide. This method dictated the crystallographic orientation at a preferable geometry.
Large and densely packed grains without PbI2 residual could be fabricated by this method to form a thin film with small surface roughness.
2.5. Photovoltaic Device Structures
Utilization of perovskite as the light harvester in photovoltaic devices emerged from the field of dye-sensitized solar cells (DSSCs)41,42, the initial structure of perovskite solar cell resembled the classical DSSCs architecture, which comprised a mesoporous n-type TiO2 film sensitized with light absorber and a liquid phase redox-active electrolyte. However, the later solid-state DSSCs development required reducing the mesoporous layer thickness significantly due to the penetrating limitation of HTM43.
Further characterization pointed it that a good quality perovskite film with strong absorption is sufficient to harvest incident light which led to new generation of cells with thinner mesoporous layer. Moreover, planer structure has been proposed and successfully fabricated and optimized.
2.5.1 Mesoporous structure
In a normal structure a perovskite solar cell infiltrates and caps a mesoporous scaffold as shown in the schematic diagram in Figure 2.5.1. There are two commonly used scaffold material: TiO2 and Al2O3. For the former, it is conventionally deposited through solution process, either by spin-coating or doctor blading. The layer is then
33 sintered at high temperature (over 450 ⁰C). It serves as a scaffold in the structure, as well as an electron transport layer. Al2O3 has a similar meso-morphology and uses deposition technique to TiO2. However, the replacement of TiO2 with an insulative
Al2O3 would not only result in a faster charge transport and unaffected photocurrent, but also a higher voltage output and efficiency. It is important to note that the thin film solar cells with inert Al2O3 as scaffolds were defined as meso-superstructed solar cells
(MSSC)44.
Figure 2.5.1 Architecture schematic diagram of mesoporous structure perovskite
2.5.2 Planar structure
The success of MSSC and its high internal quantum efficiency (IQE) which is close to 100% demonstrated that the planar configuration (Figure 2.5.2) is promising as a simpler architecture11. In addition, it has a distinct advantage for commercial production. However, it has been reported that low shunt resistant can be caused by incomplete film coverage, thus a series of studies have been undertaken to eliminate the problem11,36,38,39.
Figure 2.5.2 Architecture schematic of planar structure perovskite solar cells
34 2.5.3 Inverted structure
The structures discussed above have a commonality that the photo-generated electrons are transferred through an electron transport material (ETM) onto the glass side, while the generated holes towards the metal contact side through a holes transport material (HTM). This configuration has been defined as “standard polarity structure”. In an inverted structure (Figure 2.5.3), photo-generated carriers follow the opposite directions. For a high efficient normal structure perovskite cell, high temperature annealed TiO2 is commonly used, therefore, the high transmissive ITO is not applicable because of its thermal instability. The Organic HTMs are widely used in inverted structured solar cell preparations, which have the advantage of low temperature deposition process. Therefore, inverted architecture can be adopted in flexible devices and roll-to-roll production.
Figure 2.5.3 Architecture schematic of inverted structured perovskite solar cells
As one of the advantages, cost effectiveness is an attracting selling point for perovskite solar cells. However, the conventional HTM Spiro-OMeTAD is not cost effective. Therefore, apart from the structural evolution and optimization, large numbers of research were carried out on alternative carrier transport material, particularly on
HTM. A great number of HTMs have been proposed and successfully applied in perovskite solar cells, including small molecules45,46, conducting polymers47,48, and inorganic p-type semiconductors49. There are some general requirements to be an ideal
HTM. Firstly its work function should well align with the associative perovskite
35 material. That is, the highest occupied molecular orbital (HOMO) energy level of HTM needs to be compatible with the valence band (VB) of perovskite. Secondly, sufficient hole mobility to transport photo-generated carriers to the back contact more efficiently.
Apart from these, excellent thermal and photochemical stabilities are required as well so that the solar cells can be fully functional and durable for commercialization. In addition, tandem architectural HTM design is required in order to obtain low light absorbance to prevent the bottom cell from the parasitic absorption loss of HTM.
2.6. Tandem Application
The emergence of tandem application in photovoltaic development is due to the inability of making use of the solar spectrum efficiently in the single junction cells and the reasons are discussed as follows. Firstly, photon energy of smaller than bandgap is dysfunctional. Secondly, photon with larger energy will lose their excess energy via thermal equilibration. Tandem solar cells are designed to employ various semiconductor materials and lead each photon into an absorber where the bandgap matches the photon energy. For the combination of semiconductors with different bandgaps, the incoming light firstly strikes the large bandgap absorber where high energy photons will be absorbed and a high voltage output will be generated. While the low energy photons will be absorbed by the sub-cell underneath with a lower bandgap to generate additional electricity output.
Of all light harvesters, perovskite materials with bandgap over 1.5 eV are considered as short wavelength absorber. Perovskite solar cells are only reactive to the light spectrum from ultraviolet up to around 800 nm, while some conventional photovoltaic materials, such as crystalline silicon and germanium are responsive to a much broader spectrum of up to 1800nm. The combination of them results in a perfect
36 match for tandem architecture which uses the spectrum more efficiently. There are many structures for tandem solar cells, in terms of system configurations and they can be segmented to two categories: two- and four-terminals.
2.6.1 Four-terminal tandem structure
A typical four-terminal tandem solar cell contains two individual cells: a bottom cell and a semi-transparent top cell, they are mechanically stacked and connected by an outer circuit. The sum of the outputs will determine cell efficiency. Challenges however remain for “stacking” in the four-terminal tandem solar cell preparation.
However, the top celled fabrication of transparent contact remains a challenge. A rear contact of an opaque perovskite solar cell typically uses full area gold, silver or aluminum. It is deposited through thermal evaporation with a thickness of over 80 nm.
Sputtered metal oxides (ITO and IZO) are typically the transparent electrodes employed in perovskite tandem solar cells, in short, sputtering yields a better outcome with a faster velocity and higher energy. In addition, the electrode is to be deposited onto top of cell evenly requiring a buffer layer to mitigate the damage by sputtered particles on the sensitive carrier transport and perovskite layers. Research studies on the topics of solution processed ITO and thin metal oxide buffer layer have been reported50-52. The current state-of-the-art of semitransparent perovskite solar cells yielded over 15% of
PCE with more than 60% on average where the transmittances were in the range of 800-
1200 nm wavelength.
37
Figure 2.6.1 Architecture schematic of four-terminal tandem solar cell
The other option of four-terminal tandem solar cell was to use an optical system to split spectrum and directing it appropriately to individual solar cells. Large number of research have been undertaken to demonstrate the feasibility of perovskite coupled with silicon solar cells51,53, as well as perovskite with perovskite solar cells54.
Figure 2.6.2 Architecture schematic of four-terminal tandem solar cells using spectrum splitting
2.6.2 Two-terminal monolithic tandem structure
In monolithically integrated tandem solar cell system, two sub-cells are configured with a recombination layer; the top cell is directly processed from the bottom cell.
Although parasitic absorption still remains, this structure is favorable due to the reduced complexity in module assembly. However, the monolithic tandem requires strict
38 fabrication compatibility 55 and current matching. The latter means the thickness of the absorber needs to be re-optimized to yield an equal current in a given spectrum, otherwise, limitation remains due to the lower current of the sub-cell 56, many simulation works have been reported to optimize the structures55,57-60. Secondly, if the amorphous/ crystalline silicon heterojunction (SHJ) cell or heterojunction with intrinsic thin layer (HIT) cell which contains temperature-sensitive ITO layer was selected to be the bottom cell, the top perovskite cell needed to be processed at lower temperature.
Thirdly, the requirements for designing and fabricating recombination layer are high. Its work function needs to match both top and bottom cells. In addition, for a solution- processed top-cell which is more common than an evaporated perovskite cell, the surface morphology and hydrophilia are required to synchronize the perovskite processing. Fourthly, perovskite cell fabrication that uses spin coating is not compatible with bottom cell that has surface texture (e.g., for antireflection control in silicon cell) due to poor perovskite cell coverage by spin coating. If planar Si cell is used as the bottom cell will result in a current loss due to lack of light entrapment. A recent paper55 demonstrated a monolithic perovskite/ crystalline silicon tandem solar cell using a low- temperature process for perovskite cell preparation, and IZO/PCBM & PEIE as the recombination layer, achieving an efficiency of 21.2%.
Figure 2.6.3 Architecture schematic of monolithic tandem perovskite-based solar cells
39 2.7. Challenges and Perspectives
Perovskite solar cells have spearheaded as the frontier technology in photovoltaic research community in past few years. Although they exhibits rapidly increasing efficiencies, as well as the potential in light emitting diode (LED) and laser application, it remains unsolved puzzles in material properties and device performances.
2.7.1 Hysteresis
Hysteresis has been widely identified in reported researches61-64. A perovskite solar cell measured under various scanning conditions, pronounced hysteresis could be observed in the current density-voltage (JV) curve. Snaith et al. discussed the possible origins of hysterical phenomenon61. Firstly, a large defect density within or near the material surface or specifically generated interface states may lead to a hysteresis, these defects entrap electrons or holes during measurement. However, defects were found to be different under forward bias or short-circuit conditions, and therefore, the measured
JV curves with different scan directions were different. Although discussed by Snaith et al in earlier works, ferroelectricity of organometal trihalide perovskites is unlikely the cause for hysteresis phenomenon. Applied bias affected polarization, which exerted either a positive or negative effect on charge collection at the contacts. Thirdly, the excess ions were the other factor that might result in hysteresis; they acted as interstitial defects which migrated to either side of the film, to build up the charge, hence aiding charge collection under functional conditions. In their publication, the reporting of
“stabilized power output” has been proposed. This requires a perovskite solar cell to be held at a constant voltage corresponding to its maximum power point during measurement. Current density and efficiency were recorded during the measurement
40 period. Stabilized power output is therefore a useful parameter to discuss whether a technique is advantageous to improve device performance.
2.7.2 Toxicity
The most efficient perovskite-based solar cells typically consist of lead (Pb), which leads to environmental and health concerns65,66. Although, research shows the amount of Pb in perovskites is less than Pb in solder in silicon photovoltaics67, perovskite solar cell as an alternative energy crisis resolution, to minimize the negative impact on environment maximized the positive outcome of the photovoltaic industry. Researches have proposed replacing Pb with tin (Sn). However, previous research also showed that
Sn is also enlisted as a harmful chemical, with similar concerns to Pb65. Therefore Sn based perovskite may not be the ideal replacement. Furthermore, the organic solvents used in fabrication process, for example DMF, are highly harmful for human being.
Therefore development of less or non-toxic perovskite based materials and processes remains top priorities.
2.7.3 Stability
In addition to the material toxicity and measurement difficulty, the biggest challenge towards its commercialization and compete with conventional silicon solar cells in the market is the stability and reproducibility. Researches indicated that the intrinsic stability of perovskite materials is not the only reason for unstable photovoltaic devices68-76, the properties of carrier transport materials77-83, other buffer layers84 and electrodes85-90, as well as the inter dependent relationships between layers also contribute to the instability of devices. Moreover, the device geometry, encapsulated material and techniques also have great impact on cell stability91-94.
41 Perovskite materials have the potential to change the existing photovoltaic landscape due to their high-efficiency and cost-effective features. Due to the ease of processing, fabrication is compatible with roll-to-toll processing which has been developed for organic photovoltaic (OPV) device. On the other hand, compared with low-cost OPV devices, the efficiency demonstrated by perovskite-based solar cell is much more competitive. Furthermore, perovskite solar cells can be coupled with silicon solar cells or other different bandgap materials to form high efficiency tandem cells.
This promising material remains an attractive solution to the photovoltaic community.
Reproducible materials and processes with high yield should be developed. Moreover other efforts, particularly affordable monolithic tandem architecture and flexible substrates have the potential to make the perovskite solar cells more economically viable. Consequently, these areas should and will attract more and more research attention.
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55 3. METHYLAMMONIUM LEAD BROMIDE
PEROVSKITE-BASED SOLAR CELLS BY VAPOUR-
ASSISTED DEPOSITION
3.1. Introduction
Methylammonium lead halide perovskite solar cells have attracted enormous research interests in the past two years1,2. Much of the attention is focused on
CH3NH3PbI3, or mixed halides CH3NH3PbI3−xClx and CH3NH3PbI3−xBrx devices with a bandgap in the vicinity of 1.6eV, despite the first demonstration of perovskites solar cell
3 that used CH3NH3PbBr3 which has a higher bandgap (2.3eV). The larger bandgap and therefore a larger voltage potential of bromide-based perovskite makes it a promising candidate for a tandem system4, such as a 3-cell stack. The highest efficiency of 10.4% has been reported by using crystallization controlled spin-coating process and poly- indenofluoren-8-triarylamine (PIF8-TAA) as hole transport layer5. Other work demonstrating efficient CH3NH3PbBr3 perovskite solar device employ poly-(3- hexyl)thiophene (P3HT); N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzidine (TPD);
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM); N,N’-dialkyl perylenediimide
6-8 (PDI) or poly(triarylamine) (PTAA) as hole transport materials . The CH3NH3PbBr3 films in these works are deposited using solution-based method. Here we demonstrate a solar device based on CH3NH3PbBr3 perovskite deposited via a vapour assisted crystallization method employing Spiro-OMeTAD as a holes transport material.
Various deposition techniques have been used for the fabrication of methylammonium lead halide perovskite solar cells. One of the challenges associated with the one-step precursor solution spin-coating process is the lack of suitable solvents
56 that can dissolve the components in the precursor mixture and the high reaction rate of the perovskite components resulting in incomplete coverage and poor uniformity9-12, leading to low-resistance shunting paths and loss of light absorption in the solar cells10.
Chloride inclusion, optimization of annealing conditions as well as sequential solution based deposition method has been demonstrated to increase film coverage and uniformity10,13,14. Dual-source vacuum evaporation has also been employed allowing efficient perovskite solar device to be fabricated with excellent film coverage and uniformity15, but the high vacuum requirement precludes this method from mass adoption. Although vapour-assisted solution process ( (VASP) method has been used to
9 fabricate planar CH3NH3PbI3 perovskite solar cell , it has not been employed in a cell structure where a mesoporous layer is present. The planar perovskite cell structure without the meso-porous layer has the advantage of a simplified design reducing number of fabrication steps and simplifying the design5. This may potentially be beneficial for tandem cell construction and allows the investigation of the underlying device physics9 without the complication of the meso-porous layer. However recent research has reported that the presence of meso-porous layer, in particular the meso- porous TiO2 (mp-TiO2) when its thickness is optimised, is beneficial in reducing the hysteresis in perovskite solar cells compared with planar structure16,17.
3.2. Experimental details
25 CH3NH3Br was synthesized following a previously reported method , by mixing methylamine (33% in methanol, Sigma-Aldrich) with hydrobromic acid (48% in water,
Sigma-Aldrich) in a 1:1 molar ratio in a 250ml round bottom flask under continuous stirring at 0⁰C for 2h. The precipitate was recovered by rotary evaporation at 60⁰C, and then washed three times with diethyl ether in ultrasonic bath for 30min. The final
57 product was collected after dehydration at 60⁰C and placed in a vacuum chamber for overnight.
Solar cell devices were fabricated on fluorine-doped tin oxide (FTO) coated glass
(Pilkington, 8 Ω/square). FTO was patterned with 2 M HCl and zinc powder. Substrates were then cleaned in 2 % Hallmanex detergent, acetone and isopropanol in ultrasonic bath for 10 min in each cleaning agent followed by UVO treatment for 10 min. The compact TiO2 layer was deposited by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol at 2500 rpm for 60 s followed by annealing at 500 ⁰C for 30 min. The mp-TiO2 layer composed of 20-nm-sized particles was deposited by spin- coating at 2000 rpm for 60 s using a commercial TiO2 paste (Dyesol 18NRT, Dyesol) diluted in ethanol (2:7, weight ratio). After drying the TiO2 film at 125 ⁰C, it was heated to 500 ⁰C, annealed at this temperature for 30 min and gradually cooled to room temperature.
In the one-step solution process, CH3NH3PbBr3 was deposited by spin-coating at
2000 rpm for 60 s on the mp-TiO2 layer, and anneal at 70 ⁰C for 30 min. The precursor solution was mixed by CH3NH3Br and PbBr2 with molar ratio of 1:1 in DMF.
For CH3NH3PbBr3 films deposited using the VASP method. Firstly, PbBr2 solution in DMF with a concentration of 1 M was spin-coated on the mp-TiO2 at 2500 rpm for
60 s. After annealing at 70 ⁰C for 30 min, the film was treated by CH3NH3Br vapour at
150 ⁰C for 10 min in a closed glass petri-dish with CH3NH3Br powder surrounded on a hotplate in glovebox, then rinsed in isopropanol at room temperature.
58 To complete the solar devices, HTM was then deposited by spin-coating at 2000 rpm for 60 s. The solution was prepared by dissolving 72.3 mg (2,2’,7,7’-tetrakis-(N, N- di-p-methoxyphenyl-amine)-9,9’-spirobifluorene) (spiro-MeOTAD), 28.8 ml 4- tert- butylpyridine (4-TBP), and 17.5 ml of a stock solution of 520 mg/ml lithium bis(trifluoromethane)sulfonimide (LiTFSI) in acetonitrile in 1 ml Chlorobenzene. The samples were left overnight in dry air before 100 nm gold contacts were thermally evaporated on the back through a shadow mask. The device fabrications were carried out under controlled atmospheric condition and a humidity of 1 ppm.
X-ray diffraction (XRD) patterns were measured using a PANalytical Xpert
Materials Research diffractometer system with a Cu Kα radiation source (λ=0.1541 nm) at 45 kV and 40 mA.
The current density–voltage (J–V) measurements were performed using an IV5 solar cell I–V testing system from PV measurements, Inc. (using a Keithley 2400 source meter) under illumination power of 100 mW/cm2 by an AM1.5G solar simulator (Oriel model 94023A).
Reflectance (R) and transmittance (T) of this CH3NH3PbBr3 film were measured using a Varian Cary UV-VIS-NIR spectrophotometer at close to normal incidence. The optical properties, in particular the imaginary part of the dielectric constant (ε2), was determined by modelling with the computer software WVASE®. The real part of the dielectric constant (ε1) was extracted from ε2 using the Kramers-Kronig (KK) method.
The real and imaginary parts of the refractive index (n and k respectively) were determined from the relationships:
59 2 2 ε1 = n − k ; ε2 = 2n Eqn 3.3.1
The absorption coefficient α of the film was then calculated:
4πk α = Eqn 3.3.2 λ
3.3. Results and Discussion
A vapour-assisted solution process (VASP) method was developed in this thesis and compared with one-step solution method. High voltage CH3NH3PbBr3 solar cell using
VASP method was fabricated, on an mp-TiO2 scaffold employing Spiro-OMeTAD as a holes transport material. The perovskite film exhibits densely packed grains and excellent coverage, in particular when it is a capping layer, over the mp-TiO2 surface.
Solar devices based on this structure have achieved power conversion efficiency of
8.7% (average value of forward and reverse scan). In addition, carrier dynamics of the
CH3NH3PbBr3 films was studied using photoluminescence-quenching measurements.
3.3.1 Film properties of VASP and one-step solution process
Figure 3.3.1(a) shows the X-ray diffraction (XRD) patterns of a CH3NH3PbBr3
(blue) film deposited by VASP method, as well as CH3NH3Br (red) and PbBr2 (black) films for reference. All of the films are deposited on mp-TiO2/ compact TiO2 (c-TiO2)/ glass. The XRD pattern of CH3NH3PbBr3 indicates a cubic perovskite phase has formed6, where peak of (100) is at 14.87⁰, (110) at 21.33⁰, (200) at 30.29⁰, (220) at
42.87⁰, and (300) at 48.21⁰ are present. CH3NH3Br and PbBr2 residuals are not detected in the synthesized CH3NH3PbBr3 indicating after all of the PbBr2 is converted into
60 CH3NH3PbBr3, the remaining CH3NH3Br has been removed completely during solvent evaporation. As shown in the top-view scanning electron microscopy (SEM) image in
Figure 3.3.1 (b), CH3NH3PbBr3 film deposited by the VASP method is uniform with densely packed grains (sizes range from 200-300nm) with full coverage.
Figure 3.3.1(a) XRD pattern of CH3NH3PbBr3 film deposited by VASP method compared with that of CH3NH3Br and PbBr2 films as references. (b) SEM top view of
VASP deposited CH3NH3PbBr3 film
61 Figure 3.3.2 (a) shows the top-surface SEM images of the CH3NH3PbBr3 deposited by one-step solution method; compare with the film in figure 3.3.1 (b) which was deposited by VASP method, the one-step solution processed film contains a large number of pinholes. Figure 3.3.2 (b) shows the XRD patterns of CH3NH3Br (MABr); and CH3NH3PbBr3 (MAPbBr3) films deposited by the two methods. Both films follow the same pattern, However, the one step-solution processed CH3NH3PbBr3 film contains CH3NH3Br residual at 36.67⁰ which is absent in the CH3NH3PbBr3 film deposited via the VASP method. The absorption coefficients of the two CH3NH3PbBr3 films are shown in Figure 3.3.2 (c) showing the high absorption by VASP processed film due to better crystallinity and uniformity compared to the film prepared by one-step solution process.
62
Figure 3.3.2(a) SEM top view of CH3NH3PbBr3 film prepared by one-step solution-based process, (b) XRD patterns of CH3NH3Br (MABr), one-step solution processed and VASP deposited CH3NH3PbBr3 (MAPbBr3). (c) Absorption coefficient of the two CH3NH3PbBr3 films
The absorption coefficient in Figure 3.3.3 (a) and the photoluminescence (PL) of the film with a peak at 536 nm in Figure 3.3.3 (b) reveals a bandgap of 2.31eV which is consistent with that reported in other work18. Time resolved photoluminescence (tr-PL) spectra taken at emission wavelength of 536nm±10nm for glass/CH3NH3PbBr3 (black); glass/c-TiO2/ CH3NH3PbBr3 (blue); and glass/CH3NH3PbBr3/spiro-OMeTAD (red)
63 together with the stretched exponential fit (in the absence of quencher) and diffusion model fits (in the presence of quenchers of TiO2 and spiro-OMeTAD) are shown in
Figure 3.3.3 (b). A near band edge peak observed in absorption, see Figure 3.3.3 (a) and in EQE curve of a complete device in Figure 3.3.4 (b) suggests exciton absorption which is feasible given a reported binding energy of 76meV19 for this material and similar diffusion coefficient and diffusion length for different charge species reported in this work. Using the models described by Equation 3.3.4, the carrier lifetime of the
CH3NH3PbBr3 film is experimentally determined to be τe = 51ns while the lifetimes of the CH3NH3PbBr3 film in the presence of quencher of TiO2 and Spiro-OMeTAD are
3.5ns and 3.1ns, respectively. A commonly used diffusion model as described by
Equation 3.3.5 is used to extract diffusion coefficient from the tr-PL data by calculating the distribution of photo-excited charge carriers using models described by Equations
20-22 3.3.6 to 9 . The diffusion length of CH3NH3PbBr3 estimated in this work is around
23 1um, higher than that reported . This is due to better crystallinity in the CH3NH3PbBr3 film with no CH3NH3Br and PbBr2 residuals as no solvent is involved in the vapour deposition of CH3NH3Br. However, this is not the case for the CH3NH3PbBr3 film
23 reported in literature , whereby CH3NH3Br residuals can be detected due to the complication of solvent involved the solution deposition of CH3NH3Br. The higher lifetime reported in this work compared to other work22 also contributes to better electrical characteristics of the complete device. The experimentally determined diffusion coefficients and diffusion length of the CH3NH3PbBr3 film in this work are summarized in Table 3.3.1.
64 The tr-PL were measured by microtime 200 microscope (Picoquant) using TCSPC technique with an excitation of 470 nm laser at 4 MHz repetition rate and a detection at
536 nm120.
The charge carrier diffusion length (LD) in the perovskite layer is calculated from the Eqn 3.3.3
LD = √Dτe Eqn 3.3.3
where τe is the recombination lifetime of charge carriers in the non-quench perovskite film and D is the diffusion coefficient. τe is determined by fitting tr-PL of the non-quenched perovskite film with a stretched exponential decay function as shown in the following:
t β I(t) = I0 exp (− ) Eqn 3.3.4 τe
In order to determine the diffusion coefficient, one-dimensional diffusion equation is used to calculate the number and distribution of the charge carrier density which is generated by the laser pulse 18-20
∂n(x,t) ∂2n(x,t) = D − kn(x, t) Eqn 3.3.5 ∂t ∂x2
where k is the PL decay rate of the perovskite film without any quenching layer and
−β β−1 k = βτe t , n(x,t) is the charge carrier density. The notation "x" represents the
65 distance from the perovskite surface to a point inside the perovskite layer and x=0 at the perovskite surface. The notation "t" is a time coordinate.
The 1-D diffusion coefficient has two boundary conditions. The first boundary condition assumes the photo-excited charge carriers are generated on the perovskite surface, the initial distribution of the charge carriers is expressed by the following equation:
−αx n(x, 0) = n0e Eqn 3.3.6
where α is the absorption coefficient of the perovskite film at 470nm. The second boundary condition assumes the quench process appears only at the TiO2/Perovskite or
Spiro-OMeTAD/Perovskite interface, and all the charge carriers are quenched in this interface. The boundary condition can be expressed as:
n(L, t) = 0 Eqn 3.3.7
where L is the thickness of perovskite.
The carrier density n(x,t) and the total charge density N(t), see Eqn 3.3.8 and 3.3.9 in supporting information can be solved by using 1-D diffusion equation and the boundary conditions.
Eqn 3.3.8
66 ∞ π2D n(x, t) = 2n exp(−kt) ∑ (exp(− (m 0 L2 m=0
m 1 1 2 (−1) exp(−αL) π (m + ) + αL 1 x + ) t) 2 cos (π (m + ) )) 2 1 2 1 2 L (αL)2 + π2 (m + ) (m + ) 2 2
Eqn 3.3.9
∞ 1 m 2n L π2D 1 2 exp(-αL) π (m+ ) +(-1) αL N(t)= 0 exp(-kt) ∑ ( exp (- (m+ ) t) 2 ) π L2 2 1 2 1 m=0 ((αL)2+π2 (m+ ) ) (m+ ) 2 2
Finally, the diffusion coefficient is obtained by fitting the N(t) and the tr-PL measured from the quenched interface.
67
Figure 3.3.3 (a) Absorption coefficient (black) and PL spectrum (red) of the
CH3NH3PbBr3 film. (b) tr-PL taken at emission wavelength of 536nm±10nm for samples with and without quenchers: glass/CH3NH3PbBr3 (black), glass/c-
TiO2/CH3NH3PbBr3 (blue) and glass/CH3NH3PbBr3 /spiro-OMeTAD (red). The solid lines represent the stretched exponential fit for glass/ CH3NH3PbBr3 PL data and diffusion model fits for the CH3NH3PbBr3 in the presence of quenchers.
Table 3.3.1 Experimentally determined diffusion coefficient (D) and diffusion lengths (LD) of CH3NH3PbBr3 film in this work.
Absorption Thickness Charge 2 -1 Coefficient at 휏푒 (ns) D (cm s ) LD (nm) (nm) species 470nm (cm-1)
Negative 0.2198±0.02 1058±48 6 104 48020 51 Positive 0.2301±0.02 1083±47
68 3.3.2 Photovoltaic devices fabricated by VASP and one-step solution process
Figure 3.3.4 (a) shows the cross-sectional SEM image of the complete device with the structure of FTO glass/c-TiO2/mp-TiO2/CH3NH3PbBr3/Spiro-OMeTAD/Au. To fabricate such structure, a 40nm of c-TiO2 hole blocking layer was spin-coated on a commercially available fluorine-doped tin oxide (FTO, Pilkington, TEC8) glass substrate. A 350nm thick mp-TiO2 film was then deposited by spin-coating a diluted colloidal anatase paste to form the electron-extracting scaffold. The light absorber,
CH3NH3PbBr3, was then fabricated by VASP method. After annealing, the Spiro-
OMeTAD was spin-coated as HTM before thermal deposition of gold contact.
69 Figure 3.3.4(a) SEM cross-sectional image (b) external quantum efficiency of a complete device.
As can be seen, a dense and uniform capping layer of CH3NH3PbBr3 325±75nm in thickness is formed above the mp-TiO2 scaffold layer. This method appears to be superior compared with the solution–based method whereby pillared overlayer has been observed 6 and uniformity is difficult to be controlled (Figure 3.3.5). The thickness of capping layer by one-step solution processes various from zero to 2 micrometers. Sharp protrusions can be observed in some areas, see Figure 3.3.5 (a). This leads to poor light absorption; shunting (where CH3NH3PbBr3 is absent) and high series resistance (where
CH3NH3PbBr3 is too thick).
70
Figure 3.3.5 Device SEM cross-sectional view of complete device using (a) one- step solution based method and (b) VASP deposition method.
The electrical characteristics of a solar device fabricated by VASP are shown in Figure
3.3.6 and Table 3.3.2. Averaged (from forward and reverse scans) conversion efficiency
2 of 8.7%, VOC of 1.45V, JSC of 9.75mA/cm , and fill factor of 61.5%. Results show that the device exhibits stronger hysteresis under fast scans. Compared with one-step solution processed CH3NH3PbBr3 device, see Figure 3.3.7 and Table 3.3.3, VASP deposited CH3NH3PbBr3 device exhibits smaller degree of hysteresis. The current and spectral response, see Figure 3.3.4 (b), demonstrated the effectiveness of the
CH3NH3PbBr3 prepared by the VASP method as a light absorber (with a bandgap of
2.31eV, consistent with that deduced from experimental absorption coefficient in Figure
3.3.3 (a)). A respectable VOC has been shown to be an indicator of film quality, in particular its surface coverage6, either in planar structure or as capping layer on a scaffold24, as supported by SEM evidence in this work. Open circuit voltage of
1.440.01V and averaged FF of ~62% have been achieved for some devices. The fill factor from this study demonstrates, that spiro-OMeTAD is effective in extracting holes
71 3 2 from the CH3NH3PbBr3 film. The high RSH of > 10 ohms-cm measured also indicates good film coverage, minimizing shunting paths between TiO2 and the spiro-OMeTAD that would otherwise be present. The EQE shown in Figure 3.3.4 (b) is not indicative of the high JSC reported as it was measured at lower illumination intensity. The purpose of the EQE spectrum was to discuss the exciton absorption peak at the band edge as above.
Therefore only the standard spectral response measurement set up for larger area crystalline silicon photovoltaic device with lower than one Sun monochromatic light was used without light bias.
Figure 3.3.6 Current density-voltage curves measured in opposite sweeping directions at a rate of (a) 3V/s, (b) 0.15V/s.
Table 3.3.2 Electrical characteristics of the same device as shown in Figure 3.3.6 measured under different scan speeds and sweeping directions.
Deposition Scan Voc Jsc FF RS (ohm- RSH (ohm- Eff. method used (V) (mA/cm2) (%) cm2) cm2) (%)
VASP 3V/s reverse 1.45 9.2 68 190 5.7×104 9.1
72 VASP 3V/s forward 1.45 10.3 55 3171 3.7×103 8.3
VASP 0.15V/s reverse 1.44 8.9 64 188 6.5×103 8.4
VASP 0.15V/s 1.43 9.4 60 4573 5.5×103 8.1 forward
Figure 3.3.7 Current density-voltage curve of CH3NH3PbBr3 devices by one-step solution process at scan rate of (a) 3V/s (b) 0.15V/s
Table 3.3.3 Electrical characteristics of the same device as shown in Figure 3.3.6 and 3.3.7 measured in opposite sweeping directions.
Deposition Scan direction Voc Jsc FF RS RSH Eff. method used rate (V) (mA/cm2) (%) (ohm- (ohm- (%) cm2) cm2)
VASP 3V/s reverse 1.45 9.2 68 190 5.7×104 9.1
73
forward 1.45 10.3 55 3171 3.7×103 8.3
0.15V/s reverse 1.44 8.9 64 188 6.5×103 8.4
forward 1.43 9.4 60 4573 5.5×103 8.1
1-step 3V/s reverse 0.79 4.2 50 355 9.1×103 1.7 solution
forward 0.71 4.4 36 2211 3.2×103 1.1
0.15V/s reverse 0.80 4.3 51 288 9.6×103 1.7
forward 0.71 4.4 36 2211 3.2×103 1.1
3.4. Conclusion
To summarize, a VASP method for fabricating CH3NH3PbBr3 perovskite film for solar devices was demonstrated. The films exhibit long carrier diffusion length exceeding 1μm. Solar device employing this film has been demonstrated achieving averaged (from forward and reverse scans) conversion efficiency of 8.7%, Voc of
2 1.45V, JSC of 9.75mA/cm , and fill factor of 61.5%. In comparison with a one-step solution process, the VASP process results in better film quality in terms of crystallinity, uniformity, coverage and light absorption, hence resulting in better photovoltaic performance in the associated devices. The high voltage output of this device makes it a good candidate for tandem solar cell application. Further work to
74 improve the VASP perovskite solar cell performance involves the modifying the interfaces of charge transport layers and perovskite with the aim of increasing the charge transportation by improving the charge separation and collection, as well as reducing the charge transport resistance. This will result in an increase in the current output and fill factor. Additionally, alternative cell configurations should be studied such as the use of planar structure to simplify the fabrication process, increasing the prospects of commercialization.
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79 4. MATERIAL CHARACTERIZATION FOR
CH3NH3PbIXCl3-X AND CH3NH3PbBr3 PEROVSKITE
In this chapter, three material characterization studies are presented. The first study uses one-photon and two-photon fluorescence microscopy in conjunction with lifetime measurements to investigate the morphology and its impact on carrier extraction of organic-inorganic metal halide CH3NH3PbIXCl3-X perovskite by electron and hole transport layers. It was demonstrated that one- and two-photon fluorescence microscopy is a promising technique for studying the morphology and its impact on carrier transport for perovskite cells. The second material characterization study reports fluorescence intermittency - “blinking” observed in VASP CH3NH3PbBr3. The results provide evidence of the effect of mobile charges (ions) and the accumulation of these charges causing an Auger-like non-radiative behavior contributing to the fluorescence quenching “off” state in the blinking. The third material characterization study is photoluminescence characterization of dynamic aging process of VASP CH3NH3PbBr3 perovskite. Together with fluorescence lifetime imaging microscopy (FLIM), SEM,
XRD, ss-PL, TRPL and fluorescence imaging demonstrate that, VASP CH3NH3PbBr3 crystallises spontaneously at room temperature in air without any annealing. The small grains can merge into the larger grains at room temperature in air. Such a slow process does not result in decomposition but a slow recrystallization.
The first study was performed on solution processed CH3NH3PbIXCl3-X with different interface layers while the second and third studies were done on VASP
CH3NH3PbBr3. Further similar studies will be extended to other perovskite materials.
80 4.1. Morphology and Carrier Extraction Study of CH3NH3PbIXCl3-X
Perovskite by One- and Two- Photon Fluorescence Microscopy
4.1.1 Introduction
It has been shown that the photovoltaic performance of perovskite based solar devices is greatly dependent on the film morphology which itself depends on the deposition technique and subsequent treatment.1-5 Poor perovskite morphology is detrimental to device performance because it not only causes electrical shorting but also deleteriously impacts light harvesting; charge dissociation; transport; and
3,6-8 9-11 recombination, particularly in planar junction devices. The large CH3NH3PbI3 grains and incomplete coverage resulting in pin-holes have been observed in films prepared by non-optimized conventional spin-coating methods,12,13 such as slow crystallization due to the high boiling point of DMF (N,N-dimethylformamide, 152 Cº), and slow crystal growth due to slow nucleation rate during drying process after spin- coating.
The morphology of perovskite is usually investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) 1-4,13,14, which requires special specimen preparation and results in sample damage during scanning by the high energy incident beam. The inevitable exposure of the perovskite material to air and moisture makes it susceptible to decomposition. In contrast, fluorescence imaging via one-photon (1P) or two-photon (2P) excitation, is contactless and can be done on films with a surface-protecting layer. It does not require special specimen preparation and does not cause film damage due to the absence of high-energy beam scanning14. Laser scanning confocal microscope is capable of submicrometer axial resolution and therefore suitable for three-dimensional optical sectioning15, especially in conjunction
81 with 2P excitation due to longer absorption depth. Under the 2P excitation, two coherent photons with energies less than the band-gap of the material under investigation can be absorbed by the material if the excitation density is sufficient16. The absorbed photons then excite electrons into the conduction band resulting in an emission of band edge photons16,17. It should be noted that the process of two-photon absorption is sensitive to the excitation density related to the material’s nonlinear absorption coefficient18,19. The
2P fluorescence imaging uses the infrared laser causing less damage to the sample18,19
Confocal microscopy can be conveniently extended by fluorescence lifetime imaging
(FLIM) modality allowing for microenvironment sensing studies through excited state lifetime monitoring and providing unique insight into the electron-hole dynamics in nanoscale region. Therefore, fluorescence imaging can further reveal regimes of electron-hole recombination providing insights into the carrier recombination mechanisms in the material or device.
In this work, one-photon and two-photon fluorescence microscopy was used in conjunction with lifetime measurements to investigate the morphology and its impact on carrier extraction of organic-inorganic metal halide CH3NH3PbIXCl3-X perovskite by electron or hole transport layers, see Figure 4.1.1. In addition, the images obtained from
1P and 2P fluorescence imaging allow the separation of surface and bulk effects of carrier extraction and recombination providing useful information for device optimization. It was found that grain boundaries and imperfect interface between perovskite and electron or hole transport layer are detrimental to carrier extraction but the severity varies depending on the type of transport layer used.
82
Figure 4.1.1 Illustration of one- and two- photon excitation fluorescence imaging.20
Adapted by permission from Macmillan Publishers Ltd., Scientific Reports, copyright
2013.
4.1.2 Results and Discussion
It has been reported the CH3NH3PbIXCl3-X perovskite exhibits efficient
21-23 fluorescence . Figure 4.1.2 (a) shows the fluorescence spectrum of CH3NH3PbIXCl3-
X perovskite excited at 405 nm and measured by a spectrometer, with a Si-CCD detector24, as well as fluorescence spectra of the same material obtained by a Leica TCS
SP5 microscope using 488 nm one-photon excitation and 950 nm two-photon excitation. The fluorescence spectra in the low energy side from the microscope were attenuated due to the low sensitivity of the detector in the near infrared. The perovskite exhibits fluorescence maximum at 770 nm. This is consistent with other observations demonstrating CH3NH3PbIXCl3-X as a direct band gap material with fluorescence independent of the excitation wavelength. Therefore, perovskite based solar cells are suitable to be investigated by using the fluorescence microscope. By using a train of 950 nm femtosecond laser pulses we observed a similar fluorescence spectrum under the 2P excitation to that measured under 1P excitation, indicating effective two photon
83 fluorescence as clear 2P imaging can also be obtained at modest excitation power. This suggests that perovskite has a large two-photon absorption coefficient suitable for application based on 2P absorption and fluorescence.
Figure 4.1.2(a) The PL spectra of the CH3NH3PbIXCl3-X perovskite measured by a
Si-CCD-spectrometer (one-photon, blue) with 405 nm excitation; and measured by a
Leica TCS SP5 microscope with an excitation of 488 nm (one-photon, black) and 950 nm (two-photon, red). (b) The PL decay traces of CH3NH3PbIXCl3-X perovskite (blue), perovskite/PCBM (black) and perovskite/spiro-OMeTAD (red) from a 50 x 50 µm2 region excited by 470 nm 5 MHz pulsed laser.
The photoluminescence (PL) decay traces were measured using time correlated single photon counting (TCSPC) technique excited at 470 nm. Figure 4.1.2 (b) shows the PL decays of perovskites before and after the addition of electron (PCBM) and hole
84 (spiro-OMeTAD) transport layers in test structures as depicted in Figure 4.2.1. The perovskite exhibits nearly single exponential decay with a respectable lifetime of 79 ns
(indicating sufficient quality perovskite film for achieving 12% conversion efficiency for an associated solar device. When covered with the hole (spiro-OMeTAD) and electron (PCBM) transport materials, the lifetimes become significantly shorter, indicating effective carrier extraction or “quenching” by the transport materials, a key factor in high conversion efficiency in associated perovskite solar devices23,25-27. It is interesting to note from bi-exponential fitting that in addition to the fast components, slower components with lifetimes of 109 ns and 93 ns are present for the perovskite/PCBM and perovskite/spiro-OMeTAD structures, respectively. The long lifetime component suggests the existence of unquenched perovskites. The lifetimes
(circa 100ns) of these unquenched perovskites are longer than that (circa 80ns) of the perovskite in the absence of transport layers because the carrier densities in the unquenched perovskites are lower resulting in lower carrier recombination rates. This is consistent with previous studies showing that the recombination rate is dependent on the carrier density in the perovskite25-28.
Figure 4.1.3 shows the one- and two-photon excited fluorescence images of the perovskite film before ((a) & (b)) and after ((c) & (d)) the addition of electron quenchers, detected with a 750/40 nm band-pass filter. High spatial resolution is demonstrated using the two-photon excited fluorescence imaging. It should be noted that under one-photon excitation (488nm), the penetration depth is within the first
65nm. Therefore fluorescence originates from the vicinity of perovskite surface or quencher/perovskite interface with negligible contribution from the bulk. On the other hand, under two-photon excitation with longer wavelength absorbed deeper in the perovskite layer, bulk effects can be observed from the fluorescence response. Under
85 both excitation regimes, there is negligible fluorescence contribution from the spiro-
OMeTAD or the PCBM. The images show the comparable morphology characteristics of the perovskite film before and after the addition of electron or hole quenchers (Figure
4.2.3 (e) and (f)). With the presence of perovskite grains as well as voids or pinholes
(dark regions). The presence of pinholes in perovskite solar cells is undesirable and results in low conversion efficiency3. Bright spots observed in the void or in the vicinity of the perovskite grain boundaries in the one-photon image, Figure 4.1.3 (a), suggest recombination activities. These bright spots are most likely isolated small grains from imperfect grain formation of perovskite. However, the bright spots were not observed in two-photon image suggesting these recombination activities are localized at the surface.
86
Figure 4.1.3 One-photon (1P; 488 nm excitation) and two-photon (2P; 950 nm excitation) fluorescence imaging with 750/40 nm band-pass filter, (a) 1P and (b) 2P images of perovskite/glass; (c) 1P and (d) 2P fluorescence images of
PCBM/perovskite/glass. (e) 1P and (f) 2P fluorescence images of spiro-OMeTD
/perovskite/glass.
Interference fringes are observed in the 2P images, Figure 4.1.3 (b) & (d) due to interference between the fluorescence directly from the excitation point and the
87 fluorescence reflected at the perovskite/glass interface29. The bright outlines along the grain boundaries as observed in Figure 4.1.3 (b) & (d) are due to the extra detected fluorescence reflected by the vertical face of the grain boundaries and then incident on the objective. These optical effects can be effectively removed after the addition of thick epoxy covering layer, see Figure 4.1.4 (b).
Figure 4.1.4(a) One- photon and (b) two-photon fluorescence images of epoxy/higher concentration spiro-OMeTAD/perovskite/glass.
1P and 2P fluorescence images of the epoxy/spiro-OMeTAD/perovskite/glass test structure are shown in Figure 4.1.4. Few interesting observations can be made by comparing images between Figure 4.1.3 and Figure 4.1.4. The first observation is indicated by the appearance of bright spots within the poor quality grain in Figure 4.1.3
(c). The poor quality grain in turn is a result of film non-uniformity (characterized by disruptions to the interference effects within the grains in Figure 4.1.3 (d)) which is not uncommon as the film is fabricated using conventional spin coating and therefore susceptible to film non-uniformity30. Although no significant increase in bulk recombination is observed in these non-uniform regions (absence of significantly brighter fluorescence response under 2P imaging), interface recombination is adversely affected, characterized by the appearance of bright spots in Figure 4.1.3 (c) in the non-
88 uniform regions. The perovskite film non-uniformity can lead to PCBM film non- uniformity and/or lead to poor perovskite/PCBM interface. Either effect can result in carrier recombination before they are extracted by PCBM.
On the other hand, spiro-OMeTAD covered perovskite exhibit different behavior whereby interface recombination is most significant along the grain boundary (see bright regions in Figure 4.1.4 (a)) and less affected by film non-uniformity which only affects the bulk recombination (see Figure 4.1.4 (b)). The dark regions within the grains in Figure 4.1.4 (a) indicate efficient hole extraction suggesting good spiro-
OMeTAD/perovskite interface can be generally achieved within the perovskite grain regardless of film uniformity.
Apart from the ability to resolve surface and bulk effects, another advantage of the fluorescence imaging technique is the ability to carry out local lifetime measurements.
Figure 4.1.5 shows the PL decay traces of the dark and bright regions (0.5 x 0.5 µm2) of spiro-OMeTAD/perovskite and PCBM/perovskite test structures which separates the binary components extracted from Figure 4.1.2 (b). The fast components in Figure 4.1.2
(b) correspond to the dark regions of the fluorescence images as a result of efficient carrier extractions by the quenchers while the slower components correspond to the brighter regions whereby un-quenched carriers are left to recombine.
89
Figure 4.1.5 The PL decay traces of “bright” and “dark” regions in
CH3NH3PbIXCl3-X perovskite covered by PCBM and Spiro-OMeTAD
4.1.3 Conclusion
In this study, it was demonstrated that one- and two-photon fluorescence microscopy is a promising technique for studying the morphology and its impact on carrier transport for perovskite cells. The two-photon images exhibit higher spatial definition with the extra advantage of depth definition while one-photon images provide the information of the top surface or interface. Therefore the combination of two imaging techniques is effective at separating the surface and bulk effects of carrier transport and recombination. This technique is also effective in separating components in PL decay traces that correspond to “good” and “bad” regions of the test or cell structure. This is most crucial for understanding the operation of perovskite solar cells and the optimization of the associated solar cell devices. It is shown that grain boundaries and imperfect interface between perovskite and electron or hole transport layer are detrimental to carrier extraction. It is shown that the PCBM fabricated in this
90 work is more sensitive to film non-uniformity while spiro-OMeTAD is more sensitive to grain boundaries in terms of effective carrier extraction.
4.2. Grain Formation and Mobile Charge Induced Fluorescence
Intermittency in vapour-assisted CH3NH3PbBr3 Perovskite films
4.2.1 Introduction
Fluorescence intermittency, also referred as to blinking, randomly switching between states of high (ON) and low (OFF) emissions, is a universal property of molecular emitters found in dyes, polymers, biological molecules as well as artificial nanostructures, such as nanocrystal, quantum dots, carbon nanotubes and nanowires47-49.
Fluorescence blinking has been extensively investigated in semiconductor nanoparticles and organic molecules, providing unique insight into their photoexcited carrier dynamics47,50-55. Based on confocal microscopy and super-resolution techniques, single molecule spectroscopy provides a powerful tool to investigate the fluorescence behavior and carrier dynamics in a single nanoparticle.
This study investigated the grain formation process of vapour-assisted
CH3NH3PbBr3 perovskite film. Additionally, this section also presented the fluorescence intermittency observed in film and isolated nanoparticles. It has been found that blinking is present in the CH3NH3PbBr3 perovskite film whereby nanoparticles are in close contact with one another. On the other hand, blinking is not present in isolated CH3NH3PbBr3 nanoparticle. The time correlated single photon counting (TCSPC) carried out under various excitation densities indicates Auger recombination from charge accumulation exemplified by the mobile charge migration in particular under high excitation is responsible for the blinking.
91 4.2.2 Results and Discussion
The samples of CH3NH3PbBr3 films and isolated nanoparticles used in this study were fabricated by vapour-assisted deposition56 on a glass substrate in the absence of any inter-layer or quencher. In short, PbBr2 solutions in DMF with various concentrations were spin-coated on cleaned glass substrate, after annealing at 70 ⁰C for
30 min, films were treated with MABr vapour at 175 ⁰C for 10 min, and then substrate were rinsed in isopropoxide followed by drying in N2 stream. The isolated nanoparticles were fabricated by the same procedures except using diluted PbBr2 solution. The grain size can be estimated to be 150-250 nm by SEM image in Figure 4.2.1 (a) and (b). The
XRD pattern in Figure 4.2.1 (c) indicates that the CH3NH3PbBr3 perovskite film is well crystallized. The SEM and optical images for the CH3NH3PbBr3 perovskite film and isolated nanoparticles are shown in Figure 4.2.2. In particular, a nano-granular structure can be clearly seen in optical image in Figure 4.2.2 (b).
92 Figure 4.2.1 SEM images of (a) the top surface and (b) the cross section of VASP
CH3NH3PbBr3 film, (c) XRD pattern and (d) absorption coefficient and the photoluminescence (PL) spectrum with a peak at 536 nm of the CH3NH3PbBr3 film.
Figure 4.2.2(a) SEM and (b) fluorescence image of vapour-assisted deposited
CH3NH3PbBr3 film, (c) and (d) SEM images of isolated CH3NH3PbBr3 nanoparticles.
As shown in Figure 4.2.3, by changing the PbBr2 solution concentration, we are able to tune the desired perovskite coverage. The lowest coverage was obtained by using 1mM PbBr2 spin-coated film, followed by CH3NH3Br treatment for 10min. The film with the lowest coverage shows that, the isolated nanoparticles ranging from 30-
100nm are formed; as increasing the PbBr2 concentration to 10mM, instead of forming a lager particle or grain, the nanoparticles are conglomerate to clusters; for the 100mM vapour-assisted deposited film, the clusters are emerged together to form grains; and the grain packed more densely when the PbBr2 concentration increased to 1M. However, if the nanoparticle/ nanoparticle interface still exist in the formed grains.
93
Figure 4.2.3 Top surface SEM images of vapour-assisted CH3NH3PbBr3 films with different coverage.
Spectroscopy images with time stamp in Figure 4.2.4 are taken by using electron beam hit the perovskite film surface, when electrons approaching the nanoparticle, they recombine with photoexcited holes, the nanoparticle is in the state of “off”, when there is no electrons around the nanoparticle, the photoexcited holes recombine with photoexcited electrons, which results photon emitting, the nanoparticle is in the state of
“on” in this case. The schematic description of this process is shown in Figure 4.2.5.
94
Figure 4.2.4 Spectroscopy images with time stamp
Figure 4.2.5 Schematic diagram of blinking process in vapour-assisted
CH3NH3PbBr3 film.
A strong fluorescence PL peak can be observed at 536 nm, see Figure 4.2.1 (d), consistent to other observations57-59. The laser illumination induced degradation can be
95 excluded because the PL measurement can be performed repeatedly. At the highest excitation intensity, the PL spectra keep identically before and after the illumination; the
PL intensity can be repeated after keeping the sample in the dark for a few minutes.
The fluorescence was observed as function of time in both CH3NH3PbBr3 film and isolated nanoparticles in Microtime-200 confocal microscopy system under an excitation of 470 nm and detected through a bandpass filter at 536 nm. Figure 4.2.6 shows the time traces of the fluorescence intensity of an isolated nano-particle and a single point of the CH3NH3PbBr3 film respectively in Figure 4.2.6 (a) and (b); and their corresponding fluorescence microscopy images respectively in Figure 4.2.6 (c) and (d).
As evident in the time trace using the same microscopy system under a continuous excitation of 470 nm, blinking can be observed. It is interesting to note that the fluorescence blinking was only observed in the CH3NH3PbBr3 film. In contrast, no fluorescence blinking is observed in isolated nanoparticles.
96
Figure 4.2.6 Time traces of the fluorescence intensity of (a) an isolated nanoparticle at different excitation intensities; and (b) a single point of the CH3NH3PbBr3 film.
Fluorescence microscopy images of (c) isolated nanoparticles and (d) the CH3NH3PbBr3 film.
The fluorescence occurrence of ON and OFF periods in nanoparticles has been usually attributed to the presence of an additional charge, which results in fluorescence quenching by non-radiative Auger recombination50,51. The charge induced blinking, ON and OFF event probability density, can be described by a truncated power-law dependence52,53:
m P( t ) t exp( t / ) Eqn 4.2.1
where m is exponent and is truncation time (or saturation time). The exponent m of some semiconductor nanoparticles studied represents characteristics of blinking. The blinking in semiconductor single nanoparticle has been intensively studied and
97 exponent m has been shown different from the ideal -1.5 due to dispersive diffusion correlation times, and related to temperature, intensity and the size and shape of nanoparticles60-64.
In this work, a 470nm laser is used for the excitation source with a NA1.4 oil objective. Figure 4.2.7 shows the PL time traces of the CH3NH3PbBr3 perovskite film, under low (80 mW/cm2) and high (2400 mW/cm2) excitation intensities in Figure 4.2.7
(a) and (b) respectively. It should be noted that the occurrence of fluorescence ON event
(whereby the fluorescence intensity is above 8 counts) is higher under low excitation.
Using equation 1, its probability density is obtained, see Figure 4.2.7 (c) and the exponent mon is found to be dependent on excitation intensity, see Figure 4.2.7 (d). At lower excitation intensity, more ON events can be confirmed; therefore the ON probability is relatively larger with a small magnitude of the exponent mon. At very low excitation density (< 60 mW/cm2), the PL intensity is very weak that no blinking is observed. With increasing excitation intensity, the ON event decreases and an increased mon is obtained, from ~ -0.4 to ~ -1.5. Further increasing the excitation intensity, the mon does not change evidently, staying at ~ -1.5.
98
Figure 4.2.7 Time traces of fluorescence by the CH3NH3PbBr3 film under (a) low
(220 mW/cm2) and (b) high (1000 mW/cm2) excitation intensities. (c) The ON event probability density as a function of time under low and high excitation intensities. (d)
The exponent mon as a function of excitation intensity.
When formation of the condensed CH3NH3PbBr3 film, a large number of nanoparticles are accumulated together, which results in free electrons and/or holes that can easily drift among the nanoparticles, referred as to mobile charges65,66. These charges can accumulate on the surface of nanoparticles, also as grain boundaries, and results in enhanced Auger non-radiative recombination, that is OFF state. In this case, the ON and OFF states will be relevant to the density of mobile charges and mobility among the nanoparticles. It has been shown that the electron and hole of CH3NH3PbBr3 perovskite exhibit similar mobilities and the diffusion length is as long as micron56, which facilitates the charge drift between the nanoparticles.
99 Fluorescence blinking is not present in isolated nanoparticle as fluorescence is continuous in these isolated nanoparticles under continuous illumination. This confirms that the CH3NH3PbBr3 has balanced mobilities between electrons and holes that would otherwise cause blinking in isolated nanoparticle.
The blinking observed in this work is in the millisecond timescale, which is of a much slower dynamic process. This is very similar to the slow transient processes observed in other works in similar timescales which are attributed to the presence of mobile ions67-70. The density and lifetime of the mobile charges would also increase upon increasing excitation intensity as their migration and accumulation at the surfaces and grain boundaries are enhanced further enhancing Auger-like behavior.
4.2.3 Conclusion
In summary, the grain formation process in vapour-assisted CH3NH3PbBr3 perovskite film was investigated, and the fluorescence intermittency present was observed. The CH3NH3PbBr3 perovskite film is composed of a closely packed nanoparticles facilitating photoexcited charge migration between nanoparticles and accumulation at the surface of the nanoparticles which results in enhanced Auger-like non-radiative behavior contributing to the fluorescence quenching (OFF state). The ON probability of fluorescence is dependent on the excitation intensity and exhibits a similar power rule to semiconductor quantum dots at higher excitation intensity. In contrast, fluorescence intermittency does not present in isolated nanoparticles due to similar effective masses of electrons and holes in CH3NH3PbBr3 perovskite and the absence of mobile charges. This finding provides unique insight into the charge accumulation and migration, and thus is of crucial importance for device design and improvement.
100 4.3. Photoluminescence Characterizations of Dynamic Aging Process of
Organic-inorganic CH3NH3PbBr3 Perovskite
4.3.1 Introduction
Rapidly improving power conversion efficiencies of 22.1%77 and 10.4%71 have been reported for methylammonium lead iodide and bromide perovskite solar cells, respectively. Moreover, organic-inorganic perovskite materials have been demonstrated as promising candidates for application in efficient light emitting diodes (LEDs) and on- chip coherent light sources due to their high photoluminescence (PL) quantum efficiencies and wavelength-tunable lasing performance21,78,79. However, the relatively poor stability of the perovskite materials is one of the largest barriers towards their commercialization80-82. Significant efforts have been made to understand the degradation of devices and materials, with many reports focused on device performance versus time and humidity83-86. Zhou et al. recently reported that by exposing the perovskite solar cell fabrication process to a low level of humidity (30%), it was possible to achieve 19.8% power conversion efficiency87. Another study found that using P3HT/ SWNTs-PMMA as the hole transport material (HTM) results in improved thermal stability and resistance to moisture88. There have also been several studies focusing on the perovskite material itself89-91 . In particular, enhanced crystallinity has
90 been observed when a CH3NH3PbI3-XClx film is stored under Ar , and in mixed iodide- bromide perovskites reversible light-induced trap formation has been observed92.
Although of great importance and effort, the comprehensive physical understanding of the aging process in organic-inorganic perovskites has not yet been available.
Specifically, nanoscale investigations can provide unique insights into physical phenomena occurring within the perovskite, which may be concealed by large area
101 averaging. Fluorescence lifetime imaging microscopy (FLIM) provides nanoscale spatially- and nanosecond lifetime- resolved morphology. This technique is very suitable for perovskite solar cells, for which the solar cell performance is closely correlated with the perovskite grain morphology. In this work, we study the dynamic aging processes in CH3NH3PbBr3 films, using steady state photoluminescence (SS-PL), time resolved PL (TRPL), FLIM and fluorescence imaging microscopy. Our results reveal that the perovskite grains keep growing spontaneously at room temperature and the larger grains are formed. We show that recrystallization during aging leads to an increased density of defects and a decreased carrier lifetime, specifically in larger grains.
4.3.2 Experimental Detail
CH3NH3Br was synthesized by mixing methylamine (33% in methanol, Sigma-
Aldrich) with hydrobromic acid (48% in water, Sigma-Aldrich) in a 1:1 molar ratio in a
250 ml round bottom flask under continuous stirring at 0 ºC for 2 h. The precipitate was recovered by rotary evaporation at 60 ºC, and washed three times with diethyl ether in an ultrasonic bath for 30 min. The final product was collected after dehydration at 60 ºC and dried further in a vacuum chamber overnight.
CH3NH3PbBr3 films were deposited using the VASP method, which was originally
93 developed by Yang Yang et. al . PbBr2 (1 M in DMF) was spin-coated on to a glass substrate at 2000 rpm for 60 s. After annealing at 70 ºC for 30 min, the film was treated by CH3NH3Br vapour for 10 min by suspending above CH3NH3Br powder in a closed glass petri-dish on a hotplate at 175 ºC. The treated film was then rinsed in isopropanol at room temperature. All processing was done in a N2 glovebox.
X-ray diffraction (XRD) patterns were measured using a PANalytical Xpert
Materials Research Diffractometer system with a Cu Kα radiation source (λ=0.1541 nm)
102 at 45 kV and 40 mA. Scanning electron microscope (SEM) images were taken by an
FEI Nova Nano SEM230 with TLD mode, at 3 kV and with a spot size of 2.5, SEM samples were coated with Chromium to reduce charging. Steady state PL spectra were recorded using a spectrophotometer comprising a 405 nm laser as the excitation source and a thermoelectrically cooled Si CDD detector. Fluorescence images were taken using a modified Leica TCS SP5 microscope. A 488 nm continuous-wave laser was used for excitation. The PL decay traces and fluorescence lifetime imaging spectroscopy were measured by a microtime200 microscope (Picoquant) using TCSPC technique with laser excitation at 470 nm and detection at 536 nm. The experiment was undertaken at room temperature.
4.3.3 Results and Discussion
To monitor dynamic processes during aging of perovskite films, five
CH3NH3PbBr3/glass samples were fabricated by the VASP method in the same batch and using the same bottle of solution94. The films then went through different storage or aging conditions and were characterized by electron microscopy, optical microscopy, X- ray diffraction (XRD), photoluminescence measurements and fluorescence imaging.
Figure 4.3.1 shows SEM images of the six samples from the same batch. Figure
4.3.1 (a) shows the sample when it is freshly made while Figure 4.3.1 (b) shows the sample after 2 weeks of storage in N2. Figure 4.3.1 (c); (d); (e) and (f) show the samples after 1 day; 2 days; 1 week; and 2 weeks of aging in air where the relatively humidity
RH = 50%-60% and the temperature T = 25 ºC. Very little change in morphology and crystal size can be observed for the sample after 2 weeks of storage in N2. Similar result was observed in XRD patterns which are shown in Figure 4.3.2 (a). These results reveal that N2 suppresses any change in morphology and crystallinity.
103
Figure 4.3.1 SEM top-view images of CH3NH3PbBr3 films (a) when it is freshly made (b) after 2 weeks of storage in N2, (c) after 1 day; (d) 2 days; (e) 1 week; (f) 2
104 weeks of aging in air where relatively humidity RH=50% to 60% and temperature T =
25 ºC. (g) Histogram of grain size at different stages.
In contrast, storage in air does have an effect on grain size. In Figure 4.3.1 (c) to (f), small grains can be seen to merge with each other to form larger grains during storage in air, and there are stretch marks on these large grains, parallel with grain boundaries.
From day 7 to day 14, the stretch marks grow into contours, which have also been observed in over-annealed perovskite films3. A statistical analysis of the grain sizes present in these films is shown in Figure 4.3.1 (g).
XRD was performed on these samples to check for decomposition of CH3NH3PbBr3 and changes in crystallinity. Figure 4.3.2 (b) shows the XRD patterns of CH3NH3PbBr3 films before and after aging and storage in N2 (for 2 weeks). The peaks at the 2θ values:
15.2°, 21.4°, 30.4°, 33.9°, 37.9°, 43.4°, 46.0°, correspond to the (101), (121), (040),
95 (141), (240), (242) (060) reflections of cubic crystalline CH3NH3PbBr3 . No additional peaks are detected from all aged samples. Remarkably, higher XRD counts are obtained from samples exposed to ambient air for longer periods of time. This can be attributed to enhanced crystallinity. A similar phenomenon has also been observed in mixed iodide -chloride perovskite films90.
To check whether the luminescence of the films changes due to aging, the steady state PL (SSPL) was measured for each sample. Figure 4.3.2(c) shows that each sample exhibits very similar SSPL, with no obvious change in peak wavelength or peak width.
Furthermore, no additional PL peaks were observed between 450 and 1100 nm; for
96 example, PbBr2 has a PL peak at 2.5eVi , which would be present if PbBr2 was generated during the aging process. In contrast, there are changes in PL lifetime and PL intensity at different stages of the aging process. This is illustrated in Figure 4.3.2 (e), where lifetime values were obtained by fitting the PL decay traces in Figure 4.3.2(d).
105 These figures show that both the PL intensity and the effective lifetime decrease with increasing aging time, which together show that the PL quantum yield decreases with aging.
106
Figure 4.3.2 XRD patterns of (a) freshly made and N2 stored (2 weeks) samples, and (b) air stored samples. (c) Steady-state PL of the CH3NH3PbBr3 film at different
107 aging stages. (d) PL decay traces and (e) Change in PL intensity and PL lifetime for the films over time.
To begin to understand what is occurring on the nano-scale, we measured the fluorescence images. Figure 4.3.3 (a) and (b) show the fluorescence images of N2 stored and aged (2 weeks) CH3NH3PbBr3 films. They agree well with the SEM images in Figure 4.3.1. The N2-stored sample exhibits smaller grains with higher uniformity, while aged samples show the results of grain growth. It is interesting to note that there are several larger grains which exhibit lower PL intensity and so appear darker in the aged sample (Figure 4.3.3(b)). In contrast, the N2 stored sample exhibits fairly uniform
PL intensity.
Figure 4.3.3 Fluorescence image of (a) N2 stored (2 weeks) and (b) air stored (2 weeks) CH3NH3PbBr3 film.
To obtain further insight into the microscopic mechanism of the aging process, we performed FLIM for the N2 stored and aged samples based on a confocal microscope.
Figure 4.4.4 (a) and (b) show the FLIM images of N2 stored and aged (2 weeks) samples under 470 nm excitation and 536 nm detection. The N2 stored sample exhibits small, uniformly-sized grains, with uniform and long PL lifetimes while the aged
108 sample exhibits larger grains with lower PL intensity and shorter PL lifetimes. These observations are consistent with the SEM and optical images in Figure 4.3.1 and Figure
4.3.3, respectively. Figure 4.4.4 (c) compares the PL decay of the same N2 stored and aged samples averaged over an area. The PL decay of the aged sample exhibits a fast component which is associated with defect trapping41,97.
Figure 4.3.4 (d) compares the PL decays of a bright point (from a smaller grain) and a dark point (from a larger grain) from Figure 4.3.4(b), and the average from the observed area. Again the larger grain exhibits a fast decay component (defect trapping), and the small grain exhibits a longer lifetime and a higher PL intensity, similar to the N2 stored sample.
25,97-99 The carrier dynamics in perovskite can be usually expressed as :
푑푛 = 퐶 푛 + 퐶 푛2 + 퐶 푛3 퐸푞푢푎푡푖표푛 4.3.1 푑푡 1 1 1
where the terms correspond to defect trapping (Shockley-Read-Hall recombination via subgap trap states), free electron-hole recombination (bimolecular) and Auger recombination, respectively. Under low excitation intensities, defect trapping and electron-hole recombination are dominant. The observed faster decay in the darker grains implies a faster defect trapping time which in turn implies a higher defect density27,100.
109
110
Figure 4.3.4 FLIM images of (a) N2 stored and (b) aged (2 weeks) perovskites. The scale bar is in nanoseconds; (c) PL decay traces of the same N2 stored and aged (2 weeks) samples averaged over a 20×20 μm2 area, and (d) the PL decay traces from a small, bright point and a large, dark point from Figure 4.3.4 (b).
At low excitation the PL intensity exhibits super-linear increase with increasing excitation intensity
The intensity of steady state PL (SSPL) was measured as a function of excitation intensity in N2 stored and aged (2 weeks) samples. To minimize the impact of continuous excitation, the samples remain in the dark except for during the PL measurement which lasted for only a couple of seconds. Figure 4.3.5 shows the PL intensity vs. excitation intensity in a double-logarithmic scale. Here in the low excitation regime the PL intensity exhibits a superliner increase with increasing excitation intensity97, due to the correlation between the defect trapping and bimolecular recombination. The slope, k, of the super linearity in double-logarithmic coordinates has been shown to closely correlate with the ratio of the trap-state density to the depopulation rate of trapped states101, and for a general single photon excitation, where
k II 16,19,32,102,103 the PL intensity PL EX ,this slope should be approximately equal to 1 if the quantum yield remains constant. For a steady state PL measurement in the low
111 excitation range, the defect trapping and bimolecular recombination are two only possible recombination mechanisms, therefore the PL intensity is the difference of the total excitation intensity and the defect trapping. Thus, the increment of PL intensity with increasing excitation intensity can be expressed as
푑퐼푃퐿 = 퐶휂휎퐼퐸푥 ∙ (1 + 푓푆) ∙ 푑퐼퐸푥 퐸푞푢푎푡푖표푛 4.3.2
where 퐼퐸푋 , η, δ are excitation intensity, absorption efficiency and cross section,
104,105 respectively; C is constant , fs is the saturation filling factor of the defect states which is correlated with the excitation intensity, defect density and relaxation rate of the defect states. With increasing excitation intensity, the trapping by defect states will decrease due to saturation filling and slow relaxation of the defect states, which results in the super-linear increase of the electron-hole radiative recombination.
The quantum yield depend on the competition between radiative recombination
(bimolecular) and nonradiative recombination (defect assisted and Auger). For a general single photon excitation PL the slope should be approximately equal to 1 in double-logarithmic coordinates if quantum yield remains constant. The larger than one power factor is ascribed to the increased contribution from bimolecular recombination with increasing excitation intensity, because radiative recombination and defect trapping are quadratic and linearly proportional to the excitation and due to the saturation of the trapping states101. At low excitation, the PL intensity exhibits a super-linear increase with excitation intensity in log-log representation, suggesting that the weight of radiative recombination significantly increases, defect state trapping has decreased proportionately 101 resulting in increased quantum yield (0.03,
0.1 and 0.8 at excitations of 1, 10 and 100 mW/cm2 for the fresh sample and 0.015, 0.08 and 0.8 at 1, 10 and 100 mw/cm2.for the aged sample).
112
Figure 4.3.5 PL intensity as a function of excitation intensity for (a) N2 stored and
(b) aged (2 weeks) perovskite samples. Both figures are plotted on logarithmic scales so that the power factors can be obtained by linear fitting in the low excitation range.
The slopes of the PL vs excitation intensities are 1.625 and 1.740 for the N2 stored and aged samples, respectively. For samples of the same composition and with an identical fabrication method, it is reasonable to assume the species of the defect states and thus the relaxation rate are the same. Therefore, the saturation factor fs is determined by the density of these defects, and the larger slope corresponds to a higher
113 density of defects. This conclusion is consistent with the TRPL and FLIM measurements.
In Figure 4.3.4 (b), the larger grains from the aged sample exhibited low PL intensity (dark) due to the presence of sub bandgap trap states41,97. Numerical simulations have been done to predict defect states at several positions spanning the whole band gap of the methylammonium lead halide perovskite36,105. Our study shows no spectral shape of the PL or low-energy PL band associated with defect state emission were observed, which indicate the aging generated defects are independent with the nature of the defect. This observation agrees well with other investigations of methylammonium lead iodide perovskite films91.
4.3.4 Conclusion
In summary, we have investigated the dynamic aging processes in VASP fabricated
CH3NH3PbBr3 films. The results reveal that the uniformly small grains can merge into the larger grains at room temperature in air. Such a slow process does not result in decomposition but a slow recrystallization. The larger grains were confirmed to introduce higher density of defects, exhibiting a shorter lifetime and higher super-linear slope in log-log representation of PL intensity versus excitation intensity relation. This study indicates that the CH3NH3PbBr3 crystallises spontaneously at room temperature in air without any annealing. In nitrogen it has been shown that the crystallisation process is suppressed. Therefore, surface passivation and encapsulation is critical to maintain the quality of perovskite absorber for high performance perovskite-based devices.
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128 5. 4-terminal Tandem Solar Cells Using CH3NH3PbBr3 by
Spectrum Splitting
5.1. Introduction
Organic metal halide perovskite solar cell research has experienced a tremendous development over the past three years1. There has been a plethora of work focused on increasing the cell energy conversion efficiency2-9 with the highest of 22.1% being achieved by an intramolecular exchange process (IEP)10. Attentions have been paid to the understanding of fundamental physics and to the study of material properties10-12.
There are also great interests in integrating perovskite cell with silicon (Si) solar cell13,14. Optical splitting is promising achieving one-sun 28% energy conversion
15 efficiency in a CH3NH3PbI3/Si system . The potential of spectrum splitting for ultra- high performance using Si solar cells is also demonstrated in16 achieving 40% efficient
GaInP/GaInAs/Ge and Si spectrum system under 365 suns. Although efforts have been made on developing an ideal 1.6 eV/0.9 eV tandem combination based on
Shockley−Queisser limits, the aim of this work is to demonstrate a possibility of using the solar cell with a very large bandgap absorber (2.3 eV in the case of Br perovskite) and high absorption coefficient as the top cell in tandem system. The study of White and et al. demonstrates that the minimum top cell efficiency required reaching 30% tandem efficiency ranges from 22% for a bandgap of 1.5 eV to 14% for a bandgap of 2 eV16. Our experimental results demonstrated such a research route of very high bandgap semiconductor materials that can be combined with existing c-Si technology. This work may guide more research interest toward the Br perovskite/Silicon tandem combination, to optimize the monolithic structure, which is more practical and possible to be commercialized in the future.
129 This work differs from the approach in earlier perovskite / Si split spectrum work.
Instead of using a CH3NH3PbI3 cell, we demonstrate spectrum splitting systems that utilize a high bandgap (2.3 eV) CH3NH3PbBr3 cell in conjunction with a nominal
550nm spectrum splitter. In addition, an all perovskite CH3NH3PbBr3/CH3NH3PbI3 (1.5 eV) split spectrum system is demonstrated for the first time achieving an efficiency of
13.4%. 23.4% conversion efficiency is achieved for the CH3NH3PbBr3/Si (1.1 eV)
PERL system and remarkably, a relative increase in conversion efficiency of 16% is also achieved by the CH3NH3PbBr3/multi-crystalline screen-printed (SP) Si spectrum splitting system.
5.2. Results and Discussion
A schematic and a photo of the spectrum splitting system for the measurement of cell conversion efficiencies are shown in Figure 5.2.1. A longpass filter with a nominal cut-off wavelength of 550nm is used to direct the short wavelength light to the
CH3NH3PbBr3 cell and the long wavelength light to the CH3NH3PbI3 cell or the Si cell.
The reflectance, transmittance and absorption property of the filter at an incident of 45 degrees is shown in Figure 5.2.1 (c).
130
131 Figure 5.2.1(a) A schematic of the spectrum splitting system set up and (b) a photo of the set up inside the enclosure for the measurement of light current-voltage characteristics. (c) Measured transmittance and reflection and calculated absorption of the FELH0550 longpass filter at an incident of 45 degrees.
7 The CH3NH3PbBr3 cell was fabricated by the vapour-assisted method , with the structure of Glass / FTO / Compact TiO2 / mesoporous TiO2 / CH3NH3PbBr3 / Spiro-
17 OMeTAD / Au. The CH3NH3PbI3 cell was fabricated by the gas-assisted method . The structure is Glass / FTO/ Compact TiO2 / CH3NH3PbI3 / Spiro-OMeTAD/ Au. Two types of Si cells are used. One of which is a Si PERL cell while the other is a multi- crystalline SP cell, see Figure 5.2.2.
Figure 5.2.2(a) Si PERL (b) multi-crystalline SP Si solar cells used in the spectrum splitting systems.
Table 5.2.1 Output parameters of the spectral splitting systems using CH3NH3PbBr3
2 VOC (V) JSC (mA/cm ) FF (%) Eff. (%)
CH3NH3PbBr3 1.283 9.5 73 8.8 before splitter
132 CH3NH3PbBr3 1.265 7.2 72 6.5 after splitter
CH3NH3PbI3 0.832 20.7 70 12 before splitter
CH3NH3PbI3 0.826 12.1 68 6.9 after splitter
Si PERL 0.68 42 79 22.7 before splitter
Si PERL after 0.673 31.6 80 16.9 splitter
SP Si before 0.587 34.9 79 16.2 splitter
SP Si after 0.578 27.1 79 12.3 splitter
CH3NH3PbBr3 and CH3NH3PbI3 combined 13.4
CH3NH3PbBr3 and Si PERL combined 23.4
133 CH3NH3PbBr3 and SP Si combined 18.8
The one-sun current density - voltage (J-V) curves and their characteristics are summarized in Table 1. The spectral responses (EQE) of the CH3NH3PbBr3;
CH3NH3PbI3; Si PERL; and SP Si cells measured before and after spectral splitting are shown in Figure 5.2.3. There is stronger hysteresis in the J-V characteristics of
CH3NH3PbI3 compared to CH3NH3PbBr3 (see figure 5.2.4, table 5.2.2 and 5.2.3).
Figure 5.2.3(a) J-V curves and (b) EQE of the CH3NH3PbBr3 (green); CH3NH3PbI3
(yellow); Si PERL (grey); and SP Si cells (violet) measured before and after spectral splitting.
As evident from Figure 5.2.3 (b) (dotted versus solid green curves), the
CH3NH3PbBr3 cell absorbed less light in the ultra-violet (UV) range and near the band- edgeafter spectral splitting. The former is due to the absorption of UV light by the spectral splitter, as shown in green curve of Figure 5.1.1 (c), while the latter is due to the lower actual cut-off wavelength (< 550nm) of the splitter at an incident of 45
134 degrees. The CH3NH3PbI3 and Si cells also suffered losses due to parasitic absorption by the spectral splitter after the cut-off wavelength.
Figure 5.2.4 Hysteresis of (a) iodine cell without filter, (b) iodine cell with filter,
(c) bromide cell without filter, (d) bromide cell with filter.
Table 5.2.2 Hysteresis of CH3NH3PbBr3 cell before and after spectrum splitting.
JSC FF Eff. Scan Condition VOC (V) (mA/cm2) (%) (%)
3V/s reverse scan 1.283 9.5 73 8.8 Before spectral
135 splitting 3V/s forward scan 1.280 10.8 54 7.5
0.15V/s reverse scan 1.236 9.0 70 7.8
0.15V/s forward scan 1.245 9.6 67 8.1
3V/s reverse scan 1.265 7.2 72 6.5
3V/s forward scan 1.296 8.1 55 5.8 After spectrum splitting 0.15V/s reverse scan 1.247 6.8 75 6.4
0.15V/s forward scan 1.295 6.8 72 6.4
Table 5.2.3 Hysteresis of CH3NH3PbI3 cell before and after spectrum splitting.
JSC FF Eff. Scan Condition VOC (V) (mA/cm2) (%) (%)
3V/s reverse scan 0.832 20.7 70 12 Before spectral splitting 3V/s forward scan 0.752 20.8 63 10
136 0.15V/s reverse scan 0.865 20.6 66 11.7
0.15V/s forward scan 0.781 20.7 56 9
3V/s reverse scan 0.826 12.1 68 6.7
3V/s forward scan 0.766 12.2 65 6.1 After spectrum splitting 0.15V/s reverse scan 0.860 12.0 66 6.8
0.15V/s forward scan 0.777 12.1 58 5.4
Nevertheless, the use of the CH3NH3PbBr3 cell in the spectral splitting system boosts the performance of the CH3NH3PbI3 cell by 11% relative and Si PERL cell by
3% relative. An even higher enhancement of 16% relative or 2.6% absolute was achieved in the CH3NH3PbBr3/SP-Si system. This demonstrates the advantage of using a higher bandgap perovskite cell with a Si cell even though the 2.3 eV/1.1 eV bandgap combination is less than the ideal 1.6 eV/0.9 eV binary tandem combination as determined by the detailed balance model with cell efficiencies at the Shockley-
Queisser limits205. This is because our tandem system takes advantage of the higher voltage output by the CH3NH3PbBr3 cell and compromises less in terms of JSC than would otherwise be achievable with CH3NH3PbBr3. Although the CH3NH3PbBr3 cell efficiency is far from the maximum achievable theoretically (~16%) for a 2.3 eV cell,
137 less light is diverted from the Si cell that has excellent spectral response to a wider wavelength range (e.g., in excess of 90% for red and near infra-red light).
Furthermore, a higher relative efficiency improvement is achieved when a commercial-type screen-printed solar cell is used in the perovskite/ Si spectral splitting tandem compared to the case when a laboratory developed PERL cell is used (see table
5.2.1 and table 5.2.4). This shows the selection of screen-printed silicon and
CH3NH3PbBr3 would be a logical choice also from an economic point of view. As the performance of perovskite cells improves, lower bandgap (closer to 1.6 eV) cells will be effective in a tandem system. For future work in improving spectral splitting systems, better filter designs including the option of short pass filter will also be required using improved optical element with more accurately defined cut-off wavelength, low UV absorption, and close to unity transmittance.
Table 5.2.4 Performance of CH3NH3PbI3/Si cell via various spectral splitting arrangements
Splitter JSC VOC (V) FF (%) Eff. (%) Used (mA/cm2)
CH3NH3PbI3 0.905 18.7 66 11.2
800 nm Si PERL 0.655 16.4 80 8.6
SP Si 0.556 13.5 77 5.8
138 CH3NH3PbI3 0.915 17.7 65 10.5
750 nm Si PERL 0.657 18.7 80 9.8
SP Si 0.560 15.6 78 6.8
CH3NH3PbI3 0.884 7.9 66 4.6
550 nm Si PERL 0.673 31.6 80 16.9
SP Si 0.578 27.1 79 12.3
CH3NH3PbI3 and Si PERL combined 19.8
800 nm
CH3NH3PbI3 and SP Si combined 17.0
CH3NH3PbI3 and Si PERL combined 20.3
750 nm
CH3NH3PbI3 and SP Si combined 17.3
550 nm CH3NH3PbI3 and Si PERL combined 21.5
139 CH3NH3PbI3 and SP Si combined 16.9
5.3. Conclusion
We have demonstrated a spectrum splitting system that capitalizes on higher bandgap inorganic metal halide perovskite solar cell. . An all perovskite
CH3NH3PbBr3/CH3NH3PbI3 (1.5 eV) split spectrum system is also demonstrated for the first time. The use of CH3NH3PbBr3 cell in a spectral splitting system boosts the performance of the CH3NH3PbI3 cell by 11% relative; Si PERL cell by 3% relative and
SP cell by 16% relative resulting in tandem efficiencies of 13.4%; 23.4% and 18.8% respectively. Remarkable energy conversion efficiencies are achieved in the
CH3NH3PbBr3 /Si systems despite the less than ideal 2.3 eV/1.1 eV bandgap combination. This is due to the high voltage output by the CH3NH3PbBr3 without diverting too much light from the Si cell that has excellent spectral response to a wide range of wavelength.
As the performance of perovskite cells improves, given the relatively higher bandgap and voltage output of CH3NH3PbBr3, the future of using CH3NH3PbBr3 with screen printed silicon appears promising. Further work to improve the performance of the spectral splitting system will involve the use of optical elements with close to unity reflectance and transmittance and better defined cut-offs optimized for improved perovskite solar cells and high performance Si cells in binary or even higher order tandem configurations. Further work should improve the stability of perovskite solar cell as the life-time of the 4-terminal tandem system is entirely based on the life-time of perovskite cell.
140
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144 6. MONOLITHIC PEROVSKITE/ PEROVSKITE TANDEM
SOLAR CELLS WITH ORGANIC RECOMBINATION
LAYER
This chapter demonstrated a monolithic tandem solar cell fabricated by stacking two perovskite cells with dissimilar bandgaps in series. A novel composite carrier recombination stack serves to protect the underlying sub-cell and to provide interconnection with matching work functions. The configuration of the monolithic tandem solar cell in this report is FTO/compact TiO2/mesoporous
TiO2/CH3NH3PbI3/Spiro-OMeTAD/PEDOT: PSS/C60/CH3NH3PbBr3/Spiro-
OMeTAD/Au. Owing to the series connection, a large open circuit voltage of 1.96 V is obtained. Moreover, the modelling of external quantum efficiency (EQE) for both fabricated device and optimized cell architecture indicate a great potential of the proposed recombination layer and perovskite/perovskite tandem structure.
6.1. Introduction
Since the first demonstrations of methylammonium lead halide perovskite solar cells by Kojima et al. in 20091,2, tremendous power conversion efficiency enhancements have been achieved3-8 since then due to excellent optical properties9, long carrier diffusion lengths 10-13, and high emission yields14. Besides optimizing the chemical compositions and modifying the surface morphology, building a tandem structure is a promising concept to further improve device performance, especially in terms of voltage output. Compared with the tandem cell structure with a high efficiency silicon bottom cell and a perovskite top cell 15-19, the combination of two large band gap perovskite cells is capable of providing higher potential energy, which significantly improves
145 power conversion from solar energy to chemical energy such as solar fuels 20,21. A recombination layer (RL) plays a key role in the tandem structure, serving both as charge recombination center and as a protection layer for the sub-cell during the deposition of bottom cell. Many materials have been proposed as the RL for perovskite/silicon tandem architecture15,19. For perovskite/perovskite tandem structure, there is only one demonstrate to date20. In this thesis, a novel structure of FTO/ compact
TiO2/ mesoporous TiO2/ CH3NH3PbI3/ Spiro-OMeTAD/ PEDOT: PSS/ C60/
CH3NH3PbBr3/ Spiro-OMeTAD/ Au are demonstrated, achieving a high open circuit voltage of 1.96V. The observed serious light soaking revealed the presence of large number of defects in the device. Moreover, simulated EQE and J-V curve indicated the proposed RL has a great potential for perovskite.
6.2. Experimental Details
31 CH3NH3Br and CH3NH3I were synthesized following a previously reported method , by mixing methylamine (33% in methanol, Sigma-Aldrich) with hydrobromic acid
(40% in methanol, Sigma-Aldrich) or hydriiodic acid (48% in water, Sigma-Aldrich) in a 1:1 molar ratio in a 250ml round bottom flask under continuous stirring at 0⁰C for 2h.
The precipitates were recovered by rotary evaporation at 60⁰C. To increase the purity of products, the powder was then dissolved in ethanol, recrystallized from diethyl ether.
The final product was collected after dehydration at 60⁰C and placed in a vacuum chamber for overnight.
Solar cell devices were fabricated on fluorine-doped tin oxide (FTO) coated glass
(Pilkington, 8Ω/square). FTO was patterned with 2M HCl and zinc powder. Substrates were then cleaned in 2% Hallmanex detergent, acetone and isopropanol in ultrasonic bath for 10min in each cleaning agent followed by oxygen plasma treatment for 10min.
146 The compact TiO2 layer was deposited by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol at 2500rpm for 60s followed by annealing at 500⁰C for
30min. The mp-TiO2 layer composed of 20-nm-sized particles was deposited by spin- coating at 2000 rpm for 60s using a commercial TiO2 paste (Dyesol 18NRT, Dyesol) diluted in ethanol (2:7, weight ratio). After dried at 125⁰C, the TiO2 film was heated to
500⁰C, annealed at this temperature for 30min and gradually cooled to room temperature.
CH3NH3PbBr3 films were deposited using the vapour-assisted method. Firstly, PbBr2 solution in DMF with a concentration of 1 M was spin-coated on the mp-TiO2 at 2500 rpm for 60s. After annealing at 70 ⁰C for 30min, the film was treated by CH3NH3Br vapour at 150 ⁰C for 10min in a closed glass petri-dish with CH3NH3Br powder surrounded on a hotplate in glovebox, then rinsed in isopropanol at room temperature.
HTM was then deposited by spin-coating at 2000 rpm for 60 s. The solution was prepared by dissolving 72.3 mg (2,2’,7,7’-tetrakis-(N, N-di-p-methoxyphenyl-amine)-
9,9’-spirobifluorene) (Spiro-MeOTAD), 28.8 ml 4- tert-butylpyridine (4-TBP), and 17.5 ml of a stock solution of 520 mg/ml lithium bis(trifluoromethane)sulfonimide (LiTFSI) in acetonitrile in 1 ml Chlorobenzene. The samples were left overnight in dry air before recombination layer deposition.
PEDOT: PSS was diluted in IPA with volume ratio of 1:4, solution was spin-coated on oxidized Spiro-OMeTAD films at 5000 rpm for 45 s, then was annealed at 120 ⁰C for 15 min. C60 was dissolved in Dichlorobenzene with concentration of 10 mg/ml, solution was kept at 100 ⁰C before spin-coating, which at 2000 rpm for 30 s. followed by annealing at 100 ⁰C for 10 min.
147 CH3NH3PbI3 films were deposited by inter-diffusion method. 115 nm PbI2 was evaporated in a vacuum chamber; CH3NH3I solution in IPA with concentration of 20 mg/ml was then spin-coated at 6000 rpm for 30s. After annealing at 100 ⁰C for 60 min,
Spiro-OMeTAD was deposited with identical recipe and spin-coating condition as described above. To complete the devices, 100nm gold contacts were thermally evaporated on the back through a shadow mask. / Perovskite tandem structure.
6.3. Results and Discussion
Figure 6.31 (a) shows the energy band diagram of the proposed tandem cell. Both
MAPbBr3 cell and MAPbI3 cell have normal n-i-p structure with Spiro-OMeTAD as the hole transport material (HTM). Here the higher bandgap cell (with CH3NH3PbBr3 /
MAPbBr3) is deposited first as it closest the light receiving side followed by the deposition of lower bandgap cell (with CH3NH3PbI3 / MAPbI3), HTM and metal contact. The primary criterion of a RL is an appropriate work function. The generated holes from MAPbBr3 cell and electrons from MAPbI3 cell recombine in the RL, therefore p- and n-type material are required to be able to collect carriers from the
MAPbBr3 and MAPbI3 cell, respectively. Spiro-OMeTAD was used as the first p-type layer to transport holes from the MAPbBr3 cell, and C60 was the n-type electron transporter in the MAPbI3 cell. As a solution processed RL, orthogonal solvents are
22 required for each adjacent layer. C60 has a very low solubility , so dichlorobenzene was used to ensure acceptable concentration and minimized impact on the layers underneath. A thin and pinhole-free layer of PEDOT: PSS is required to protect the
Spiro-OMeTAD from dissolving in the solvent for the C60 layer. Another requirement for the RL composition is low temperature deposition, which prevents accelerated degradation of the MAPbBr3 cell, which has been fabricated on the substrate. The
148 annealing temperature used in this study is lower than 120 ⁰C, with a short period of 10 minutes.
Figure 6.3.1(a) Schematic energy band diagram and (b) device architecture of perovskite/ perovskite tandem structure.
The tandem device architecture is shown in Figure 6.3.1 (b), a compact TiO2 was spin-coated on patterned FTO substrate, followed by mesoporous TiO2, and subsequently CH3NH3PbBr3 was deposited by vapour-assisted method. To reduce the thickness and increase conductivity of PEDOT: PSS, IPA has been used to dilute the original precursor. After annealing, PEDOT: PSS forms a dense film to protect underlayers. Then, C60 in dichlorobenzene was deposited by spin-coating. CH3NH3PbI3 film was deposited by inter-diffusion process. The SEM cross sectional image in Figure
6.3.2 (a) of CH3NH3PbI3/ CH3NH3PbBr3 tandem solar cell shows that the thickness of
149 compact TiO2, mesoporous TiO2, CH3NH3PbI3, Spiro-OMeTAD, PEDOT: PSS, C60,
CH3NH3PbBr3, Spiro-OMeTAD and gold are approximately 40, 350, 350, 185, 45, 80,
150, 250 and 100nm, respectively. In addition, a rather rough but fully covered C60 film is observed, probably due to the poor wettability of C60 solution on the PEDOT: PSS film. The roughness has been taken into account in the subsequent EQE simulation.
Total transmission of MAPbBr3 cell and RL was modelled and good agreement was obtained with experimental results, therefore validating our optical model. The external quantum efficiency spectrum is presented assuming 100% internal quantum efficiency. .
The J-V curves and photovoltaic performance of individual cells and tandem cell are shown in Figure 6.3.2 (b) and Table 6.1. The CH3NH3PbI3 cell was fabricated by
23 interdiffusion method . A thin layer PbI2 was firstly deposited on C60 by thermal evaporation, then CH3NH3I solution was spin-coated on the substrate, followed by anneal at 100 ⁰C for 30 min. There is a quick reaction between PbI2 and CH3NH3I, which was confirmed by the instant color change. However, due to the limited penetration of CH3NH3I, thick PbI2 results in unreacted PbI2 residual, which leads to low device performance. Therefore the optimized PbI2 thickness is 115 nm. The
CH3NH3PbBr3 cell was fabricated by vapour-assisted method with a mesoporous
24 structure as discussed in chapter 3 . PbBr2 solution in DMF was spin-coated on mesoporous TiO2, annealed film was treated with hot (175 ⁰C) CH3NH3Br vapour for
10 min, followed by Spiro-OMeTAD deposition. The single junction MAPbBr3 cell shows a voltage output of over 1.4 V, which is attributed to the large bandgap of
CH3NH3PbBr3. The J-V curve of tandem device shows a low fill factor, and the most possible reason is non-ideal film quality of RL. Huge roughness of C60 layer results in a large number of defects at the interface of RL and the MAPbI3 cell, leading to increased charge recombination. This observation is consistent with the stabilized power output.
150 In Figure 6.3.2 (c), the efficiency was stabilized at 5.9% after 500 s light soaking, and current increased with similar trend. The severe light soaking was not observed in both single junction MAPbI3 and MAPbBr3 cells, indicating that further optimization of RL could yield better device performance.
151 Figure 6.3.2(a) Cross-sectional SEM image of tandem cell and (b) JV curves for individual cell and monolithic tandem. (c) Stabilized efficiency and current output of tandem cell.
Figure 6.3.3 Hysteresis of monolithic tandem solar cell (at 0.1 V/s scan rate)
Table 6.3.1 Photovoltaic performance of single junction and tandem cell.
Scan Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
Reverse 1.04 17.30 71 12.7
CH3NH3PbI3 Forward 1.04 17.66 61 11.1
Reverse 1.41 6.16 64 5.2
CH3NH3PbBr3 Forward 1.35 5.81 55 4.4
Reverse 1.96 6.40 41 5.1 Monolithic tandem Forward 1.87 6.00 40 4.6
152
Due to the limitation of the facilities, EQE was measured without light bias (Figure
6.3.5 (a)). In this case, at short wavelength, current output was limited by MAPbI3 cell which suffer from limited incident light, while at long wavelength, the current was limited by MAPbBr3 cell, because the photons with less energy cannot be absorbed by large bandgap material.
The optical constants, refractive index n and extinction coefficient k, of each individual layer is extracted by fitting the ellipsometry data as well as transmission spectra over the wavelength range of interest. The measured and modelled transmittance
(T) of the MAPbBr3 cell and RL are shown in Figure 6.3.4. Good agreement was obtained between the experimental and modelled data with the discrepancy attributable to the non-uniformity of the mesoporous layer and the roughness of C60 layer, see
Figure 6.3.2 (a), which propagates throughout the whole structure. This also validates the optical model.
Figure 6.3.4 Measured and modelled transmittance of the MAPbBr3 cell and recombination layer.
153 The determined optical constants and thickness of each layer were used to calculate the total reflection and absorption occurring within each layer of the device using a standard transfer matrix model25. Under the assumption that 100% IQE (Internal
Quantum Efficiency) across the measured spectrum could be achieved26,27, External
Quantum Efficiency (EQE) could be determined by
퐸푄퐸 퐼푄퐸 = = 100% 1 − 푅푒푓푙푒푐푡푖표푛 − 푇푟푎푛푠푚푖푠푠푖표푛
The modelled EQE spectra of the monolithic tandem structure, as well as each individual cell are shown in Figure 6.3.5 (b).
The current-voltage characteristic of each sub cell is derived according to the non- radiative limit theory, taking into account parasitic power consuming parameters such as series resistance, shunt resistance and ideality factor28,29. The J(V) characteristic of the monolithic tandem device was subsequently derived from the J(V) curves of both sub cells30.
154
Figure 6.3.5(a) measured EQE curve without light bias (b) simulated EQE, and (c)
EQE of optimized cell architecture.
The simulated results also reveal that, by increasing the thickness of CH3NH3PbBr3 capping layer to 750 nm, and CH3NH3PbI3 layer to 165 nm, the tandem structure can reach the maximum output, which VOC approaching 3 V (2.95 V). Simulated EQE shows in Figure 6.3.5 (c). However, both MAPbI3 and MAPbBr3 perovskite layers in this work were fabricated by two-step process, so the film thicknesses were limited by
155 the penetration of methylammonium halide in vapour or liquid phase. Therefore, further research will be focused on one-step deposition for perovskite layer.
6.4. Conclusion
A novel structure of perovskite/ perovskite tandem solar cell was demonstrated, and an organic RL was proposed. The solution deposited RL matches the work function of both CH3NH3PbI3 and CH3NH3PbBr3 material, and act as a protection film for under layers from dissolving by further fabrication. A high open circuit voltage of 1.96V has been achieved by the demonstrated structure, and simulation indicates a great potential of the combination of large and small bandgap perovskites. Further research will be carried out on optimizing the RL film quality to improve the fill factor of the tandem cell.
Further design and fabrication work will focus on the use materials with more appropriate bandgaps enabling better current matching between the top and bottom cells. Both strategies will result in an increase in efficiency of the tandem cell.
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161 7. CONCLUSION
In this chapter, significant results and original contributions from this thesis are summarized. Recommendations for future research are also discussed.
7.1. Discussions
A VASP method for fabricating CH3NH3PbBr3 perovskite film for solar devices was developed. The films exhibit long carrier diffusion lengths of exceeding 1μm. Solar devices employing these films have been demonstrated achieving an average (from forward and reverse scans) conversion efficiency of 8.7%, VOC of 1.45V, JSC of
9.75mA/cm2, and fill factor of 61.5%. Compared with one-step solution process, the
VASP method results in a more superior film quality in terms of crystallinity, uniformity, coverage and light absorption, hence leading to a better photovoltaic performance in the associated device. The high voltage output of this device makes it a good candidate for tandem solar cell application. The efficiency reported here is the highest for the CH3NH3PbBr3 cells using the Spiro-OMeTAD as the HTM, and the second highest reported for CH3NH3PbBr3 solar cells at the moment. It is also the highest for CH3NH3PbBr3 cells fabricated on mesoporous structure.
Moreover, these high quality films are ideal for materials characterization study because of their long diffusion lengths, high crystallinities and low defect densities.
Firstly, an investigation was carried out on the mobile charge induces fluorescence intermittency in bromide perovskite material. The blinking observed in this work was in millisecond timescale, which confirmed the presence of mobile ions in CH3NH3PbBr3.
The density and lifetime of the mobile charges would also increase upon increasing excitation intensity. This is because their migration and accumulation at the surfaces
162 and grain boundaries were enhanced leading to an improved Auger-like behavior.
Secondly, the dynamic aging process of CH3NH3PbBr3 was studied by photoluminescence characterizations. The results unraveled that the uniformly small grains could merge into the lager grains at room temperature in air. Such a slow process does not result in decomposition but a slow recrystallization. The larger grains were concluded to introduce higher density of defects and exhibit a shorter lifetime. In addition, the morphology and carrier extraction of CH3NH3PbIXCl3-x was studied by one- and two-photon fluorescence microscopy. It demonstrated a promising technique for studying the morphology and its impact on carrier transport for perovskite cells, the integration of one- and two-photon fluorescence imaging techniques is useful at separating the surface and bulk components in PL decay traces corresponding to different quality regions of the solar cell. These fluorescence characterization works yielded a better understanding in the operating mechanism of perovskite solar cells, and the major factors that govern the device performance. Besides, they also directed the path towards developing better deposition methods.
Perovskite solar cells are an ideal candidate for tandem application. In this thesis, the tandem concept was demonstrated by a four-terminal spectrum splitting system using a large bandgap bromide perovskite solar cell coupled with a lower bandgap iodine perovskite cell, as well as a silicon solar cell. This work ascertained the feasibility of utilizing CH3NH3PbBr3 as top cell in tandem architecture. Finally, a monolithic perovskite/ perovskite solar cell with organic recombination layer was designed and fabricated where a high voltage output of 1.96 V was accomplished by the proposed structure.
163 7.2. Novelty and contribution
7.2.1 Novelty of the thesis
This thesis contained a number of original contributions:
1. A novel deposition method for high efficiency CH3NH3PbBr3 solar cells was
presented, this is the first time that the vapour-assisted solution process was
used for the preparation of CH3NH3PbBr3 film. The efficiency reported here
was the highest for CH3NH3PbBr3 cells using the Spiro-OMeTAD as the HTM,
and the second highest reported for CH3NH3PbBr3 solar cells at the moment. It
was also the highest for CH3NH3PbBr3 cells fabricated on mesoporous structure.
2. One- or two-photon fluorescence imaging was utilized in this thesis. It is a
technique which was capable of fingerprinting film morphology and carrier
extraction without special specimen preparation or electron beam damage of
perovskite structures. The one- or two-photon fluorescence imaging was
promising technique for perovskite solar cell characterizing and the findings in
this study provided an additional insight in the understanding of carrier
extraction and future information extracted from using this novel technique will
be very useful for the optimization of perovskite solar devices.
3. Grain formation and the observation of fluorescence intermittency were
investigated in this thesis. This study provided unique insight into the charge
accumulation and migration, and thus this was of crucial importance for device
design and improvement.
4. A comprehensive study has been carried out on dynamic aging process in
vapour-assisted fabricated CH3NH3PbBr3 film. This study indicated that the
CH3NH3PbBr3 crystallizes spontaneously at room temperature in air without
164 any annealing. Under nitrogenized condition it has been shown that the
crystallization process was suppressed. Therefore, surface passivation and
encapsulation were critical to maintain the quality of perovskite absorber for
high performance perovskite-based devices.
5. A spectrum splitting system using high bandgap CH3NH3PbBr3 perovskite solar
cell was demonstrated in this thesis. It yielded remarkable energy conversion
efficiencies when coupled with a Si solar cell whether it was high performance
silicon passivated emitter rear locally diffused (PERL) solar cell; or a
commercially relevant multi-crystalline-screen-printed-Si solar cell.
6. A perovskite/ perovskite monolithic tandem solar cell with a novel
recombination layer was designed and fabricated. This is, so far, the only layer-
by-layer deposited perovskite-based tandem cell using different absorber.
7.2.2 Contribution of the author
1. CH3NH3PbBr3 cell by VASP method in chapter 3: R. Sheng designed the
experiments, fabricated and characterized the thin film, as well as the solar
devices.
2. One- or two- photon fluorescence imaging work in chapter 4.1: R. Sheng
fabricated the samples and characterized XRD, absorption, SEM, she was also
involved in interpretation of fluorescence images. One-photon and two-photon
fluorescence microscopy, photoluminescence measurement and the associated
data analysis were done by X. Wen.
3. Grain formation and observation of fluorescence intermittency work in chapter
4.2: R. Sheng fabricated the samples and characterized XRD, absorption, SEM,
she was also involved in data analysis and interpretation. Fluorescence
165 microscopy, photoluminescence measurement and the associated data analysis
were done by X. Wen.
4. Dynamic aging of CH3NH3PbBr3 work in chapter 4.3: R. Sheng designed the
experiments, fabricated the samples and characterized XRD, absorption, SEM,
she was also involved in data analysis and interpretation. Fluorescence
microscopy, photoluminescence measurement, FLIM and the associated data
analysis were done by X. Wen.
5. Spectrum splitting work in chapter 5: R. Sheng fabricated the CH3NH3PbBr3
perovskite solar cell, CH3NH3PbI3 and silicon cells were provided by others. R.
Sheng designed the set-up and did the measurement, analyzed the results.
6. Spectrum splitting work in chapter 5: R. Sheng fabricated the CH3NH3PbBr3
perovskite solar cell, CH3NH3PbI3 and silicon cells were provided by others. R.
Sheng designed the set-up and did the measurement, analyzed the results.
7.3. Perspectives of future research
Further research studies on perovskite/ perovskite tandem solar cell can be carried out in the following directions:
1. Optical modelling can be done to optimize the best film thicknesses combination
for each layer in the tandem structure to achieve current match.
2. Material synthesis for stable perovskite materials with precisely engineered
bandgap are required to be developed. It can maximize the efficiency of
spectrum consumption.
3. For solution processed perovskite fabrication method, orthogonal solvent is
necessary to minimize the possibility that further deposition may dissolve the
layers underneath.
166 4. New deposition techniques, for example, thermal evaporation or other “dry”
method can be developed to fabricate more uniform film, and in turn, to
fabricate lager area cells.
5. More efficient recombination layer can be proposed to eliminate the excessive
recombination loss.
6. New deposition techniques, for example, thermal evaporation or other “dry”
method can be developed to fabricate more uniform film, and in turn, to
fabricate lager area cells.
7. More efficient recombination layer can be proposed to eliminate the excessive
recombination loss.
167 APPENDIX
List of Author’s Publications
Journal Publications
Rui Sheng, Anita Ho-Baillie, Shujuan Huang, Sheng Chen, Xiaoming Wen,
Xiaojing Hao, Martin A. Green "Methylammonium Lead Bromide Perovskite-
Based Solar Cells by Vapour-Assisted Deposition". The Journal of Physical
Chemistry C, 2015, 119 (7), 3545-3549.
Rui Sheng; Anita Ho-Baillie; Shujuan Huang; Mark Keevers; Xiaojing Hao;
Liangcong Jiang; Yi-Bing Cheng; Martin A. Green, “Four-Terminal Tandem
Solar Cells Using CH3NH3PbBr3 by Spectrum Splitting”. The Journal of
Physical Chemistry Letters 2015, 3931-3934.
Rui Sheng; Xiaoming Wen; Shujuan Huang; Xiaojing Hao; Sheng Chen; Yajie
Jiang; Xiaofan Deng; Martin A. Green, Anita Ho-Baillie, “Photoluminescence
characterisations of a dynamic aging process of organic-inorganic
CH3NH3PbBr3 perovskite”. Nanoscale 2016, 8 (4), 1926-1931.
Rui Sheng, Maximilian Hoerantner, Zhiping Wang, Yajie Jiang, Wei Zhang,
Amedeo Agosti, Anita Ho-Baillie, Shujuan Huang, Xiaojing Hao, Martin Green,
Henry Snaith. “Monolithic Perovskite/ Perovskite tandem solar cells with
organic recombination layer”. To be submitted.
Xiaoming Wen, Rui Sheng, Anita Ho-Baillie, Aleš Benda, Sanghun Woo,
Qingshan Ma, Shujuan Huang, Martin A. Green "Morphology and Carrier
Extraction Study of Organic-inorganic Metal Halide Perovskite by One- and
Two-photon Fluorescence Microscopy". The Journal of Physical Chemistry
Letter. 2014,5 (21), 3849-3853
168 Sheng Chen; Xiaoming Wen; Rui Sheng; Shujuan Huang; Xiaofan Deng;
Martin A. Green; Anita Ho-Baillie, “Mobile Ion Induced Slow Carrier Dynamics
in Organic–Inorganic Perovskite CH3NH3PbBr3”. ACS Applied Materials &
Interfaces 2016, 8 (8), 5351-5357.
Yajie Jiang; Martin A. Green; Rui Sheng; Anita Ho-Baillie, “Optical modelling
data for room temperature optical properties of organic–inorganic lead halide
perovskites”. Data in Brief 2015, 3, 201-208.
Yajie Jiang; Martin A. Green; Rui Sheng; Anita Ho-Baillie, “Room temperature
optical properties of organic–inorganic lead halide perovskites”. Solar Energy
Materials and Solar Cells 2015, 137 (0), 253-257.
Jianfeng Yang; Xiaoming Wen; Hongze Xia; Rui Sheng; Qingshan Ma;
JinCheol Kim; Patrick Tapping; Tahaaki Harada; Tak W. Kee; Fuzhi Huang; Yi-
Bing Cheng; Martin Green; Anita Ho-Baillie; Shujuan Huang; Santosh Shrestha;
Robert Patterson; Gavin Conibeer. “Acoustic-optical phonon up-conversion and
hot-phonon bottleneck in lead-halide perovskites”. Nature Communications
2017, 8, 14120.
Jiewei Liu; Sandeep K. Pathak; Nobuya Sakai; Rui Sheng; Sai Bai; Zhiping
Wang; Henry J. Snaith. “Identification and Mitigation of a Critical Interfacial
Instability in Perovskite Solar Cells Employing Copper Thiocyanate Hole-
Transporter”. Advanced Materials Interfaces 2016, 3, 1600571-n/a.
Yajie Jiang; Xiaoming Wen; Ales Benda; Rui Sheng; Anita Ho-Baillie; Shujuan
Huang; Fuzhi Huang; Yi-Bing Cheng; Martin A. Green, “Time-resolved
fluorescence anisotropy study of organic lead halide perovskite”. Solar Energy
Materials and Solar Cells 2016, 151, 102-112.
169 Xiaofan Deng; Xiaoming Wen; Shujuan Huang; Rui Sheng; Takaaki Harada;
Tak W Kee; Martin A. Green; Anita Ho-Baillie, “Ultrafast Carrier Dynamics in
Methylammonium Lead Bromide Perovskite”. The Journal of Physical
Chemistry C 2016, 120 (5), 2542-2547.
Xiaoming Wen; Anita Ho-Baillie; Shujuan Huang; Rui Sheng; Sheng Chen;
Hsien Chen Ko; Martin A. Green, “Mobile Charge-Induced Fluorescence
Intermittency in Methylammonium Lead Bromide Perovskite”. Nano Letters
2015, 15 (7), 4644-4649.
Arman Mahboubi Soufiani; Fuzhi Huang; Peter Reece; Rui Sheng; Anita Ho-
Baillie; Martin A. Green, “Polaronic exciton binding energy in iodide and
bromide organic-inorganic lead halide perovskites”. Applied Physics Letters
2015, 107 (23), 231902.
Rui Lin, Matthew Wright, Kah Howe Chan, Binesh Puthen-Veettil, Rui Sheng,
Xiaoming Wen, Ashraf Uddin “Performance improvement of low bandgap
polymer bulk heterojunction solar cells by incorporating P3HT”. Organic
Electronics 2014, 15, 2837-2846
Conference Publications
Xiaofan Deng; Xiaoming Wen; Rui Sheng; Shujuan Huang, Takaaki Harada;
Tak W Kee; Martin Green; Anita Ho-Baillie, “Ultrafast charge generation and
relaxation dynamics in methylammonium lead bromide perovskites Conference:
Proc. SPIE 9668, Micro+Nano Materials, Devices, and Systems”, Volume:
9668
Sheng Chen; Xiaoming Wen; Shujuan Huang; Rui Sheng; Martin Green; Anita
Ho-Baillie, “Illumination dependent carrier dynamics of CH3NH3PbBr3
170 perovskite” Conference: Proc. SPIE 9668, Micro+Nano Materials, Devices, and
Systems,, Volume: 9668
Conference oral presentation
Rui Sheng, Anita W. Y. Ho-Baillie, Shujuan Huang, Mark Keevers, Xiaojing
Hao,1 Liangcong Jiang, Yi-Bing Cheng and Martin A. Green “4-terminal
tandem solar cells using CH3NH3PbBr3 by spectrum splitting” PSCO15
Lausanne, Switzerland, September 2015.
Conference poster presentation
Rui Sheng, Qingshan Ma, Sanghun Woo, Anita Ho-Baillie, Shujuan Huang,
Xiaojing Hao, and Martin Green“Fabrication and characterization of
CH3NH3PbBr3 perovskite solar cells” SSSC14 conference, Oxford. September
2014.
171