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Printable Mesoscopic Perovskite Solar Cells
A comprehensive exploration of printable perovskite solar cells and their potential for commercialization
In Printable Mesoscopic Perovskite Solar Cells, a team of distinguished researchers delivers an accessible and incisive discussion of the principles, technologies, and fabrication processes associated with the manufacture and use of perovskite solar cells. The authors detail the properties, characterization methods, and technologies for halide perovskite materials and devices and explain printable processing technologies, mesoscopic anode and cathodes, and spacer layers for printable perovskite solar cells.
In the book, you’ll find expansive discussions of the stability issues inherent in perovskite solar cells and explore the potential for scaling and commercializing the printing of perovskite solar cells, complete with real-world industry data.
Readers will also find:
- A thorough introduction to the background and fundamentals of perovskite solar cells
- Comprehensive explorations of the characterization methods and technologies used with halide perovskite materials and devices
- Practical discussions of printable processing technologies for perovskite solar cells
- Fulsome treatments of the stability issues associated with perovskite solar cells and potential solutions for them
Perfect for materials scientists, solid state physicists and chemists, and electronics engineers, Printable Mesoscopic Perovskite Solar Cells will also benefit surface chemists and physicists.
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Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Title Page
Copyright
Biography
Preface
1 Background and Basic Knowledge of Perovskite Solar Cells
1.1 Background
1.2 The Principle of Solar Cells
1.3 The Typical Structures of PSC
References
2 Characterization Methods and Technologies for Halide Perovskite Materials and Devices
2.1 Introduction
2.2 Printing Layer Quality
2.3 Material and Crystal Properties
2.4 Spatially Resolved Steady‐state Photophysical Methods
2.5 Transient Optoelectronic Methods
2.6
I
–
V
Performance: Transient and Steady State
References
3 Printable Processing Technologies for Perovskite Solar Cells
3.1 Introduction
3.2 Solution‐Based Technologies
3.3 Conclusion and Outlook
References
4 Mesoscopic Anodes and Cathodes for Printable Perovskite Solar Cells
4.1 Introduction
4.2 Fabrication Methods
4.3 Comact Layer (TiO
2
)
4.4 Mesoporous Anodes (n‐Type Semiconductor: TiO
2
, etc.)
4.5 Mesoporous Cathodes (NiO and Co
3
O
4
)
4.6 Back‐Contact Porous Carbon
4.7 Photovoltaic Measurements
4.8 Conclusion
References
5 Insulating Layers for Printable Mesoscopic Perovskite Solar Cells
5.1 Introduction
5.2 ZrO
2
‐Insulating Mesoscopic Layers
5.3 Al
2
O
3
‐Insulating Mesoscopic Layers
5.4 SiO
2
‐Insulating Mesoscopic Layers
5.5 Multilayer Insulating Mesoscopic Layers
5.6 Conclusion and Perspective
References
6 Perovskite Materials and Perovskite Solar Cells
6.1 Perovskite Materials
6.2 Compositional and Interfacial Engineering of Perovskite Solar Cells
References
7 The Efficiency Progress in Printable Mesoscopic Perovskite Solar Cells
7.1 Introduction
7.2 Solvent Engineering and Annealing
7.3 Composition Engineering
7.4 Additive Engineering
7.5 Interfaces Engineering
7.6 Conclusion and Outlook
References
8 Stability Issues and Solutions for Perovskite Solar Cells
8.1 Substrate
8.2 Electron Transport Layer
8.3 Hole Transport Layer
8.4 Back Electrode
8.5 Encapsulant
8.6 Halide Perovskite Light Absorbing Layer
8.7 Summary
References
9 Manufacture, Modules, and Applications
9.1 Introduction
9.2 Manufacture
9.3 Modules
9.4 Applications
9.5 Summary
References
10 Perspective
10.1 Commercializing
10.2 Exceeding SQ Limit
10.3 Efficiency Breaking Out of SQ Limit
References
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 Solar cell parameters for carbon‐based cells with and without Cu:N...
Table 4.2 Solar cell parameters for standard carbon cell without and with Co
Chapter 5
Table 5.1 Photovoltaic parameters of FP‐MPSCs based on ZrO
2
with different t...
Table 5.2 Photovoltaic parameters of FP‐MPSCs based on ZrO
2
with different b...
Chapter 7
Table 7.1 Typical physical and chemical properties of common organic solvent...
Chapter 9
Table 9.1 Summary of techniques that have been used for the deposition of th...
Table 9.2 Architecture, design, and performance of some of the most signific...
Chapter 10
Table 10.1 Photovoltaic parameters of the state‐of‐the‐art fully printable m...
List of Illustrations
Chapter 1
Figure 1.1 The conduction and valence band edges of the p‐type and n‐type re...
Figure 1.2 (a) The PV cell and (b) the production of electrical current due ...
Figure 1.3 (a) The DSSC device architecture and the working mechanism of dye...
Figure 1.4 (a) Schematic representation of the bulk heterojunction OSC. (b) ...
Figure 1.5 (a) Crystal structure of cubic perovskite with general chemical f...
Figure 1.6 Device architecture for (a) mesoscopic (b) triple layer, (c) regu...
Chapter 2
Figure 2.1 Illustration of focused ion beam (FIB) milling process.
Figure 2.2 Schematic of four‐point probe sheet resistivity measurements setu...
Figure 2.3 (a) UV–vis absorption spectrum of MAPbI
3
. Inset shows the d[(
αhν
...
Figure 2.4 Raman shift spectra of the (a) stress‐annealed pyrolytic graphite...
Figure 2.5 Principle of electron beam interaction.
Figure 2.6 SEM images of FTO/c‐TiO
2
/m‐TiO
2
of unfilled PSC stack (left) m‐Zr...
Figure 2.7 SEM image and EDX maps of a mesoporous cell cross section, indica...
Figure 2.8 Schematic diagram of the basic setup of an atomic force microscop...
Figure 2.9 AFM images of spray‐coated compact TiO
2
layers on FTO scanned thr...
Figure 2.10 (a) Representative image of a drop on the layer of investigation...
Figure 2.11 Microscopic PL images of two mesoscopic PSC (top view from the g...
Figure 2.12 EL image (a) as well as PL image under open circuit (b) and shor...
Figure 2.13 (a) Real‐time monitoring of PL intensity and spectral peak posit...
Figure 2.14 (a) The PL measurement in real‐time on the glass/ZrO
2
/graphite d...
Figure 2.15 Block diagram of LIT system.
Figure 2.16 (a) The plot of the charge recombination lifetime and (b) mean e...
Figure 2.17 OCVD analysis on FTO/mp‐TiO
2
/carbon‐graphite (referred to as wit...
Figure 2.18 Schematic setup of a pulse‐based TRPL setup.
Figure 2.19 Alternative TRPL setup to measure the PL decay from the excitati...
Figure 2.20 PL spectrum (a) and PL decay (b) of perovskite layers deposited ...
Figure 2.21
J
–
V
curve of a mesoscopic PSC curve, measured in reverse (red) a...
Figure 2.22 External quantum efficiency and spectral response patterns of 12...
Figure 2.23 Equivalent circuit of the two‐diode model.
Figure 2.24 Effect of parallel resistance
R
p
on the
V
oc
vs. light intensity ...
Figure 2.25 Effect of saturation current
J
01
(a) and
J
02
(b) on the
V
oc
vs. ...
Chapter 3
Figure 3.1 Timeline of fabrication methods for PSCs and PSMs. The methods ar...
Figure 3.2 (a) Schematic diagram of spin coating. Source: Rong et al. [13] R...
Figure 3.3 Schematic diagram of meniscus coatings. (a) Blade coating; (b) Sl...
Figure 3.4 (a) Schematic diagram of blade coating. Source: Mallajosyula et a...
Figure 3.5 (a) Schematic diagram of slot‐die coating. Source: Ding et al. [2...
Figure 3.6 (a) Schematic diagram of bar coating with optical microscope imag...
Figure 3.7 (a) Schematic diagram spray coating. Source: Swartwout et al. [38...
Figure 3.8 (a) Schematic diagram of two main inkjet‐printing methods: contin...
Figure 3.9 (a) Schematic diagram of screen printing. Source: Rong et al. [13...
Figure 3.10 (a) Schematic diagram of the perovskite films by a two‐step CBD ...
Figure 3.11 (a) Schematic diagram of soft‐cover deposition. Source: Ye et al...
Figure 3.12 (a) Schematic diagram of brush painting by “Maobi.” Source: Mao ...
Chapter 4
Figure 4.1 Cross‐sectional illustration of multi‐porous‐layered electrodes p...
Figure 4.2 Fabrication scheme of multi‐porous‐layered electrodes perovskite ...
Figure 4.3 Structure of laboratory cells; (a) a single MPLE; (b) MPLEs on a ...
Figure 4.4 Screen‐printing design example of MPLE‐PSC submodules for univers...
Figure 4.5 Schematic image of spray‐pyrolysis deposition of compact TiO
2
lay...
Figure 4.6 Photocurrent density–voltage (
J
–
V
) curves of solid‐state dye‐sens...
Figure 4.7 (a) Photonic–electronic‐materials effects of anode particle size ...
Figure 4.8 Acidic‐route synthesis scheme of nano‐TiO
2
colloid (
d
= c. 18 nm)...
Figure 4.9 Basic‐route synthesis scheme of nano‐TiO
2
colloid (
d
= c. 30 nm) ...
Figure 4.10 Powder‐based preparation of TiO
2
screen‐print paste for mesoporo...
Figure 4.11 A picture of screen print procedure: (a) hands with squeegee on ...
Figure 4.12 Screen print procedure: (a) setting printing mesh, squeegee, and...
Figure 4.13 Schematic soaking image of perovskite precursor solution (a) and...
Figure 4.14 Photovoltaic characteristics of reverse (red) and forward (blue)...
Figure 4.15 Repeated
J
–
V
measurements of MPLE‐PSC with time interval. Source...
Figure 4.16 IPCE activation effects: (a) activation setup; (b) IPCE spectra ...
Chapter 5
Figure 5.1 The crystal model of (a) monoclinic ZrO
2
, (b) α‐Al
2
O
3
, and (c) cr...
Figure 5.2 (a) High‐purity ZrO
2
powders, (b) High‐purity ZrO
2
beads.
Figure 5.3 Schematic structure of carbon‐based, hole‐conductor‐free FP‐MPSCs...
Figure 5.4 (a) Cross section of the triple‐mesoscopic FP‐MPSCs. (b) Energy l...
Figure 5.5 SEM images of (a) Al
2
O
3
and (b) ZrO
2
mesoscopic layers; cross‐sec...
Figure 5.6 Nitrogen adsorption–desorption isotherms and pore‐size distributi...
Figure 5.7 (a) Typical
J
–
V
curves for FP‐MPSCs with different thickness of Z...
Figure 5.8 SEM images of ZrO
2
insulating layers with particle size of (a) 5 ...
Figure 5.9 (a) High‐resolution TEM image of perovskite/S20 spacer film compo...
Figure 5.10 (a) Cross‐sectional SEM image of FP‐MPSCs with ZrO
2
as spacer la...
Figure 5.11 (a) Photovoltaic parameters and (b) dark current of FP‐MPSCs bas...
Figure 5.12 Nyquist plot (a, b) and Bode plot (c) of FP‐MPSCs with different...
Figure 5.13 Parameters obtained from high frequency (∼10
2
–10
6
Hz) semicircle...
Figure 5.14 (a) Schematic architecture of FP‐MPSCs based on Al
2
O
3
as spacer ...
Figure 5.15 Schematic illustration to fabricate a carbon‐based, hole‐conduct...
Figure 5.16 (a) Carbon‐based, hole‐conductor‐free FP‐MPSCs based on the devi...
Figure 5.17 The schematic structure and cross‐sectional SEM image of PSCs wi...
Figure 5.18 Schematic diagram showing the transfer process of electron and h...
Figure 5.19 (a)
J
–
V
curves and (b) corresponding photovoltaic parameters of ...
Figure 5.20 (a) Cross‐sectional SEM image of FP‐MPSCs based on SiO
2
spacer l...
Figure 5.21 (a)
J
–
V
curves and (b) key photovoltaic parameters of FP‐MPSCs w...
Figure 5.22 SEM images of (a) mp‐TiO
2
film, SiO
2
layers prepared from slurry...
Figure 5.23 (a) Formation processes of bifunctional Al
2
O
3
interlayer; schema...
Figure 5.24 XPS spectra of Al 2p (a) and Ti 2p (b) and UPS spectra (c) for T...
Figure 5.25 (a)
V
oc
as a function of various Al
2
O
3
/ZrO
2
thickness. (b) Cross...
Figure 5.26 (a) Schematic structure of FP‐MPSCs based on the structure of Ti...
Figure 5.27 (a)
R
CE
at the counter electrode interface, (b)
R
ct
, and (c) the...
Figure 5.28
J
–
V
curve and schematic structure of FP‐MPSCs based on TiO
2
/ZrO
2
Figure 5.29 Derived equivalent circuit components obtained from impedance un...
Figure 5.30 Illustration to show problems of current spacer layer and perovs...
Figure 5.31 (a) The novel (TEG)
2
(CH
3
NH
3
)PbI
3
perovskite precursor phase (1) ...
Chapter 6
Figure 6.1 Crystal structures of the three different MAPbI
3
phases. (a) Cubi...
Figure 6.2 (a) Correlations between tolerance factor and crystal structure o...
Figure 6.3 (a) Schematic of [100] oriented families of layered organic–inorg...
Figure 6.4 (a) VASP synthesis of perovskite films. Source: Reproduced with p...
Figure 6.5 The different defects present at surface and grain boundaries of ...
Figure 6.6 Consequences of the formation of excess PbI
2
in the perovskite fi...
Figure 6.7 Various phases of FAPbI
3
and CsPbI
3
and variation of internal ene...
Chapter 7
Figure 7.1 Historic architecture evolution of PSCs, starting from dye‐sensit...
Figure 7.2 (a) Phase transformations between perovskite precursors and perov...
Figure 7.3 Illustration of consecutive morphology controlling operations and...
Figure 7.4 (a) The schematic diagram of perovskite crystallization in the me...
Figure 7.5 Growth of MAPbI
3
in mesoscopic layers. (a) Morphology evolution o...
Figure 7.6 Crystallinity and film properties of MAPbI
3
in mesoscopic layers....
Figure 7.7 The performance parameters of devices based on Cs
0.1
Rb
0.05
FA
0.85
P...
Figure 7.8 Scheme (a) and cross‐section SEM image (b) of the Cu‐based solar ...
Figure 7.9 Selected diffraction pattern from time‐resolved GIWAXS measuremen...
Figure 7.10 (a)
J
–
V
curve using the 2D/3D perovskite with 3%AVAI in HTM‐free...
Figure 7.11 (a) Structure characterizations of different perovskite films. (...
Figure 7.12 Schematic view of the crystal growth process of perovskite CH
3
NH
Figure 7.13 (a) Schematic illustration of in situ GIWAXS experimental setup....
Figure 7.14 (a) Structure of the printable mesoscopic PSCs based on carbon. ...
Figure 7.15 (a) Formation process of bifunctional Al
2
O
3
interlayer; schemati...
Figure 7.16 (a) the device structure of a typical printable PSC based on tri...
Figure 7.17 Posttreatment and in situ formation of 2D perovskites. (a) Schem...
Figure 7.18 The schematic structure of printable mesoscopic PSCs with mp‐TiO
Chapter 8
Figure 8.1 Stability concerns and solutions related to the ETL in PSCs. (a) ...
Figure 8.2 Stability concerns and solutions related to the HTL in PSCs. (a) ...
Figure 8.3 Stability concerns and solutions related to the back electrode in...
Figure 8.4
Encapsulations for PSCs
. (a) Cover glass‐free encapsulation by co...
Figure 8.5 Phase stability of halide perovskites. (a) Light soaking led to t...
Figure 8.6 Grain boundary modification increased the stability of PSCs by in...
Figure 8.7 The long‐term stability of p‐MPSCs at MPP over 9000 hours. Source...
Chapter 9
Figure 9.1 Manufacture of low‐capital cost mCPSC mini‐modules in “Serigrafía...
Figure 9.2 Evolution of the best‐reported lab‐cell (≤0.1 cm
2
) efficiencies a...
Figure 9.3 Screen printing. (a) A schematic of the process; (b) An example o...
Figure 9.4 (a) A microscopic image of a 130‐34 mesh, i.e. 130 threads per cm...
Figure 9.5 Labelled high‐speed camera image of the squeegee forcing the mesh...
Figure 9.6 (a) Dependence of viscosity of the TiO
2
ink on paste:terpineol ma...
Figure 9.7 Dependence of the film thickness on: (a) the print speed; (b) the...
Figure 9.8 Infiltration methods for mCPSC. (a) Schematic representation of t...
Figure 9.9 Generic schematic of a module with five single cells.
Figure 9.10 Example of modules prepared via the registration method. (a) Cro...
Figure 9.11 The mCPSC module based on the scribing method. (a) P1 on a subst...
Figure 9.12 Schematic representation of a series‐connected module with three...
Figure 9.13 PCE of unit cells (experimental and simulated) and of
Z
modules....
Figure 9.14 Reported results using Poisson's equation. (a) A study on single...
Figure 9.15 3D plot of generated power as function of P2 and
W
a
of a module ...
Figure 9.16 (a) The scheme of the encapsulation of mCPSCs based on hot melt ...
Figure 9.17 Comparison of device performance under different illumination co...
Guide
Cover Page
Title Page
Copyright
Biography
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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Printable Mesoscopic Perovskite Solar Cells
Edited by Hongwei Han, Michael Grätzel, Anyi Mei, and Yue Hu
Editors
Prof. Hongwei Han
Huazhong Univ. of Science & Technology
National Laboratory for Optoelectronics
1037 Luoyu Road
430074 Wuhan
China
Prof. Michael Grätzel
Ecole Polytechnique Federale de Lausann
Institute of Chemical Science
Station 6
CH‐1015 Lausanne
Switzerland
Prof. Anyi Mei
Huazhong Univ. of Science & Technology
National Laboratory for Optoelectronics
1037 Luoyu Road
430074 Wuhan
China
Prof. Yue Hu
Huazhong Univ. of Science & Technology
National Laboratory for Optoelectronics
1037 Luoyu Road
430074 Wuhan
China
Cover Images: Background image: © Hongwei Han, Huazhong University of Science & Technology
Inset image: © Niethammer Zoltan/Shutterstock
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Library of Congress Card No.: applied for
British Library Cataloguing‐in‐Publication Data
A catalogue record for this book is available from the British Library.
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Print ISBN: 978‐3‐527‐34958‐6
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Biography
Hongwei Han is a Professor at Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology. He is a pioneer in printable mesoscopic perovskite solar cells.
Michael Grätzel is a Professor at Ecole Polytechnique Federale de Lausanne. He is a pioneer in the field of molecular photovoltaics and the development of perovskite solar cells.
Anyi Mei, PhD, is a Professor at Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology. His research is focused on printed mesoscopic solar cell materials and devices.
Yue Hu, PhD, is an Associate Professor at Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology. Her research interest is dye‐sensitized solar cells and perovskite solar cells.
Preface
The sudden outbreak of COVID‐19 disrupted the normal life of people around the world in 2020. We were combating the virus in the most primitive way of staying at home to fight against the pandemic. Out of sheer boredom, I was thinking maybe we could make full use of the spare time to summarize the research achievements and insights on printable mesoscopic perovskite solar cells (PSCs) (referred to simply as p‐MPSC, also honored to be known as “Han cells” or “Wuhan cells”) since we can't go out to continue our research. By doing so, we can set sail for our work again in a shift manner once the pandemic is over. Therefore, after a quick discussion with our team, I emailed Prof. Michael Grätzel at École Polytechnique Fédérale de Lausanne (EPFL, Swiss Federal Institute of Technology in Lausanne) in Switzerland. He is a pioneer in dye‐sensitized solar cells (DSSCs) and we invited him to compile the book with us. Prof. Michael Grätzel readily accepted our invitation and recommended us to invite other internationally recognized scholars to participate in the work.
Human beings are fragile in face of nature. Any sign of slight disturbance in the environment can impact people's life. Over the past century, the greenhouse effect caused by the excessive emission of carbon dioxide from overexploitation and usage of fossil fuels has tipped the balance and led to global warming, glacier and permafrost melting, and sea level rising, which not only endanger the balance of natural ecosystems, but also pose threats on the survival of mankind. COVID‐19 reminds us that humankind should launch a green revolution and move faster to create a green way of development and life, preserve the environment, and make Mother Earth a better place for all. On September 22, 2020, Chinese president Xi Jinping had announced at General Debate of the 75th Session of the General Assembly of the United Nations, “China will scale up its Intended Nationally Determined Contributions by adopting more vigorous policies and measures. We aim to have CO2 emissions peak before 2030 and achieve carbon neutrality before 2060. We call on all countries to pursue innovative, coordinated, green and open development for all, seize the historic opportunities presented by the new round of scientific and technological revolution and industrial transformation, achieve a green recovery of the world economy in the post‐COVID era and thus create a powerful force driving sustainable development.”
The new generation of ultra‐low‐cost photovoltaic (PV) technology will be the key to drive carbon neutrality, to fundamentally reduce the proportion of coal power, and to advance the development of the “ultimate power source in twenty‐first century” – hydrogen energy. As the leading technology of PV industry, p‐MPSC has received widespread attention from scientific research and business sectors because of its simple fabrication process, excellent performance, low‐cost materials constituents, and enormous commercial value since its invention. p‐MPSC is based on triple‐layer mesoporous films and takes halide materials as its light‐absorbing materials and mesoporous carbon as electrode. All components can be fabricated by screen printing or slot coating method which shows great advantages over industrial production and upscaling module fabrication. This whole process can meet the demand of ultra‐low‐cost solar cells for PV industry.
The development history of organic–inorganic metal halide perovskite materials can be traced back to 1893. H.L. Wells in America was the first to successfully synthesize the perovskite material based on CsPbX3. In 1958, C.K. Møller in Denmark took a further step to confirm that CsPbX3 is a perovskite structure. In 1978, D. Weber in Germany fabricated organic–inorganic hybrid perovskite materials for the first time. Subsequently, D.B. Mitzi and C.R. Kagan from IBM Corporation in America applied this organic–inorganic perovskite material into LED and TFT devices in 1994 and in 1999, respectively. However, the materials couldn't be widely used because of their poor stability. The perovskite material was firstly used in solar cells as liquid perovskite‐sensitized solar cells by T. Miyasaka Group in Japan in 2009. But unfortunately, it didn't draw much attention since it can only work for dozens of seconds under light‐soaking conditions. Nam‐Gyu Park Group in South Korea and Michael Grätzel Group in Switzerland applied this perovskite materials into solid‐state DSSCs for the first time until 2012, and the device showed a good stability after 500 hours of preservation in the dark conditions. Since then, there has been a wave of research on PSCs.
The first PSC to show promising light‐soaking stability was obtained by the p‐MPSC based on a triple‐layer mesoporous films structure and carbon counter electrode. Aided by the natural strengths in triple mesoscopic films structure like inhibiting ionic immigration and thermal expansion mismatch and by introducing the bifunctional organic molecular, Han Group in China obtained a stability of 1000 hours in ambient air and successfully passed the main items of IEC61215:2016 qualification tests. Meanwhile, the device fabricated by the team has been working for more than 13 000 hours at a maximum power point of 55 ± 5 °C under the standard sunlight without obvious decay. M.K. Nazeeruddin Group in Switzerland also gained p‐MPSCs with a stability of 10 000 hours at short‐circuit conditions. Furthermore, WonderSolar LLC. in China has made tremendous efforts to upscale p‐MPSCs toward commercialization, both of which have shown wide application prospect of p‐MPSCs. So far, the instability of perovskite materials has been satisfactorily solved through the design of device structure with triple‐mesoscopic layers.
Compared with conventional p–n junction devices, the mesoscopic structure with nano‐size pores favors charge extraction among various dynamic competitive processes due to the micron‐scale charge diffusion length of perovskite. When the incident light is fully absorbed by the perovskite material in ETL, only holes will remain in the perovskite medium in the part of mesoscopic ZrO2 layer and carbon layer, so far as there is no need for conventional hole transport layer within p‐MPSCs. The hole‐conductor‐free character and the carbon electrode help reduce the cost of solar cells. Meanwhile, all functional layers including the back electrode in p‐MPSCs are fabricated via solution‐processed methods while evaporation is excluded, which reduces the device fabrication cost. The inexpensive components and the full solution‐processable character minimize the cost and make p‐MPSC a great choice for low‐cost photovoltaics.
This book consists of 10 chapters and the lead authors of each chapter are the experts on the front line in MSPC industry. We introduce the structure characteristics and application on PV device of perovskite materials combining physical and chemical basic principles and the research achievements of the lead authors and industry professors. We focus on the characteristics of p‐MPSCs. Starting from inorganic framework materials and perovskite materials, we summarize the characterizations of p‐MPSCs and analyze the kinetics of carriers in solar cells under mesoscale conditions. Based on that, we put up with the strategies of enhancing the efficiency and stability of p‐MPSCs and make prospect of p‐MPSC commercialization.
Hereby, we would like to express our special thanks for the contributions and support from Prof. M.K. Nazeeruddin Group in Switzerland, Prof. Andreas Hinsch Group in Germany, Prof. Seigo Ito Group in Japan, Prof. Trystan Watson Group in the UK, and Jian Zhang Group, Wenjun Wu Group, and Han Group in China.
The authors sought to express accurately in words with clear illustrations during the compiling, and the editors reviewed and proofread each chapter carefully. However, due to limited time and knowledge, errors and omissions are inevitable. Advice and corrections from readers and peers will be deeply appreciated for our future reprint.
In HUST, Feb. 2021Hongwei Han
1Background and Basic Knowledge of Perovskite Solar Cells
Maria Vasilopoulou1, Abd Rashid B. Mohd Yusoff2, and Mohammad K. Nazeeruddin3
1 National Centre for Scientific Research Demokritos, Institute of Nanoscience and Nanotechnology, Agia Paraskevi, 15341 Attica, Greece
2 Pohang University of Science and Technology(POSTECH), Department of Chemical Engineering, Pohang, Gyeongbuk, 37673, Republic of Korea
3 École Polytechnique Fédérale de Lausanne (EPFL), Institute of Chemical Sciences and Engineering, Group for Molecular Engineering of Functional Materials, Rue de l'Industrie 17,, CH‐1951, Sion, Switzerland
1.1 Background
The deployment of renewable energy sources lies within the context of prohibiting climate change and achieving greenhouse gas reduction targets [1]. Solar energy is clean and abundant; therefore, its usage can fulfill the need for sustainable development in order to protect the environment by limiting the consumption of fossil fuels. Photovoltaic systems convert the sun's energy directly into electricity through the application of semiconductor materials that utilize the so‐called photovoltaic effect or photovoltaic phenomenon.
The photovoltaic phenomenon was first reported in 1839 by 19‐year‐old Edmund Becquerel, who was able to produce weak electrical current in a prototype cell by employing a silver‐coated platinum electrode immersed in certain electrolytes, which was exposed to sunlight. He called this phenomenon the “photovoltaic effect,” which is the basic principle that governs solar cell operation that converts sunlight into electricity [2]. However, it was about forty years later, in 1876, when the first solid‐state photovoltaic cell was demonstrated by William Adams and Richard Day [3]. It employed a junction formed between selenium (Se) and platinum (Pt) which produced electricity without having moving parts and led to the development of modern solar cell systems. Adams and Day also found that “the ultra‐red or the ultraviolet rays have little or no effect on the production of energy.” They also observed that the produced electricity was highly dependent on the illuminating power of the light, “being directly related to the square root of that illuminating power” [4]. A few years later, in 1883, Charles Fritts fabricated the first large‐area solar cell by pressing a selenium layer between two thin gold metal foils [5].
In the following years, photovoltaic effects were also observed in other semiconductor materials such as copper–copper oxide (Cu:CuO) thin‐film heterostructures, lead sulfide (PbS), and thallium sulfide (TlS). In these early devices, a thin semi‐transparent metal layer was deposited on the semiconductor material to provide an asymmetric Schottky junction without prohibiting light from reaching the semiconductor. The discovery of silicon (Si) p–n junction in the 1950s offered a significant boost to the development of solar cells as these p–n junction structures exhibited better rectifying ability than the Schottky ones and hence delivered better photovoltaic behavior. The first Si solar cell was reported by Chapin, Fuller, and Pearson in 1954 and converted sunlight to electricity with an efficiency of 6% [6]. These pioneers used their solar panels to power a small toy Ferris wheel and a solar‐powered radio transmitter. A few years later, in 1957, they increased the efficiency to 8% [7]. Notably, a solar cell based on cadmium sulfide (CdS) p–n junction was demonstrated in 1954, also producing electricity with an efficiency of 6%.
During the next decades, interest in photovoltaics expanded to other semiconductor materials such as gallium arsenide (GaAs), indium phosphide (InP), and cadmium telluride (CdTe), which, unlike Si, possess a direct bandgap, and theoretical work indicated that they could offer higher efficiency. Afterward, the growing awareness of the need to make use of an alternative to fossil fuel sources of electricity led photovoltaic production to be expanded at a rate of 15–25% per year. This has also driven a sufficient cost reduction which made photovoltaics competitive for increase in supply in grid‐connected loads at peak use and in contexts where the conventional electricity supply was quite expensive, for example, in remote low‐power applications such as telecommunications, navigation, and rural electrification. However, new markets have opened up such as building integrated photovoltaics, where besides the efficiency and cost of the photovoltaic system, its easy integration in building facades and roofs can be facilitated by using alternative solution‐based materials and fabrication methods.
1.2 The Principle of Solar Cells
1.2.1 Silicon Solar Cells
A solar cell is an optoelectronic device based on a p–n junction formed between two oppositely doped regions of a single semiconductor material, a p–type which has an excess of free holes and n‐type having an excess of free electrons [2]. The p‐type and n‐type regions in a Si p–n junction are created by using appropriate trivalent (acceptor, having three valence electrons like boron, B; aluminum, Al; gallium, Ga; and indium, In) and pentavalent (donor, like phosphorus, P; antimony, Sb; and arsenic, As) impurities (dopants), respectively, via ion implantation, epitaxy, or diffusion of dopants. Because the Si semiconductor owns four valence electrons, doping with acceptors creates an excess of holes, while doping with donors results in the formation of nearly free electrons. Consequently, the concentration of free holes is much higher in the p‐type region compared to the n‐type region and vice versa (free electrons are much more in the n‐type region), which creates a high concentration gradient of charge carriers across the junction line.
Considering the energy diagram, the Fermi level (EF) shifts toward the valence band (VB) maximum in the p‐type region and toward the conduction band (CB) minimum in the n‐type region, hence creating a discontinuity in energy across the p–n junction. As a result, free holes (majority carriers) diffuse from the p‐type region toward the junction line, while electrons diffuse from the n‐type region and cross the p–n junction where they subsequently recombine with holes leaving negatively charged acceptor and positively charged donor ions at the p‐type and n‐type regions, respectively. These charged ions continue to increase and, because they are immobile, they build an electric field which is directed from the n‐type to p‐type region of the junction.
Due to this internal electric field, minority carriers (which are holes from the n‐type region and electrons from the p‐type region) drift toward the opposite region and eventually balance and stop the diffusive flow of majority carriers. At this stage of equilibrium, the two charged ionic “layers” on either side of the p–n junction form the so‐called space charge region of the diode. This central region is also called as “depletion region” because it is depleted of free carriers; holes have left the space −xp0 ≤ x < 0 in the p‐type region and electrons have left the space 0 ≤ x ≤ xn0 in the n‐type region, where x = 0 corresponds to p–n junction line. As a result, for xp0 ≤ x ≤ xn0, we have np < ni2, where n, p are the concentration of free electrons and holes in the doped semiconductor, and ni (equal to pi) is the concentration of electrons (holes) in the intrinsic material before doping. The width of the depletion region is W = xn0 + xp0. Note that, deep in the p‐type and in the n‐type regions, the semiconductor remains quasi‐neutral, having a concentration of free holes p = NA and of free electrons n = ND in these quasi‐neutral p and n regions, where NA and ND are the concentrations of acceptor and donor dopants, respectively.
The internal electric field of the p–n junction defines a voltage barrier which is called the device's “built‐in” potential, Vbi, that prohibits further movement of free carriers, and the diode enters an equilibrium phase where the drift current of the minority carriers becomes equal to the diffusion current of majority carriers, hence eliminating each other. This potential is defined by the following equation:
If we consider the band diagram of the semiconductor just before the p–n region is formed, the conduction and VB edges will line up at both sides, while the Fermi level will exhibit a discontinuity at the junction (due to different positioning between EFp and EFn that correspond to the Fermi levels of p‐type and n‐type semiconductors and lie near the VB maximum and CB minimum, respectively, while EFi corresponds to the Fermi level of an intrinsic semiconductor and lies at the middle of the bandgap, Figure 1.1a). Statistical mechanics demands that at equilibrium where the diffusion of opposite carriers is prohibited by the internal electric field, the Fermi level of the system is unique and becomes aligned at both sides of the p–n junction (Figure 1.1b). If we apply an external forward bias with the positive voltage, Va, applied to the p‐region, as illustrated in Figure 1.1c, the field in the depletion region decreases by Va, so that it will not balance anymore the diffusion of the majority of holes and electrons flowing toward the opposite directions of the p–n junction with the drift current of the minority carriers. This will result in an increased majority hole and electron flow toward opposite directions that both contribute to a net current density (Jtot) flowing from the p‐type to the n‐type regions of the junction. On the contrary, if we apply a negative voltage, Va, (reverse bias) to the p‐type region, the electric field in the depletion region will be strengthened and the associated drift current of the minority carriers will be larger than the diffusion current. As a result, the diode is flowed by a small (because the concentration of minority carriers is extremely low) reverse current (also called as reverse saturation current, Js). Figure 1.1d illustrates the qualitative behavior of the diode current density, Jtotal, as a function of applied bias, Va, and shows that diodes based on p–n junctions rectify the current flow as follows:
Figure 1.1 The conduction and valence band edges of the p‐type and n‐type regions of a semiconductor (a) before contact and (b) at the equilibrium when p–n junction is formed. (c and d) The illustration of current generation under forward bias of a p–n junction and (e) the current density–voltage characteristic of a rectifying diode.
where e is the elemental charge, kB is the Boltzmann constant, and T is the absolute temperature. Equation (1.2) is known as “Shockley” (or diode) equation.
In a Si photovoltaic cell, when a photon is absorbed from the semiconductor material, it creates a free electron in the CB, hence leaving a free hole in the VB (Figure 1.2a). Due to the presence of the “internal” electric field of the p–n junction, the photogenerated holes in the crystal are forced to drift toward the direction of the electric field and accumulate on the p‐type region, whereas the photogenerated electrons drift to the opposite direction and accumulate on the n‐type region, thereby producing a potential difference termed as open‐circuit voltage (VOC) of the photovoltaic cell. Under reverse bias, the accumulated photogenerated holes and electrons flow toward the respective electrodes (the hole selective and electron selective contact, respectively), and thus a significant reverse current (termed as photocurrent, Jphoto) (Figure 1.2b) flows through the device. A representative current density–voltage (J–V) characteristic of a solar cell operating in the dark and under illumination is shown in Figure 1.2c. In the operation in the dark, we obtain the rectifying behavior of a diode based on the p–n junction. The curve lying beneath the dark curve shows what happens under illumination where a reverse saturation current is obtained even in the absence of an external voltage. This current is called short‐circuit current (JSC). If we apply a forward bias, it will compensate the internal electric field, and we will reach a point where it becomes zero and thus there is no current flow within the device. At this point, we obtain the VOC because it is as if the electric circuit is open.
For efficient solar cell operation, both JSC and VOC should be maximized. Moreover, the so‐called fill factor (FF) should approach unity. It is the product of the current and voltage, according to the following equation:
where JMAXVMAX gives the maximum power.
Figure 1.2 (a) The PV cell and (b) the production of electrical current due to the photovoltaic phenomenon taking place in a p–n junction. (c) The J–V characteristics of a solar cell in dark and under illumination and (d) the equivalent electrical circuit of a solar cell.
Finally, the obtained power conversion efficiency (n or PCE) is calculated according to the following equation:
A photovoltaic cell can be represented by an equivalent electrical circuit presented in Figure 1.2d. By applying Kirchhoff's law, we obtain the device current:
where IL is the photocurrent, I0 is the reverse saturation current of the diode, Rs is the series, Rsh is the shunt resistance, and n is the so‐called ideality factor. For optimum operation, Rs should be minimized, whereas Rsh should be maximized.
Using the sun as the photon source (the energy of solar photons ranges from 0.3 to over 4.0 eV), a high‐efficiency solar cell can be produced only if we overcome the compromise between photocurrent and photovoltage: a high photocurrent device harvests the majority of the sunlight, whereas a high photovoltage solar cell only harvests the high‐energy (UV) photons. There is, hence, a limit in the maximum theoretically achieved efficiency in a single junction device, which is called the Shockley and Queisser limit and is equal to 30% [8]. This can be only achieved by an ideal solar cell harvesting UV to near‐infrared (IR) photons up to 1.1 eV. Silicon solar cells have progressed considerably over the five decades of their existence, and the to‐date efficiency record of 26.7% for single‐crystal‐based Si photovoltaics [9] is approaching the theoretical limit.
1.2.2 Dye‐sensitized Solar Cells
Dye‐sensitized solar cells (DSSCs) are much different in their architecture and working principle compared to the p–n junction Si photovoltaics. They were first demonstrated in 1972, after the discovery that organic dyes (i.e. chromophores) coated on the zinc oxide (ZnO) electrode of an electrochemical cell could produce electricity upon illumination [10]. Regarding the working mechanism, it was based on the observation that the absorbed photons with energy above the bandgap of the chromophore formed electron–hole (i.e. exciton) pairs in the excited molecule followed by exciton dissociation and electron injection into the wide bandgap metal oxide semiconductor, hence producing electricity (Figure 1.3a). The first demonstration was followed by intense study on ZnO‐single crystal electrodes aiming to increase efficiency of these premature DSSCs. However, due to the limited absorption within the visible range of commonly used chromophores and small surface area of ZnO electrode, the PCE remained as low as 1% [12]. Higher efficiencies were obtained upon increasing the porosity of ZnO electrode to enhance the absorption of dye over electrode and, subsequently, the light‐harvesting efficiency (LHE) of the system. However, large improvements were obtained in 1991 at Ecole Polytechnique Fédèrale de Lausanne (EPFL), when Michael Grätzel replaced the ZnO electrode with a titanium dioxide (TiO2) nanoporous layer with a roughness factor of c. 1000; this skyrocketed the DSSCs efficiency up to 7% [13]. These cells, also known as Grätzel cells, were originally co‐invented in 1988 by Brian O'Regan and Michael Grätzel at UC Berkeley but were highly optimized by Grätzel's team during the next years [14].
The main constituent of the DSSC cell is a thick (of the order of a few micrometers, μm) mesoporous network of TiO2, which consists of nanoparticles around 10–30 nm in diameter, prepared as paste in ethanol or water solutions. The paste can be deposited via spin coating, screen printing, or doctor blade onto a glass substrate coated with fluorinated tin oxide (FTO) that serves as the anode electrode (electron selective contact or anode); afterward, the TiO2 layer is sintered at 400 °C to avoid decomposition. In this “mesoporous” layer, the particles form a network with a high porosity of 40–60%. The mesoporous TiO2 is sensitized with the organic dye (D) and infiltrated with a redox‐active electrolyte [15, 16]. During the next years, this electrolyte was replaced by a solid‐state hole transport material (HTM) to avoid device failure due to electrolyte leakage. The photogenerated electrons (produced when dye is photoexcited with sunlight) are transferred from the excited states of the dye into the TiO2 and then to the front contact (FTO). The oxidized dye (D+) is subsequently regenerated by the redox active electrolyte through a multi‐charge‐transfer process; generally, four iodide molecules as well as two holes take place in the regeneration of dye by creating one triiodide and one iodide species (i.e. I3−/I− redox couple, Figure 1.3b) [11, 17]. The triiodide then transfers holes to the counter electrode (platinum, Pt), where it is reduced to three iodide species. Hence, the circuit is complete and current flows through the device. In the case of solid‐state hole transport‐based DSSCs, the regeneration of dye occurs via initial hole transfer to the highest occupied molecular orbital (HOMO) or VB of the hole transporter and then to the cathode electrode to complete the circuit [18].
Figure 1.3 (a) The DSSC device architecture and the working mechanism of dye‐sensitized solar cell. (b) Illustration of the energy levels at the photoactive heterojunction in a dye‐sensitized solar cell. Light is absorbed in the dye at energies higher than the optical bandgap. The open‐circuit voltage (VOC) is set by the difference in the quasi‐Fermi level for electrons EFn in the TiO2 and the equilibrium redox potential of the electrolyte, I3−/I−. The loss‐in‐potential from going from the optical bandgap (hv) to the open‐circuit voltage is clearly illustrated. Source: Reproduced with permission from Snaith et al. [11]/John Wiley & Sons.
Grätzel's cell was based on a 10‐μm‐thick, optically transparent, and colorless with high surface area TiO2 nanoparticle consisting of film. Deposition of a monolayer of a trimeric ruthenium complex, RuL2(μ‐(CN)Ru(CN)L2′)2, where L is 2,2′bipyridine‐4,4′‐dicarboxylic acid and L′ is 2,2′bipyridine resulted in a significant coloration of TiO2 film due to the strong absorption of the ruthenium complex within the visible (absorption onset at 750 nm and extinction coefficient at 478 nm of 1.88 × 107 cm2 mol−1). As a result, the device harvested a high portion of nearly 46% of the incident solar light energy flux and yielded PCE values between 7.1% and 7.9% in simulated solar light (due to a large JSC above 12 mA cm−2) as well as 12% in diffuse daylight. It also exhibited an exceptional stability by sustaining at least five million turnovers without decomposition. In 1993, Nazeeruddin et al. achieved a record efficiency of 10% by using m‐X2Bis (2,2,‐bipyridyl‐4, 4/‐dicarboxylate) ruthenium (II) complexes as dye sensitizers with absorption onsets up to 800 nm [19]. Further improvements were reported by the EPFL team in 2011 when they demonstrated 12.3% efficient DSSCs that incorporated a cobalt Co(II/III) tris(bipyridyl)–based redox electrolyte in conjunction with a custom synthesized donor–π‐bridge–acceptor zinc porphyrin dye as sensitizer [20]. These authors later demonstrated the record efficiency of 13% holding DSSC, featuring a prototypical structure of a donor–π‐bridge–acceptor porphyrin sensitizer that maximized electrolyte compatibility and improved light‐harvesting properties [21].
1.2.3 Organic Solar Cells
Organic solar cells (OSCs) are layered structures consisting of an organic photoactive layer sandwiched between two opposite electrodes (Figure 1.4a) [22]. The photoactive layer is based on a blend of a polymer donor with either a fullerene or a non‐fullerene acceptor (NFA), hence forming the so‐called bulk heterojunction (BHJ) architecture. This is because of the large Coulombic attraction between electron and hole pairs, the so‐called Frenkel excitons, due to the low dielectric constants (about 3.5) of common organic semiconductors [23].
For these excitons to dissociate into free carriers, the presence of a two‐component system containing the electron donor (D) and the electron acceptor (A) that is similar to that of a p–n junction is necessary [24, 25]. The electron donor should possess a large ionization energy (IE), whereas the electron acceptor should have a high electron affinity (EA) for the following reason: Upon photon absorption, an electron is excited from the HOMO to the lowest unoccupied molecular orbital (LUMO) of the donor, hence forming a Frenkel exciton (Figure 1.4b). This exciton then diffuses toward the D:A heterointerface where it dissociates via an energy gradient into a hole and an electron; the latter transfers to the LUMO of the acceptor material and forms a charge transfer (CT) complex, which will be favorable to occur when:
Figure 1.4 (a) Schematic representation of the bulk heterojunction OSC. (b) Organic solar cell energy‐level diagram depicting the HOMO and LUMO levels of donor and the formation of charge‐transfer states between the donor and acceptor. The process of geminate recombinations is also illustrated. Source: Reproduced with permission from Servaites et al. [22]/Royal Society of Chemistry.
where and is the electron affinity of the acceptor and the donor, respectively, and UD is the binding energy of the exciton (of the order of few hundreds of meV). This sets a theoretical maximum for the attainable voltage output in these cells as being the difference between the donor HOMO and acceptor LUMO, given by the following formula:
representing important material design considerations. The electron/hole pair forming the CT state can also be referred to as geminate pair and is highly dependent on the Coulombic attraction between opposite carriers and on the distance that separates these species [26]. If the latter becomes larger than the coulomb capture radius, the geminate pair dissociates into free carriers; otherwise, the geminate pair will recombine across the donor:acceptor interface, the so‐called geminate recombination, which constitutes a common loss mechanism in OSCs [27, 28]. The dissociated electrons and holes can then be transported through the acceptor and donor domains, respectively, to the respective electrodes, with electrons being collected at the cathode and holes at the anode.
In the most efficient OSC architecture, the BHJ one, an interpenetrating network of pure donor and acceptor domains, forms the active layer because it strikes a balance between promoting exciton dissociation at D:A interfaces and transports charges through the bulk. This implies that the final and leading loss mechanism in OSCs is charge recombination during charge transport to the electrodes. Key objectives in OSCs are the minimization of energy and charge losses both upon exciton dissociation and through transport within the BHJ device (the latter is crucial due to the low carrier mobility and low diffusion length of the order of 10 nm in organic semiconductors) [1] in order to enhance the efficiency of devices.
A challenging step that contributes to efficient OSCs operation is the charge transport and collection at the electrodes. As most photoactive materials exhibit large energetic differences of their molecular levels, i.e. the HOMO of the donor and the LUMO of the acceptor with the corresponding electrodes, a typical OSC device configuration, fabricated on a transparent rigid (e.g. glass) or flexible substrate, usually comprises a hole transport/extraction layer (HTL/HEL) and an electron transport/extraction layer (ETL/EEL), which are all sandwiched between a high work‐function (WF) (anode, the hole‐selective contact) and a low WF (cathode, the electron‐selective contact) electrode aiming at reducing these differences. In principle, the cathode electrode should have a sufficiently low WF (lower than the LUMO of the acceptor in the active layer), whereas the anode electrode should have a sufficiently high WF (higher than the HOMO of the active layer). An energetic mismatch at any device interface may lead to barriers for charge extraction, to high contact resistance, and to space charge formation and undesirable recombination, thus severely reducing device performance. Other issues that may influence charge transport/extraction, even when interfacial energetic alignment is excellent, are the surface energy mismatch between the various layers and poor interfacial compatibility/morphology.
Examples of electron donor materials used in the photoactive layer of BHJ OSCs are the poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl] (PCDTBT), and poly((4,8‐bis[(2‐ethylhexyl)oxy] benzo[1,2‐b,4,5‐b′]dithiophene‐2,6‐diyl)(3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl] thieno [3,4‐b] thiophenediyl)) (PTB7). Materials for electron acceptors are either fullerene such as [6,6]‐phenyl C71 butyric acid methyl ester (PC71BM) and 1′,1″,4′,4″‐tetrahydro‐di[1,4]methanonaphthaleno[5,6]fullerene‐C60(IC60BA) or NFAs such as 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d,2′,3′‐d′]‐s‐indaceno[1,2‐b,5,6‐b′]dithiophene (ITIC‐2F). Although the first OSCs reported in 1984 by Tang exhibit very low efficiency of 1% (they were two‐layer structures based on a phthalocyanine/perylene carboxylic derivative bilayer absorber) [29], recent developments on NFAs have led to a rapid increase in PCEs, with values of ∼17.1% in ternary blend single‐junction cells [30] and ∼17.3% in double‐junction tandem OSCs [31].
1.2.4 Perovskite Solar Cells
Organometallic halide perovskites are materials with a general formula ABX3 (Figure 1.5a) [34]. Typical examples of A‐site substitution are monovalent cations such as organic methylammonium (CH3NH3, MA) and formamidinium (CH(NH2)2, FA) or an inorganic cation such as cesium (Cs) and rubidium (Rb), for B‐site occupation, a heavy divalent metal such as lead (Pb) or tin (Sn) and X is a halogen anion (i.e. Cl, Br, and I). The first reported perovskite solar cells (PSCs) had adopted the mesoporous configuration of their DSSC counterparts, and they were hence termed mesoscopic PSCs (MPSCs). They were considered as solid‐state DSSCs in which the dye sensitizer has been replaced by an organic–inorganic lead halide perovskite. These first demonstrations of PSCs were based on the 3‐dimensional (3D) methylammonium lead iodide (MAPbI3) perovskite, and yielded in mid‐2012 PCEs between 6% and 10% [35–37], which were doubled within a few subsequent years only. Immense research to improve device performance by combining novel perovskite materials and their processing techniques led to a remarkable certified PCE of 27.7% in a tandem configuration [38].
In a typical mesoscopic device architecture (Figure 1.5b), a thin (∼50 nm) compact TiO2 layer is first coated on the FTO on glass substrate [32]. Subsequently, the mesoporous TiO2 layer is deposited on top of the compact layer to serve as the electron transport material (ETM) as well as the scaffold for the infiltration of the perovskite absorber. After its post‐annealing at 450 °C, the deposition of the perovskite absorber layer (300–500 nm thick) takes place. The device is completed by the deposition of a thin HTM capped with a metal electrode, such as gold (Au) or silver (Ag). The working principle of these devices is quite similar to those of DSSC counterparts and can be split into three steps, i.e. light absorption by perovskite absorber, charge dissociation and separation, and charge transport to respective electrodes. In the first step, light is absorbed by a thin perovskite layer. An optimum bandgap of perovskite (∼1.55–1.6 eV) and a high absorption coefficient (∼105 cm−1) ensure that maximum visible light is absorbed by the perovskite layer [39]. High‐efficiency PSCs require negligible light absorption by the TCO and charge transport/extraction layers such that most absorbed light reaches the perovskite absorber layer. Next is charge dissociation and separation. Herein, the low exciton binding energy of MAPbI3, in the range of a few eV only [40], suggests that photon absorption leads to the generation of free charge carriers. This is extremely desired for a high‐efficiency solar cell operation as no external force is required to separate the photogenerated electron–hole pair. The low binding energy and generation of free charge carriers are among the key reasons for high‐efficiency PSCs [41]. In other excitonic solar cells, where photogenerated charge pairs (excitons) are bound with higher binding energy (300–500 meV), losses during exciton dissociation and migration account for significant losses [42].
Figure 1.5 (a) Crystal structure of cubic perovskite with general chemical formula ABX3. Typical device architecture of perovskite solar cells. (b) Device architecture of mesoscopic PSCs. (c) The regular planar (n‐i‐p) PSC configuration where electrons are collected at the conducting substrate and (d) the inverted planar (p‐i‐n) architecture where holes are collected at the FTO substrate. Source: (a) and (b) Reproduced with permission from Krishna et al. [32]/Royal Society of Chemistry. (c) and (d) Reproduced with permission from Lopez‐Varo et al. [33]/John Wiley & Sons.
The third step that completes the photovoltaic operation in PSCs is charge extraction toward respective electrodes. This requires the injection of at least one type of charge carriers from the perovskite absorber layer into the transport material (and blocking of the other type). Charge selective layers that allow only one type of charge carriers to be extracted are hence employed alongside the perovskite layer to facilitate their extraction. These selective contacts heavily influence interfacial recombination, charge accumulation and extraction, and consequently play a critical role in determining photovoltaic parameters in PSCs.
Other architectures besides the mesoscopic cells are the regular planar (n‐i‐p) or inverted planar (p‐i‐n) structures (Figure 1.5c,d) in which the mesoporous TiO2 layer is omitted and the perovskite planar absorber is deposited directly either on the electron transport (n‐i‐p) or the hole transport (p‐i‐n) material, which guides photogenerated charges toward their respective electrode [33, 43]. These planar devices can be processed at temperature below 150 °C and, therefore, are advantageous for mass production.
An interesting aspect while designing a PSC is its tunable electrical properties. From a typically intrinsic perovskite, their electrical conductivity can be tuned to n‐ or p‐type by manipulating its defects density. Typical 3D halide perovskite, for example, MAPbI3 or FAPbI3, shows an optimum direct bandgap (Eg) of ∼1.6 and 1.5 eV, respectively [2–4]. Theoretical calculations predict that an optimized single junction MAPbI3 PSC can deliver PCE of ∼31%. This efficiency corresponds to a photocurrent density (JSC) of 26 mA cm−2, an open‐circuit voltage (VOC) of 1.3 V, and FF of 91% [44]. However, this requires that there be no non‐radiative losses within the bulk of the perovskite as well as the device interfaces. The PSCs still have to overcome losses due to bulk and interfacial defects to reach their Shockley Queisser limit.
Another important device architecture with triple‐mesoscopic layer, which replaces expensive metal contacts such as Au or Ag with carbon (C), is printable MPSCs, also called as Han Cells or Wuhan Cells [45–47]. Such structures, even though lagging in performance compared to their mesoporous or planar counterparts, provide improved operational stability and also are cost‐effective . Moreover, carbon composite electrodes can offer an affordable, conductive alternative with abundant sources and blends of carbon black (CB) and graphite (G) being also chemically resistant toward oxidation/reactions and can be printed in batch or continuous roll‐to‐roll processing, hence offering the possibility for fully printable large area devices [48, 49].
1.3 The Typical Structures of PSC
1.3.1 Mesoscopic Structure
MPSCs are based on a mesoporous ETL such as TiO2 and SnO2 or an insulating scaffold like Al2O3 and ZrO2[50]. The perovskite absorber is then infiltrated in the mesoporous metal oxide scaffold layer, followed by a solid‐state hole conductor, which is deposited on top of the perovskite layer (Figure 1.6a). The reader is referred to Section 1.2.4 for more details about the MPSC architecture. Notably, the most efficient PSCs are the mesoscopic ones as they allow larger portion of the incident light to be absorbed while also exhibiting a high surface area between the perovskite absorber and the bottom ETL. Even though in early years high‐temperature sintering of the metal oxide layer was generally applied, it was later revealed that using metal oxides alternative to TiO2 such as SnO2 that demand post‐treatment at lower temperatures can lead to the possibility of MPSCs fabrication upon flexible substrates and multifunctional device architectures [51].
Figure 1.6 Device architecture for (a) mesoscopic (b) triple layer, (c) regular planar n‐i‐p, and (d) inverted planar p‐i‐n structure.
1.3.2 Triple‐mesoscopic Layer Structure
Besides the mesoscopic structure using a single mesoporous metal oxide at the bottom electrode, solar cells with a double layer consisting of mesoporous oxides such as TiO2 and ZrO2, where the perovskite film is infiltrated, have been successfully demonstrated (Figure 1.6b) [46]. In the first demonstration of this device structure, which was printable, the metal halide perovskite was infiltrated into the mesoporous TiO2/ZrO2 scaffold by drop‐casting a solution through the printed porous carbon (C) layer, which served as the top cathode electrode [46]. The perovskite precursor solution contained PbI2 in g‐butyrolactone mixed with MA and 5‐aminovaleric acid (5‐AVA) cations that formed a mixed‐cation (5‐AVA)x(MA)1−xPbI3 perovskite. The hole‐conductor–free cell with the simple mesoscopic TiO2/ZrO2/C triple layer as a scaffold to host the perovskite absorber yielded a PCE of 12.84% and good long‐term stability.
However, for these printable cells, it is difficult to control the crystallization mechanism of perovskite owing to the complicated triple‐layer mesoporous structure. Recently, a solvent evaporation‐controlled crystallization method has been reported that enabled ideal crystallization of the perovskite absorber in the mesoscopic structure [52]. It was based on the adjustment of the evaporation rate of solvent during annealing in the mesoporous structures. The FP–MPSCs exhibited a homogeneous pore filling in the triple‐layer structure without any additives and achieved a stabilized PCE of 16.26% using ternary‐cation perovskite absorber.
1.3.3 Regular Planar n‐i‐p Structure
This structure is quite similar to thin‐film inorganic and OSCs. It consists of a bottom transparent electron selective electrode (anode), an n‐type thin compact ETL, the perovskite absorber layer, the p‐type HTL, and the top high WF metal cathode (hole selective electrode) (Figure 1.6c
