Perovskite Solar Cells: Principle, Materials And Devices

Principle, Materials and Devices

Eric Wei-Guang Diau, Peter Chao-Yu Chen;;;

• Publisher: WSPC
• Year: 2017
• ISBN:
  9789813222533
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

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Energy and climate change are two of the most critical issues nowadays. These two topics are also correlated to each other. Fossil fuels are the main energy supplies that have been used in modern history since the industrial revolution. The impact of CO 2 emission has been a major concern for its effect on global warming and other consequences. In addition, fossil fuels are not unlimited. Due to the increasing demands for energy supplies, alternative renewable, sustainable, environmentally friendly energy resources are desirable.

Solar energy is an unlimited, clean, and renewable energy source, which can be considered to replace the energy supply of fossil fuel. The silicon solar cell is one of the dominant photovoltaic technologies currently, which converting sunlight directly into electric power with around 20% efficiency. This technique was been widely used in mainstream solar energy applications for decades, though the relatively energy-demanding production process remained with challenges to be resolved.

Recently, emerging photovoltaic technologies such as organometal halide hybrid perovskite solar cell has attracted tremendous attention due to their promising power conversion efficiencies (over 22%) and ease of fabrication. Their progress roadmap is unprecedented in photovoltaic history from the material development and efficiency advancement perspective. Beyond the rapid progress achieved in the last few years, it is expected that this novel technology would make an impact on the future solar cell market providing long-term stability and Pb content issues are addressed. These challenges rely on a better understanding of materials and device function principles. The scope of this book is to provide a collection on the recent investigations from fundamental process, materials development to device optimization for perovskite solar cells.

--> Contents:

• Additive-Assisted Controllable Growth of Perovskites (Yixin Zhao and Kai Zhu)
• Control of Film Morphology for High Performance Perovskite Solar Cells (Cheng-Min Tsai, Hau-Shiang Shiu, Hui-Ping Wu and Eric Wei-Guang Diau)
• Sensitization and Functions of Porous Titanium Dioxide Electrodes in Dye-Sensitized Solar Cells and Organolead Halide Perovskite Solar Cells (Seigo Ito)
• P-Type and Inorganic Hole Transporting Materials for Perovskite Solar Cells (Ming-Hsien Li, Yu-Hsien Chiang, Po-Shen Shen, Sean Sung-Yen Juang and Peter Chao-Yu Chen)
• Hole Conductor Free Organometal Halide Perovskite Solar Cells: Properties and Different Architectures (Sigalit Aharon and Lioz Etgar)
• Stability Issues of Inorganic/Organic Hybrid Lead Perovskite Solar Cells (Dan Li and Mingkui Wang)
• Time-Resolved Photoconductivity Measurements on Organometal Halide Perovskites (Eline M Hutter, Tom J Savenije and Carlito S Ponseca Jr)

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--> Readership: Graduate students and researchers in chemistry, materials science and photovoltaics. -->
Keywords:Perovskite Solar Cells;Hole Transporting Materials;Stability;THz SpectroscopyReview:0

1	Additive-Assisted Controllable Growth of Perovskites

Yixin Zhao* and Kai Zhu^†,‡

*School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

^†National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA

^‡[email protected]

Table of Contents

List of Abbreviations

I.Introduction

II.Cl-Based Additives

A.Role of MACl in One-Step Solution Processing

B.Extremely Volatile Cl-Based Additive for Room Temperature Perovskite Processing

C.Adverse Effect from Nonvolatile Cl-Based Additives

III.Excess MA Additive in Nonstoichiometric Precursor

IV.Additives for PbI₂ in Two-Step Method

V.Conclusions

VI.Acknowledgments

VII.References

List of Abbreviations

DSSC	dye-sensitized solar cell
EDX	energy-dispersive X-ray
FTO	fluorine-doped tin oxide
IPA	isopropanol
Jsc	short-circuit current density
MAI	methylammonium iodide
MAPbI3	methylammonium lead iodide
PCE	power conversion efficiency
PSC	perovskite solar cells
PV	photovoltaic
SEM	scanning electron microscopy
TGA	thermogravimetric analysis
TGA-MS	thermogravimetric analysis-mass spectroscopy
Voc	open circuit voltage
XPS	X-ray photoelectron spectra
XRD	X-ray diffraction

I. Introduction

Organic–inorganic hybrid perovskites (e.g., CH₃NH₃PbI₃ or MAPbI₃) were first demonstrated as a functional light absorber in the standard liquid-junction dye-sensitized solar cell (DSSC) configuration using a thick (~10 μm) mesoporous TiO₂ film by Miyasaka and coworkers in 2009.¹ In this seminal work, the facile crystallization/formation of a dark methylammonium lead iodide (MAPbI₃) sensitized TiO₂ layer from the precursor solution via simple spin coating demonstrated the promising advantages of this new semiconductor absorber compared to the complicated dye or quantum dot synthesis process. However, because of the much lower power conversion efficiency (PCE < 4%) compared to conventional DSSCs (PCE > 10%), most DSSC researchers did not focus much attention on this new absorber. Two years later, Park and coworkers significantly improved the PCE to 6.5% by using a combination of thinner mesoporous TiO₂ film, more stable liquid electrolyte, and higher-concentration precursor solution for deposition of MAPbI₃ perovskite.² This study also revealed an unusual concentration dependence of the perovskite crystallization/formation process. At that time, the crystallization process/mechanism of MAPbI₃ was not well understood; MAPbI₃ is believed to in situ crystallized on the TiO₂ surface, but interestingly, it cannot form when the precursor concentration is too low. Such observation implies a complicated crystallization of MAPbI₃. Although the crystallization process was not well understood, the solid-state perovskite solar cells (PSCs) soon surged in PCE to a critical value of ~10% in 2012.^3,4 This performance level almost doubled the PCE of state-of-the-art solid-state DSSCs and approached that of liquid-junction DSSCs. In these two pioneering reports, the one-step solution growth of MAPbI₃ from PbI₂-MAI (where MAI is methylammonium iodide) or the “mixed-halide” MAPbI_3–xCl_x from PbCl₂-3MAI exhibited a similar performance level. However, MAPbI_3–xCl_x soon demonstrated its success in fabricating high-efficiency PSCs with a planar structure, and also led to a long, extensive, still active debate about the role of Cl (sometimes referred to as “Cl doping fever”) in lead halide perovskites. In contrast, the regular one-step method using PbI₂-MAI precursor solution seemed less ideal for producing high-quality MAPbI₃ thin films for PSCs. Consequently, most research groups soon switched to a two-step sequential solution deposition method to prepare high-quality MAPbI₃ films, in which perovskite is formed by the intercalation of MAI into PbI₂ precursor films.⁵ However, the complete PbI₂-to-MAPbI₃ intercalation conversion via the two-step method is challenging in the absence of a mesoporous scaffold for providing MAI diffusion pathways.

Figure 1 shows a schematic illustration of the growth mechanism for one-step and two-step methods.⁶ In the one-step method, the MA⁺ and [PbI₃]^– together with the coordinated solvent molecules crystallize into the perovskite structure under moderate thermal annealing. However, the formation of MAPbI₃ occurs simultaneously with the evaporation of solvent molecules, leading to shrinkage of the precursor films. Thus, a key factor for obtaining high-quality perovskite thin films is to control the crystallization process and solvent extraction/evaporation to avoid or mitigate the precursor film shrinkage, which is prone to create structural defects in the final perovskite films. In contrast, in the two-step method, the precursor film of PbI₂ undergoes substantial volume expansion (about a factor of two) after the MAI intercalation. This volume expansion can readily result in a rough surface, as opposed to the problem of volume shrinkage in the one-step method.

Figure 1. The schematic shrinkage and volume expansion mechanism during the crystallization of MAPbI₃ perovskite in one-step and two-step methods. Taken from Ref.6 with permission of the Royal Society of Chemistry.

The additive-assisted growth methods are generally effective at controlling the shrinkage and volume expansion of precursor films in both one-step and two-step solution deposition of perovskites. In general, the additives used in one-step methods form smooth additive-containing intermediates before the final perovskite films are developed. These intermediates can effectively control perovskite crystallization kinetics and modify the film shrinkage during the perovskite film formation process. In two-step methods, the additive is usually to control the volume expansion or transformation of PbI₂-based precursor films into final perovskite films.

II. Cl-Based Additives

A.Role of MACl in One-Step Solution Processing

To overcome the technical obstacle to deposit high-quality perovskite films in one-step method, solution additives (especially Cl compound) have demonstrated to be effective agents to control the growth of perovskite thin films. Among various additives, the CH₃NH₃Cl or MACl, when added to the standard PbI₂-MAI precursor solution for one-step deposition, can effectively adjust the kinetics of the crystallization process of forming pure MAPbI₃ perovskite thin film, leading to enhanced crystallinity, absorption, and significantly improved coverage on a planar substrate.⁷ Noteworthy is that the amount of MACl additive can vary over a wide range, enabling a large processing window for reproducible results.

Figure 2. (a) Images illustrating the impact of different amounts of MACl and annealing duration (at 100°C) on the appearance of MAPbI₃ perovskite films. (b) Typical XRD patterns of the corresponding films shown in (a). Taken from Ref.7 with permission of the American Chemical Society.

Figure 2 shows the effect of varying the amount of MACl added to the one-step precursor PbI₂-MAI solution on the appearance of the perovskite MAPbI₃ films, which are annealed at 100°C for various durations as indicated. The films prepared from the regular perovskite precursor solution containing only PbI₂ and MAI would immediately turn to a brown film after annealing to remove the solvent. With an increasing amount of MACl, it takes more time to turn the greenish precursor perovskite film to a brown/dark brown color. For example, it takes more than 25 min for the perovskite film to turn brown when 2 molar ratio of MACl is used (...