Controlling Crystal Growth of CH3NH3PbI3 Perovskite for Air-Processed High-Efficiency Photovoltaic Devices

CH3NH3PbI3鈣鈦礦晶體生長調控:研發可空氣中加工的高效率光伏器件

Student thesis: Doctoral Thesis

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Award date21 Aug 2017

Abstract

Perovskite solar cells (PVSCs) have attracted growing interests among the photovoltaic community due to their excellent optoelectronic properties, such as high optical absorption coefficient, long charge carrier diffusion length and high charge carrier mobility, which have led to a rapid improvement in device power conversion efficiency (PCE) from 3.8% in 2009 to a certified PCE of 22.1% in 2016. Despite the encouraging advances in device efficiency, understanding the crystallization mechanism and developing air-processed PVSCs are still grave challenges that remained to be addressed for future commercialization.
This dissertation describes in detail the research strategies and major achievements during my PhD study on the above mentioned challenges in PVSCs field. It can be divided into three areas: (1) Investigation and optimization of solution processed high-quality CH3NH3PbI3 perovskite thin films with one-step and two-step approaches; (2) Impact of perovskite bulk intrinsic properties (mobile ions) and interlayer materials on photo-electrical conversion efficiency and device stability; (3) Investigation of ambient effect on perovskite crystal and thin film growth, and development of highly efficient air-processed CH3NH3PbI3 PVSCs.
Firstly, the morphological evolutions of perovskite films prepared by one-step and two-step solution processes are investigated and optimized with different engineering approaches. For one-step method, the perovskite precursor material lead iodide (PbI2) and methylammonium iodide (MAI) are mixed with stoichiometric ratio. Fast reaction between MAI and PbI2 in air induces noncontinuous and branching structures in perovskite films. For two-step method, the MAI is deposited on the top of PbI2 film. The conversion reaction from PbI2 to CH3NH3PbI3 perovskite film is determined by the interdiffusion process between PbI2 and MAI stacking layer precursors, which is controllable in air. Considering the formation mechanism and differences in morphologies of perovskite films prepared by one-step and two-step methods (Chapter 3), two-step method is suggested to be a promising approach for developing air-processed PVSCs. Therefore, the following parts of this thesis focus on investigating and optimizing the PVSCs fabricated by two-step method.
Secondly, in Chapter 4 and Chapter 5, different MAI loading times on PbI2 films are introduced to precisely control the degree of the conversion reaction and also enhance the device performance. Then a correlation between the device performance and fundamental electronic properties of the PVSCs has been systematically investigated. By studying the structural effect and recombination pathways in PVSCs, it is found that the grain size, the bimolecular and trap-assisted recombination processes, as commonly addressed in literatures, are not responsible to the variation in photovoltaic performance. Interestingly, mobile ion concentrations in a range of 1016 cm-3 to 1017 cm-3 is extracted by probing the transient ionic current in PVSCs. Combining with transient photocurrent (TPC) and steady-state photoluminescence (PL) approaches, it is found that the mobile ions in perovskite films reduce the photocurrent, increase the amount of non-radiative recombination and trigger the deformation of perovskite to PbI2. This part of work suggests that more attention should be paid to the intrinsic bulk properties of perovskite films (mobile ions) which play a significant role in determining the device performance and stability.
Besides the intrinsic bulk properties of perovskite films, the interlayer materials also affect the device stability (Chapter 6). To improve the stability of PVSCs, robust ZnO is proposed to be used as electron transporting materials. The formation and stability of perovskite films deposited on ZnO nanoparticles (ZnO-NPs) interlayer are investigated. It is demonstrated that CH3NH3PbI3 perovskite decomposes to PbI2 instantly upon thermal annealing due to the hydroxide groups attached on ZnO-NPs surface which is confirmed by X-ray photoelectron spectroscopy (XPS) measurement. To prevent the perovskite film from decomposition, two buffer layers, small molecular PC61BM and polymeric PEI, have been employed between perovskite and ZnO-NPs. The polymeric buffer layer PEI is found to be more effective to separate these two materials and allows larger perovksite crystal growth with additional post-annealing process, which has significantly improved the fill factor and overall power conversion efficiency of the device. This part of work paves a way for future development of air-processed and air-stable PVSCs by using robust metal oxides as charge transporting interlayers.
To make the PVSCs applicable to low-cost manufacturing process, the possibility of air-processed highly efficient PVSCs has been investigated. Besides the general wisdom of the effect of moisture, for the first time we find that the oxygen in air has severe impact on the quality of the solution-deposited perovskite films. Different from the moisture that induces fast crystallization of PbI2 upon deposition, oxygen exacerbates the wettability of the PbI2 solution on substrates. To reduce the impact of oxygen and moisture on the formation of PbI2, we find that simply preheating the substrate and PbI2 solution can obtain fully covered and uniform PbI2 film deposited in air. This is possibly due to the increased vapor pressure of the solvent at higher temperature to reduce the ingress of oxygen and moisture during the PbI2 film deposition. Using this simple method, an air-processed PVSC made under a humid atmosphere of 70% RH has a record PCE of 18.11%. This work not only reveals the origin of the detrimental effects on perovskite film formation in ambient air, but also provides a simple practical strategy to develop air-processed high-efficiency PVSCs for future commercialization.

    Research areas

  • Perovskite solar cell, Crystal growth, Solution-processed, Air-processed, Photovoltaic cells