Composition and Interface Engineering for Efficient and Stable Inverted Perovskite Solar Cell

摘要

Hybrid organic-inorganic perovskite solar cells (PSCs) have been a rising star in solar energy conversion devices. In the recent five years, the certified champion power conversion efficiency (PCE) of PSCs has leaped to 25.2% from 15%, surpassing that of multicrystalline silicon (22.8%) and CIGS solar cells (23.4%), and approaching the record of single crystal silicon solar cell at 26 %. This is due to the unique structural flexibility of perovskite material over traditional inorganic photovoltaic system in that there are vast possibilities to alter the chemical compositions, fabrication processes and associative materials for device optimizations and scientific investigations. Inverted PSCs are one type of PSCs that exhibit great scalability and good device stability and performance due to the large selection space of material systems. My thesis is mainly focused on studying the composition and interface of perovskite and charge transport materials and their impacts on stability and performance of resulted solar cell device. Perovskite solar cell adopts layer-by-layer structure consist of bottom glass or flexible substrate with transparent conducting oxide (TCO) as bottom electrode. On top of TCO, a layer of hole transporting layer (HTL) or electron transporting layer (ETL) is applied to extract charge from perovskite layer on top. This ETL or HTL will be used as substrate to deposit subsequent perovskite layer which could dramatically affect the film quality due to specific surface properties. Also, the carrier concentration, energy level and carrier mobility of such layer determines the charge extraction as well as charge transport properties in contact with perovskite materials. Perovskite layer is normally deposited by solution process. Due to the polycrystalline nature of resulted perovskite thin film, the orientation and morphology of perovskite film is decisive in the process of light absorption, charge generation and transport. Also, the composition of perovskite precursor solutions can be easily adjusted for achieving certain objectives. A subsequent ETL or HTL is then placed before final evaporation of top electrode normally being metallic materials. In addition, the interfaces between each layers can also be critical in solar cell operation and attracts tremendous scientific effort. To achieve efficient solar cell operation, ETL/Perovskite/HTL layers and their interfaces requires thorough investigations and engineering attempts. In my thesis research, I mainly studied inverted perovskite solar cell based on NiOx as HTL and PCBM as ETL, the basic device structure is glass/FTO/NiOx/Perovskite/PCBM/Ag. I mainly adopted two approaches for efficient and stable photovoltaic devices. First, the composition of perovskite has been studied to understand the physical and chemical impact of compositional variation of related materials on solar cell performance and stability. Due to the structural tolerance of perovskite material, the composition can be varied at a certain range without significantly disrupting the structure and properties. I unravel the effect of different cations including cesium (Cs), methylammonium (MA) and formamidinium (FA) in small band gap FA-based perovskite formula. I systematically studied the crystal growth process with different perovskite formula and found an important crystalline intermediate-mediated film growth that has significant impact on film quality. Increased relative amount of MA against Cs resulted in the formation of MAI-PbI2-DMSO intermediate which retards the crystallization and hinders the transformation of photoactive perovskite phase. On the other hand, Cs rich formula leaded to PbI2-DMSO intermediate which facilitates the process. The impact of compositional variation on electronic structure and hence energy alignment has also been revealed. The inclusion of smaller size MA and Cs shifted the conduction band to be better aligned with electron transporting PCBM. The collective ef