Investigation On High Performance Organic-inorganic Hybrid Perovskite Solar Cells

As the world is facing the problem of energy crisis,the study of novel thin film solar cells is pretty important for the development and application of renewable energy. Recently, organic-inorganic hybrid perovskite solar cell, as a rising star in the field of novel thin film solar cells, has drawn much attention, due to its abundant raw materials, easy manufacturing process, low cost, light weight and flexibility. Only several years’ development, the power conversion efficiency(PCE) of this kind of solar cell has been increased from 3.8% to 22.1%, which was thought to be a promising candidate for the commercial silicon solar cells. However, there are still some problems need to be solved. On the one hand, most high efficiency perovskite solar cells are based on the conventional structure, in which electron transporting layer of TiOx needs to be made on high temperature, 2, 2’, 7, 7’-Tetrakis(N, N-p-dimethoxyphenylamino)-9, 9’-spirobifluorene(Spiro-OMeTAD) or poly-triarylamine(PTAA) are both very expensive hole transporting materials, the fabrication process is more complex and the flexibility cannot be realized. On the other hand, the stability issue is still a big challenge, which strongly restricts its commercialization process. In order to solve these problems, we carried out our work based on the low-temperature solution-processed inverted planar structure, in which several perovskite systems are involved, the traditional MAPbI3-xClx or MAPbI3 system, MAPb I3-xBrx system and FAPbI3 system. We mainly increase the PCE and stability by fabrication process modification, interface engineering and device structure design. The main contents for each chapter are listed as follows:In chapter 1, we firstly introduced the necessity of investigation on novel thin film solar cells and the background of perovskite solar cells come into being. Secondly, we systematically introduce the main content of perovskite solar cell, including its history, perovskite materials and properties, device structure and working mechanism, the fabrication technique, parameters to evaluate the device performance, latest progress and future trend. At last, we stated our research purpose and contents.In chapter 2, we introduced some fundamental work in the initial sta ge of our research project, including synthesizing raw materials of methylamine iodide(MAI), methylammonium bromide(MABr), formamidinium iodide(FAI)and formamidinium bromide( FABr), investigating the fabrication process of MAPbI3-xClx, MAPbI3 MAPbI3-xBrx, FAPbI3 and FAPbBr3 perovskites, modifying the fabrication process of standard device(ITO/PEDOT:PSS/Perovskite/PCBM/Ag), and investigating a series of related characterization methods, like SEM, XRD, UV-vis absorption and so on. All of these basic studies laid a solid foundation for my research project and all the other works in our research group related to perovskites.In chapter 3, solution-processed bathocuproine(BCP) was employed as cathode interfacial layer in inverted planar MAPbI3-xBrx perovskite solar cells. Compared with evaporated BCP, solution-processed BCP shows superior cathode modification effect due to its smaller charge transfer resistance. As a result, a high fill factor exceeding 85% and a power conversion efficiency exceeding 13% were achieved in CH3NH3 Pb I3-xBrx based perovskite solar cells. The high-efficient MAPb I3-xBrx perovskite solar cells with high fill factors presented in this work have potential to be integrated with CIGS or other kinds of solar cells for tandem devices.In chapter 4, we investigated the FAPbI3 based perovskite solar cells. Since FAPbI3 perovskite shows superior properties compared to the MAPbI3(or MAPbI3-xClx)system, such as broadened absorption range, higher phase transition temperature and better photostability, the investigation on FAPbI3 based perovskite solar cells are much more important. In this work, FAPbI3 was employed as light harvester in inverted planar structure perovskite solar cells for the first time. When the slowed-down annealing process was conducted, the FAPbI3 perovskite layer showed high crystallinity, large grain size and full surface coverage. Our slowed-down annealed FAPbI3-based device shows superior performance to the modified MAPbI3-based solar cell, especially for the short circuit current density. A high PCE of 13.56% was achieved, with a short circuit current density 21.48 m A/cm2. The modified device showed very good reproducibility and photo-stability as well. This work paves the way for low-temperature fabrication of efficient inverted planar structure FAPbI3 perovskite solar cells.