Research On The Structure Of CH₃NH₃PbI₃ Preovskite Films Prepared Using A Vapor-assisted Solution Process

In recent years, organic-inorganic hybrid perovskite type solar cell developed rapidly because of the advantages of high efficiency and low cost. The efficiency of all solid state perovskite solar cell increases to more than 20%, with a large market potential and a bright future. Lead halide perovskite is a direct band gap material, and in the whole visible light wave band showed strong light absorption ability and the optical absorption coefficient is higher than 104cm-1. Perovskite is bipolar, electrons and holes can be transferred simultaneously in perovskite and the transportation distances of electrons and holes are very large, which could reach more than 100 nm,even more than 1 micron. The large transportation distances make the perovskite films in the devices can be very thick to absorb the sunlight energy adequately. The photoelectric conversion efficiency of perovskite solar cell is mainly determined by the properties of perovskite thin film, hole transport layer and electron transport layer.The morphology and crystallinity of perovskite thin film affect the transport and recombination of the carriers directly. It has a great influence on the photoelectric conversion efficiency, so the preparation process of perovskite thin film is very important for the preparation of perovskite type solar cell.Preparation of perovskite thin film is flexible, and the preparation method of perovskite thin film includes one-step solution and two-step solution, etc. One-step solution includes one-step solution spin-coating method, vapor deposition solution of two sources, etc. Two-step solution includes the immersion method, two-step spin-coating method, vapor-assisted solution process, etc. Although the spin-coating solution to prepare perovskite thin films is simple and fast, the low coverage of perovskite films prepared by the spin-coating solution, and large difference existed in morphology, leads to poor reproducibility. Vapor deposition solution of two sources has many advantages, such as less impurity defects in thin films, dense structure and excellent uniformity of surface, while requires high vacuum environment,complicated instrumentation and large energy consumption. Compared with spin-coating and vapor deposition by two sources, the perovskite thin films preparedby vapor-assisted process possess higher surface coverage, lower surface roughness and larger grain size. The quality of films prepared by vapor-assisted solution process is superior to these prepared by the traditional solution method without extra requirement of vacuum. So vapor-assisted solution process is economical and environment-friendly. However the conversion efficiency of perovskite type solar cell prepared by this method is much lower than these prepared by the spin-coating and vapor deposition by two sources. It is necessary to optimize the technology to enhance the device efficiency.In this thesis, the vapor-assisted solution process was further improved, and the experimental and preparation process were optimized. The influence of the substrate temperature to perovskite thin film crystallinity and morphology is studied. The perovskite thin films were prepared using a modified vapor-assisted solution process with different substrate temperatures, and were characterized by Grazing Incidence X-ray diffraction(GIXRD), UV-Visible absorption spectroscopy, and scanning electron microscope(SEM). The results show that the crystallinity, morphology and the optical absorption of perovskite thin film prepared using the modified vapor-assisted solution process are sensitive to the substrate temperature during the preparation. In general, a low substrate temperature in the modified vapor-assisted solution process is more desirable to prepare perovskite films with good crystallinity and high optical absorption for better device performance.