| Perovskite solar cells（PSCs）have experienced more than ten years of development,and their efficiency has soared from an initial 3.9%to over 25%,becoming the most promising photovoltaic category.Vapor deposition method is a common process for preparing semiconductor thin films,which has the advantages of good repeatability,large area uniformity and low environmental pollution.However,there are still relatively few studies on the preparation of perovskite thin films by vapor-phase methods.During the vapor-solid reaction process of preparing formamidine（FA）-based perovskite thin films by two-step vapor deposition,it is difficult for FAI to diffuse to the bottom of the Pb I2 precursor film,resulting in a large amount of Pb I2remaining at the bottom,and the accumulation of excess FAI on the surface tends to produce yellow phase FAPb I3,which is not conducive to the material’s light absorption.In order to overcome this problem,a Cs Br/Pb I2 mixed precursor film was prepared by a dual-source co-evaporation method,and the light-assisted vapor-solid reaction was carried out with the MACl/FAI mixed vapor to obtain the finally prepared high-quality perovskite film,and high-efficiency rigid and flexible PSC devices.This thesis carries out research work from the following aspects:The first attempt was made to prepare FA-based PSCs with high device performance by the vapor-assisted solution process（VASP）.However,due to the complex composition of the precursor film in the VASP process and the difficulty of the precursor film to wet the substrate,the following parts mainly used two-step sequential vapor deposition method to prepare the perovskite film.To achieve this,we firstly regulated the perovskite composition and prepared perovskite films of FAPb I3,MAFAPb I3,Cs MAFAPb I3,and Cs MAFAPb(BrxI1-x)3 systems.Finally,the Cs Br/Pb I2precursor film prepared by dual-source co-evaporation was reacted with MACl/FAI vapor under the assistance of light,and a high-quality Cs MAFAPb(BrxI1-x)3 perovskite film was obtained.Then we optimized the process parameters of the two-step vapor deposition method.Firstly,the evaporation conditions of the precursor film were optimized,and then the amount of MACl/FAI in the vapor-solid reaction and the heating process of the upper and lower substrates were optimized.After optimization,we finally fabricated a small-area（0.1125 cm2）device with an efficiency of 20.46%and a large-area（1 cm2）device with an efficiency of 19.96%.At the same time,by optimizing the preparation process,the efficiency of the flexible device based on the PEN-ITO substrate prepared by the vapor phase method reaches 17.42%.Finally,we conduct a detailed study on the assisting light applied during the vapor-solid reaction.We found that high-quality formamidinium-based perovskites could be obtained with the assistance of a 13 m W/cm2 white light source,and the device performance is significantly improved compared with the dark conditions.The experimental results of high field polarization show that the assisting light can promote the ion migration in the perovskite film.Time-of-flight secondary ion mass spectrometry（To F-SIMS）analysis showed that the vertical distribution of I-and Cs+ions was uniform in the perovskite films prepared under white and blue light.Space-charge-limited current（SCLC）test analysis shows that the electron defect density is lowered in the perovskite prepared under white and blue light conditions,which is consistent with the higher efficiency of the corresponding devices.In other perovskite films prepared under assisting light with longer wavelengths or in the dark,formamidinium ions accumulate on the surface of the film,and Cs+ions accumulate on the bottom,resulting in a larger lattice constant of the film,which is detrimental to device performance.The synchrotron radiation grazing incidence wide-angle X-ray scattering（GIWAXS）results show that the films prepared with assisting light contain less residual Pb I2,and the lattice constant of the film is smaller,and the residual strain on the surface of the film is smaller,which is beneficial to improve the device performance.To summarize,in this thesis,the formation of perovskite films is thermodynamically and kinetically controlled by a two-step vapor deposition method and a light-assisted method,which greatly reduces the density of defect states in the perovskite films and improves the carrier lifetime and device performance.In addition,this method can be applied to the preparation of large-area perovskite films,providing a new process technology for the large-scale production of PSCs in the future. |
