Preparation Of CsPbIBr₂ Inorganic Perovskite Material And Its Photovoltaic Properties

In the past decade,metal halide perovskite materials have attracted great attention due to their excellent photoelectric properties,and have been widely used in fields such as solar cells and light emitting diodes（LED）.Compared with their organic-inorganic hybrid counterparts,all-inorganic perovskites have exhibited excellent thermal stability;besides,all-inorganic perovskites have relatively broad band gaps,which renders them to be good top cell materials in tandem solar cells.Among the all-inorganic perovskites,CsPbIBr₂ perovskite has suitable band gap（2.05 eV）and relatively excellent thermal and moisture stability.In recent several years,solar cells based on CsPbIBr₂ perovskite have achieved much progress,but there is still a big performance gap as compared with hybrid perovskite.The obstacles include:CsPbIBr₂films are too thin to achieve sufficient light absorption because low-concentration precursor solutions are often used in order to obtain high-quality CsPbIBr₂ perovskite films,which results in low short-circuit current in the cell.Large voltage loss occurs because of the high defect density in the perovskite film.In order to solve the above-mentioned key problems,the work of this thesis focuses on the following two aspects:First,Solvent engineering was utilized to obtain CsPbIBr₂ films with high quality,sufficient thickness,and high reproducibility.Second,Alkali metal（K）was doped into CsPbIBr₂ films to suppress the defect formation and improve the transport and collection of photogenerated carriers in the device.The main findings are detailed as following:（1）The effect of annealing temperature（40℃-360℃）on the quality and photovoltaic characteristics of CsPbIBr₂ film was studied.The results show that the perovskite phase can be formed at low temperature（40℃-200℃）,but CsPbIBr₂films exhibit small crystal grains,poor crystallinity,and poor compactness.Thus,the device efficiency is less than 2%.As annealing temperature is increased,the grain size,surface coverage,and crystallinity of the perovskite film are gradually improved,and the best film was obtained at 280°C.Correspondingly,the highest efficiency of 3.3%was achieved in the device fabricated at the annealing temperature of 280°C.Although further increasing the annealing temperature can lead to increased particle size,the compactness and crystallinity of the perovskite film are deteriorated,resulting in a drop in cell efficiency to<3%.Subsequently,the concentration of the precursor solution was optimized.Experimental results show that the increased concentration is beneficial for the crystallization,compactness,light absorption,and photovoltaic characteristics of CsPbIBr₂perovskite film.However,the perovskite film becomes less dense as the concentration increases from 1.0 M to 1.1 M,resulting in a drop in efficiency from 3.71%to 3.1%.（2）After determining the best annealing temperature and precursor solution concentration,anti-solvent treatment was introduced during spin coating to further optimize the crystallization kinetics of CsPbIBr₂.It was demonstrated that only dripping anti-solvent at the time point when the precursor sate turns from liquid into gel can result in films with high quality.Accordingly,effects of different kinds of anti-solvents including isopropanol,ethyl acetate and chlorobenzene on the quality and photoelectric properties of CsPbIBr₂ film were explored.Compared with the other two anti-solvents,isopropanol can effectively balance the rates of nucleation and grain growth,thereby obtaining a perovskite film with significantly improved coverage and grain size.The conversion efficiency of corresponding device attained 4.65%.（3）Film quality and photovoltaic properties of CsPbIBr₂ were modified in a controllable way through K⁺doping.The crystallinity and particle size of the CsPbIBr₂ film can be improved by doping K⁺within an appropriate level.The enhanced absorbance and much longer carrier lifetime are recorded in doped CsPbIBr₂ with 1mol%K vs Cs as compared with the undoped one.As a result,the device efficiency was improved form 3.71%to 4.18%.When the K⁺doping level increases to 2%-3%,however,the occurrence of pits/holes in the film leads to dramatic reduction in the energy conversion efficiency.