Composition Optimization And Crystallization Control Of Perovskite Thin Films

Organo-lead halide perovskites（such as CH₃NH₃PbI₃,MAPbI₃ and NH₂CH=NH₂PbI₃,FAPbI₃）have drawn much attentions due to their suitable band gap,moderate cost and low temperature solution-process ability.These merits make them one of the most promising candidates for the next-generation optoelectronic devices.In 2009,the photoelectric conversion efficiency of perovskite solar cells was only 3.8%,while the certificated efficiency has reached 22.1% in a short period.That efficiency is already acceptable for the commercial applications.And now the stability of perovskite solar cells is the bottleneck that highly restricts its potential application.In this thesis,two strategies,such as optimizing the perovskite composition and adjusting the crystallization process,were employed to enhance the stability of the ogano-lead halide perovskite solar cells.Methylammonium lead triiodide（CH₃NH₃PbI₃,MAPbI₃）is the mostly studied hybrid perovskite absorber material for PSCs,but its stability is relatively poor.One promising way is to regulate its organic components,such as introducing FA into MAPbI₃ perovskite structure.However,the introduction of the FA normally generates non perovskite phase δ-FAPbI₃,degrading the performance of the devices.In this work,a solid-liquid ion exchange method is developed to in-situ fabricate phase pure FAxMA1-x PbI₃ perovskite film.In this experiment,we have optimized the reaction temperature to balance the ion exchange speed and phase preservation.As a result,the component of the pure phase perovskite could be continuous regulated.The efficiency of the optimized FAxMA1-xPbI₃ perovskite solar cells has reached 15.5%,with less hysteresis.In addition to the stability of the perovskite material itself,the moisture in the external environment could also affect its stability.It is considered that water molecules could easily pass through the grain boundaries into the perovskite films,causing the decomposition of the bulk films.Therefore,controlling the number of grain boundaries in the film is another important strategy to improve its stability.In this work,we introduce the organic chloride to form an intermediate phase,and the intermediate phase isdecomposed at high temperature to get improved perovskite film.The introduction of organic chlorides significantly prolonged the crystallization time of perovskite films,resulting in the formation of large grain perovskite films by Oswald ripening.The reduction of the density of grain boundaries on the one hand reduced the internal recombination of the carriers and improved the photoelectric efficiency to 17%;On the other hand,the stability of perovskite devices has also been significantly improved.The device is still able to maintain 90% of the initial efficiency after 400 hours in the surrounding environment,while the relatively small grains perovskite solar cells have been unable to work after 350 hours.Through the research of this paper,we can understand how the composition and crystallinity of perovskite material affect the stability of the materials,which provides the necessary experimental basis and theory for further improving the stability of perovskite solar cells.