On The Mechanism Of Photo-Instability Of Lead Trihalide Perovskites

Lead halide perovskites possess excellent photoelectric properties,including large light-emission efficiency and absorption efficiency,high light-efficiency,long carrier diffusion distance and lifetime and good defect-tolerance.And there are a series of preparation methods could fabricate lead halide perovskite films with high quality and low defect density.Therefore,lead halide perovskites have developed rapidly in the fields of optoelectronics such as solar cells,light-emitting diodes?LED?,lasers and light detectors.In particular,the power conversion efficiency of perovskite solar cells has exceeded 25%,and the external quantum efficiency of perovskite LED has also reach more than 20%.However,the stability of lead halide perovskites is vulnerable to environmental factors,including water,oxygen,light,heat,which lead to the degradation in materials.Therefore,the current working stability of lead halide perovskite cannot meet the standard requirements of market.Currently,there are few studies on the photo-instability of lead halide perovskites,including photo curing effect,photo dissociation effect,photo-induced phase transition effect,photo-induce phase separation effect,and researchers have not reached a unified consensus on the mechanism behind photo-instability.Therefore,understanding the photo-instability of lead halide perovskites and explaining the underlying mechanism have theoretical guiding value for the material modification and practical photoelectronic devices.In this thesis,we investigate the photo-instability effects of lead halide perovskites CH₃NH₃PbBr₃ films and CsPbBr₃ films under different excitation densities,by observing and recording the relationship between photoluminescence?PL?intensity and lifetime and illumination time.Based on detailed experimental data,we further explaining the underlying mechanism behind the photo-instability effects.The main contents of this paper include:1.Firstly,we fabricated CH₃NH₃PbBr₃ films with smooth and compact surface by chloroform-assisted solution process.According to the evolution of PL intensity and lifetime over time under different excitation densities,we observe that at the low excitation densities,the films undergo the photoluminescence intensity enhancement?PLIE?effect and photoluminescence intensity decrease?PLID?effect subsequently;however at the middle excitation densities,we only find PLIE effect occurring in the films;at the higher excitation densities,only PLID effect takes place in the CH₃NH₃PbBr₃ films.Based on experimental study,we put forward that photo-induced photoluminescence intensity enhancement?PLIE?and photoluminescence intensity decrease?PLID?both occur and compete with each other in the perovskite,and one effect prevail the other depending on specific excitation density.We propose a model related with the formation and migration of Br_i⁰ to explain the mechanism behind the photo-instability effects occurred in CH₃NH₃PbBr₃ films.2.Secondly,we prepared CsPbBr₃ films and CsPbBr₃:Li films doping with 25 mol%Li⁺by PVP-assisted solution process.According to the evolution of PL intensity and lifetime over time under different excitation densities,we observe that at the low excitation densities,the CsPbBr₃ films and CsPbBr₃:Li films both maintain good photo-stability under continuous illumination;however,at the higher excitation densities,the former and the latter both exhibit PLID effect,and the PL intensity of the latter decreases more slowly.According to the photo-instability effects,we put forward that the photo-induced PLID effect is attributed to the migration of Br^-and formation of Pb⁰.We further assert that the doping Li⁺could suppress the ion migration of Br^-,optimizing the photo-stability of all-inorganic lead halide perovskite CsPbBr₃ films.