Study On Preparation Of Bismuth-doped Perovskite CsPb_1-xBi_xI₃ Via Dual-source Coevaporation And Its Mechanism Of Black Phase Transition

All-inorganic perovskite CsPbI₃ solar cells have attracted extensive attention due to their decent thermal and photostability.The B-site doping strategy can render CsPbI₃ in the black phase at a relatively low annealing temperature and greatly enhance the stability of the black phase.In this paper,the feasibility of preparing bismuth-doped CsPbI₃ was studied via dual-source coevaporation method.In order to understand the difference between the products obtained via coevaporation and their solution-processed counterparts,we designed controlled experiments using solution-processed method,exploring in depth the nucleation process of solution-processed CsPb_1-xBi_xI₃,and finally figured out the mechanism of the black phase transition at low temperature for CsPb_1-xBi_xI₃.The research contents and conclusions are as follows:（1）The solid solution Pb_1-xBi_xI₂ were first prepared via melting.After confirming that it can be evaporated on substrates without stoichiometric deviating,the bismuth-doped perovskites were prepared by dual-source coevaporation using CsI and Pb_1-xBi_xI₂ and were characterized.It is found that the products obtained by coevaporation under a normal growth condition are neither the black phase nor the yellow phase of CsPb_1-xBi_xI₃,but a zero-dimensional perovskite whose structure can be expressed as Cs₄Pb_1-xBi_xI₆.After increasing the evaporation rate of the solid solution Pb_1-xBi_xI₂,the mixed phase of CsPb_1-xBi_xI₃ and Cs₄Pb_1-xBi_xI₆ is obtained.The CsPb_1-xBi_xI₃ obtained by dual-source coevaporation require high temperature of about 340℃to be transformed into black phase,which is almost the same with undoped CsPbI₃,indicating the effect to lower the temperature of the black phase transition is not observed by this experiment.（2）In order to explain the temperature-lowered black phase transition mechanism within the solution-processed CsPb_1-xBi_xI₃,two different solution-processed preparation methods were adopted.Firstly,the CsPb_1-xBi_xI₃was prepared by solution method using CsI and solid solution Pb_1-xBi_xI₂ as precursors.It was found that the resulting films still need a high temperature of340℃to be transformed into black phase.Then the CsPb_1-xBi_xI₃ was prepared by using CsI,PbI₂ and a small amount of Bi I₃ as three precursors.The black phase transition temperature below 100℃was observed.Combined with various characterizations and analyses,we propose that the key factor determining the low-temperature black phase transition of bismuth-doped perovskite CsPbI₃ is grain size and the colloid size within the precursor solution.