Optical Properties Investigation And Structural Phase Transitions Of MA_xFA_1-xPbI₃ Perovskite Materials

Recently,as a new type of opto-electronic material,halide perovskite has large absorption coefficient,long carrier diffusion length and long carrier lifetime and,thus,it becomes the focus in the research field.Lead perovskite has become the mostly investigated material for solar cell due to its narrower bandgap.So far,the power conversion efficiency of perovskite solar cell has exceeded 24%,which means that it is appropriate for commercial use in this respect.However,its optical and electrical properties could be severely affected by the temperature,humidity and light amid the environment.Therefore,the research of stability improvement of pervoskite has been the hot spot in this field.In this thesis,the optical properties and structral phase transitions of MA_x FA_1-x-x PbI₃（MA:CH₃NH₃⁺,FA:CH（NH₂）₂⁺）have been investigated.Samples used herein are confirmed to be cubic phase according to XRD characterization firstly.The influence of A site cations(MA_x FA_1-x)on the optical properties has been investigated by analyzing the temperature dependent（30 K²95 K）photoluminescence spectra and excitation power dependent photoluminescence spectra at low and rome temperature.To explore the phase transitions in perovskite material,phase transition range has been defined,and the phase transition per unit temperature（?E/?T,E here is the peak position changes in phase transition range,T is temperature）has been compared.It is found that the bandgap of MA_x FA_1-x-x PbI₃ become narrower as FA increase.The ratio of MA to FA is linearly related to the bandgap of MA_x FA_1-x-x PbI₃ except for the pure A site cation conditions.Bandgaps of MAPbI₃ and FAPbI₃ diviate from this trend actually.The type of A site cation has significant impact on temperature dependent photoluminescence spectra of MA_x FA_1-x-x PbI₃.It demonstrates the largest red shift of photoluminescence peak while the proportions of MA and FA are comparable when the temperature increases;the red shift range will shrink if either of the two kinds of cations dominant in the structure;MAPbI₃ and FAPbI₃ display abrupt phase transitions compared to the mixed A site cation perovskites.The red shift range here can be used to define the phase transition region.Analyses of phase transitions show that perovskites possess best stability if proportions of MA and FA are comparable;and the materials display poor stability while either of the two kinds of cations dominants in the structure.The stability here derivates from the strain inside the crystal.Strains induced from MA and FA,which have different sizes,have different directions.And they make the crystal stable and count for the gentle red shift of temperature dependent photoluminescence spectra.Above results are paramount for the understanding of optical properties and structural phase transitions of perovskites.And they are meaningful for improving the phase stability and performance of perovskite optoelectronic devices.