Two-Step Synthesis Method And Performance Optimization Of CsPbI₂Br Perovskite Films

With the proposal of"carbon dioxide peaking"and"carbon-neutral",photovoltaics power generation has attracted widespread attention,and solar cells gradually become a hot research area.The highest certified power conversion efficiency of perovskite solar cells was 25.8%since they were first reported in 2009.Although the power conversion efficiency（PCE）of perovskite solar cells have been ceaselessly broken through,the stability of perovskite always restricts its commercial development.So that,the research has gradually focus on how to improve the stability of perovskite solar cells.CsPbI₂Br with better thermal stability have attracted much attention in recent years.However,the two-step method was rarely applied to synthesize CsPbI₂Br.In this research,CsPbI₂Br film was prepared by sequential thermal evaporation method,and FA⁺was doped into CsPbI₂Br by the"evaporation-solution"two-step method to improve the humidity stability of CsPbI₂Br.Specific research contents are as follows:（1）The effects of process parameters such as vacuum degree,distance between evaporation source and substrate,evaporation rate,material pre-melting time,crucible type,and substrate temperature on the morphology and density of PbI₂ films were investigated.It is found that substrate temperature has obvious influence on the morphology of PbI₂.When the substrate temperature is 120℃,the thickness of PbI₂ can be controlled better and the morphology of the PbI₂ layer is relatively density.Based on that,co-evaporation and sequential thermal evaporation method were used to prepare CsPbI₂Br perovskite films.Finally,the sequential thermal evaporation method can fabricate pure phase CsPbI₂Br films,and the best PCE of those solar cells can reach 8.20%,which is much higher than that prepared by the co-evaporation devices（1.09%）.（2）The hole transport layer Spiro-OMe TAD was replaced by CuSCN with better chemical stability,and the CsBr buffer layer was thermally evaporated to reduce the corrosion of CsPbI₂Br by diethyl sulfide which is the solvent of CuSCN.The PCEs of the CsPbI₂Br solar cells with CsBr buffer layer were improved compared with those CsPbI₂Br solar cells without CsBr buffer layer.When the thickness of the CsBr buffer layer was 6 nm,the best PCE of the CsPbI₂Br solar cells was 9.96%.Meanwhile,the thermally evaporated CsBr layer can also passivate the surface defects of CsPbI₂Br to improve the PCEs of perovskite solar cells.（3）Pure Cs_0.5FA_0.5PbI₂Br film was obtained by the"evaporation-solution"two-step method when the annealing temperature was 220℃and the FABr concentration was 60 mg/m L.The highest PCE of Cs_0.5FA_0.5PbI₂Br devices was 8.22%.Compared to CsPbI₂Br film,the humidity stability of Cs_0.5FA_0.5PbI₂Br was greatly improved.When the unencapsulated Cs_0.5FA_0.5PbI₂Br device was stored in the environment（RH=35-40%）for about 1152 hours,and the efficiency dropped to 35%of the initial PCE.Meanwhile,Cs_0.5FA_0.5PbI₂Br still maintained good thermal stability and photostability.After being annealed under 140℃for 1 h or treated by 7.5 W/cm² ultraviolet light for 12 h,the phase and PCEs of Cs0.5FA0.5PbI2Br devices have no obvious changed.