Stoichiometry dependent phase evolution of co-evaporated formamidinium and cesium lead halide thin films

Due to its scalability, thermal evaporation is an important processing route for perovskites in order to ensure the transition from research to commercialization. In this study, we focus on vacuum co-deposition of (i) FAPbI 3 , (ii) (Cs,FA)PbI 3 and (iii) (Cs,FA)Pb(I,Br) 3 at room temperature and investigate the influence of stoichiometrical variations on the development of α and δ phases. Considering the standard perovskite term APbX 3 , where A = FA and/or Cs and X = I and/or Br, we use EDX to determine the specific ratios of A/Pb, Cs/FA as well as X/Pb, respectively. We find, that at room temperature, the FAPbI 3 δ phase is not easily suppressed. But, in both, FAPbI 3 and (Cs,FA)PbI 3 , more AX leads to a strong increase in α phase growth while more PbI 2 leads to an increase in δ phase. Incorporation of Cs slightly reduces the observed AX/PbX 2 threshold, at which the δ phase is suppressed. Finally, when Br is introduced to the layer, this threshold is reduced far below 3. Ensuring single-phase growth during evaporation of perovskite absorbers for solar cells is a critical step towards industrialization, since the mechanisms of δ-phase suppression need to be fully understood.