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)PbI3 and (iii) (Cs,FA)Pb(I,Br)(3) at room temperature and investigate the influence of stoichiometrical variations on the development of alpha and delta 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) delta phase is not easily suppressed. But, in both, FAPbI(3) and (Cs,FA)PbI3, more AX leads to a strong increase in alpha phase growth while more PbI2 leads to an increase in delta phase. Incorporation of Cs slightly reduces the observed AX/PbX2 threshold, at which the delta phase is suppressed. Finally, when Br is introduced to the layer, this threshold is reduced far below 3.