Structural evolution of sequentially evaporated (Cs,FA)Pb(I,Br)3 perovskite thin films via in situ x-ray diffraction

Abstract

Evaporation of perovskite thin films for solar cell applications is a solvent-free, well controllable, and scalable deposition path with promising prospects for commercialization. Compared to commonly applied simultaneous co-evaporation of various halide precursor salts, sequential evaporation followed by an annealing step allows to better control the amount of deposited precursors, and has the potential to largely improve reproducibility. In this work, Cs/formamidinium (FA)-based lead iodide perovskites are deposited via sequential evaporation in a vacuum chamber and the phase formation and evolution of different precursor-stacking sequences and annealing conditions are investigated with in situ X-ray diagnostics. In addition, some Br is added to investigate the effect of halide intermixing. The stacking sequence is found to strongly influence the formation of dominant phases as well as the preferential orientation and HGHmorphology of the as-deposited films. These variations in turn affect the diffusion and conversion during thermal annealing and ultimately the conversion ratio of the final perovskite layers. For example, it is found that starting the stacking sequences with the A cations (CsI, FAI) favors a fast and complete conversion of the perovskite phase. However, the result is the formation of perovskite layers with large voids.