Interplay of grain size, crystal orientation, and performance in mixed-ion lead halide perovskite films

Mixed-cation lead mixed-halide perovskite thin-films are a promising alternative to methylammonium lead iodide due to higher long-term stability, photostability and bandgap tenability. However, the crystallinity of the absorber layer plays an important role for charge carrier collection as well as for an efficient transport between the different device layers. In this contribution, we investigate the influence of grain size by changing the molar concentration of the perovskite precursor containing Rb, Cs, MA, FA, Pb, I and Br on the charge carrier dynamics. We find that with increasing molar precursor concentration, the grain sizes increase and the perovskite grains become more oriented with an improved charge carrier lifetime. In particular, films with small grains show mostly random grain orientation angles, whereas films with larger grains are oriented with {100} planes around an angle of 20 degrees relative to the surface normal. These findings may he the crucial factor in engineering high-quality films leading to high power conversion efficiencies.