Impact of excess PbI2 on the structure and the temperature dependent optical properties of methylammonium lead iodide perovskites

Abstract

We investigate the impact of excess PbI₂ in the precursor solution on the structural and optical properties of thin films of the model hybrid perovskite methylammonium lead iodide (MAPbI₃). We find that excess of PbI₂ in the precursor solution results in crystalline PbI₂ in the final thin film that is located at the grain boundaries. From UPS we find that this crystalline PbI₂ phase has no direct impact on the electronic structure of MAPbI₃. In contrast to this, temperature dependent absorption measurements indicate a systematic change in the temperature dependence of the exciton binding energy in the perovskite. We also observe a decrease in the critical temperature and a concomitant smearing out of the tetragonal–orthorhombic phase transition as a function of excess PbI₂. Our results thus help to better understand the exact role of PbI₂ in the perovskite layer and pave the way for a more tailored design of perovskite solar cells.