Photoluminescence of Bound‐Exciton Complexes and Assignment to Shallow Defects in Methylammonium/Formamidinium Lead Iodide Mixed Crystals

Abstract The high defect tolerance of metal halide perovskites, in terms of their exceptional optoelectronic properties, is assumed to be due to the very fact that most native point defects are shallow, which does not contribute to the non‐radiative recombination of free carriers. Here, a systematic study is presented, which concerns the evolution of shallow‐defect signatures observed at low temperatures in the photoluminescence (PL) spectra of mixed organic‐cation lead iodide perovskite single crystals (FA_xMA_1−xPbI₃, where MA stands for methylammonium and FA for formamidinium). Below ≈100 K, a number of peak‐like features become clearly apparent in the PL spectra at energies lower than the strong free‐exciton emission, which are related to the radiative recombination of bound exciton complexes associated with native shallow defects (donors and/or acceptors). Based on state‐of‐the‐art ab initio calculations, a tentative assignment is provided for all PL features to different shallow‐defects (Pb, I, and MA vacancies as well as I interstitials) typically present in hybrid perovskites. The defect‐related signatures exhibit a clear trend regarding the mixed‐crystal composition, indicating that the material becomes less prone to defect formation with increasing FA content.