Ultrafast Spectroscopy of Photoexcitations in Organometal Trihalide Perovskites

Studying the room temperature broadband ultrafast transient response of photoexcitations in three perovskite films, namely MAPbI₃, MAPbI_1.1Br_1.9, and MAPbI_3?xCl_x (MA = CH₃NH₃), allowed unravelling the branching ratio between photogenerated carriers and excitons, a key factor for optoelectronic applications of perovskites. An instantaneously generated mid‐IR photoinduced absorption (PA) band, PA₁ is observed in all three perovskites, as well as a strong derivative‐like band of photoinduced bleaching (PB) and PA (PA₂) close to the corresponding absorption band edge. From the distinguished different decay dynamics of the PA bands in MAPbI₃, PA₁ is interpreted as due to the exciton transition, whereas PA₂ and PB are due to band‐filling effect caused by the photocarriers. In contrast, all bands in MAPbI_1.1Br_1.9 and MAPbI_3?xCl_x share the same dynamics and are therefore due to the same species, namely photogenerated excitons. The transient photoinduced polarization memory (POM) for both excitons and photocarriers as well as the steady‐state photoluminescence (PL) emission are observed in MAPbI₃, but not in MAPbI_1.1Br_1.9 and MAPbI_3?xCl_x because they possess cubic symmetry at room temperature. The estimated long excitons diffusion length (≈150 nm) in MAPbI₃ opens up the possibility of photocarriers generation at interfaces and grain boundaries even when the exciton binding energy is large compare to k_BT.