Pronounced Exciton Dynamics in the Vacancy-Ordered Bismuth Halide Perovskite (CH₃NH₃)₃Bi₂I₉ Observed by Ultrafast UV–vis–NIR Transient Absorption Spectroscopy

Abstract

We present an investigation of the ultrafast charge carrier dynamics for the one-third metal-deficient lead-free perovskite (CH 3 NH 3 ) 3 Bi 2 I 9 on mesoporous TiO 2 . Excitation of the perovskite at 400 or 505 nm leads to characteristic second-derivative-type spectral features in the transient absorption spectra, suggesting substantial contributions of bound excitons, in contrast to the widely used lead-based perovskites. The immediate appearance of broad NIR absorption is assigned to TiO 2 conduction band electrons formed by instantaneous dissociation of a subpopulation of excitons at the perovskite/TiO 2 interface. Excitation with excess energy above the perovskite’s band gap opens up an additional fast (70 fs) exciton dissociation channel with about 26% amplitude. Antisolvent-assisted synthesis of (CH 3 NH 3 ) 3 Bi 2 I 9 reduces the crystallite size to about 500 nm but has only a minor effect on the carrier dynamics. The results suggest that photovoltaic applications of this material will likely require bulk-heterojunction architectures to efficiently split the excitons into free carriers.