Benign mid-gap halide vacancy states in 2D-bismuth-based halide perovskite microcrystals for enhanced broadband photodetectors

Lead halide perovskites are widely recognized for their exceptional defect tolerance, setting the benchmark for high-performance optoelectronic applications. Conversely, low-toxicity perovskite-inspired materials (PIMs) typically exhibit suboptimal optoelectronic performance, primarily due to their intrinsic susceptibility to defects. In this study, we address this limitation by exploring the effects of halide vacancies in PIMs through the synthesis of non-stoichiometric Cs₃Bi₂Br₃I_5.2 microcrystals (MCs) with a trigonal crystal structure, incorporating iodine vacancies. Density functional theory simulations reveal that these iodine vacancies introduce benign mid-gap states that facilitate charge transport without perturbing band-edge excitons. As a result, the MCs exhibit sharp photoluminescence emission with a linewidth of 140 meV and a minimal Stokes shift of 147 meV, indicative of efficient band-edge recombination. Transient absorption measurements confirm photo induced mid-gap absorption, while space charge limited current measurements demonstrate low trap densities of 1.1 × 10¹¹ cm⁻³, despite the presence of iodine vacancies. We further fabricated self-driven broadband photodetectors using 2D-Cs₃Bi₂Br₃I_5.2 MCs, achieving a high responsivity of 0.9 A W⁻¹ with a photoresponse extending to 800 nm. While ultrafast carrier localization remains a performance-limiting factor, the room-temperature carrier mobility exceeds 1 cm² V⁻¹ s⁻¹, positioning Cs-Bi-Br-I as a highly promising low-toxicity absorber for advanced optoelectronic and light-harvesting applications.