Reversible structural transformation of metastable lead-free organic–inorganic hybrid bismuth halide single crystals

Abstract

Lead-free organic–inorganic hybrid bismuth halides have been recognized as promising alternatives to lead-based perovskites for low-toxic and environmentally-friendly optoelectronic applications. These materials showcase unique optoelectronic properties, ascribed to their low-dimensional electronic structures, such as a long carrier lifetime, a strong quantum-well effect, self-trapped states, and good stability. However, there is still a lack of understanding of the structure dependency of these properties. Investigations of hybrid bismuth halides are thus highly urgent in the pursuit of developing functional optoelectronic devices utilizing these properties. Herein, we report a new one-dimensional (1D) polymeric organic–inorganic hybrid bismuth halide, PDABiI₅ (PDA = 1,3-propane diammonium), which forms single crystals of centimeter size with a narrow optical bandgap of 1.77 eV. Interestingly, a reversible moisture-induced dimensional transformation was observed on 1D PDABiI₅ through its conversion into 0D dimeric (PDA)₂Bi₂I₁₀·2H₂O, coming with a significant bandgap widening and photoluminescence shifts. This transformation is attributed to water molecules forming hydrogen bonds with the ammonium groups, promoting the deformation of Bi–I bonds from [BiI₅²⁻]_n chains to edge-sharing [Bi₂I₁₀]⁴⁻ dimers. This phenomenon presents exciting prospects for environmentally responsive device applications. Furthermore, optoelectronic characterizations demonstrated a long carrier lifetime exceeding 2 μs, a defect density of 1.49 × 10¹⁰ cm⁻³ and a high μτ product of 4.06 × 10⁻⁴ cm² V⁻¹. This work provides insights into the unique properties of PDABiI₅ and (PDA)₂Bi₂I₁₀·2H₂O and their structure–property correlation, and offers promising avenues for the development of eco-friendly and functional optoelectronic devices.