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

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<middle dot>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 mu s, a defect density of 1.49 x 1010 cm-3 and a high mu tau product of 4.06 x 10-4 cm2 V-1. This work provides insights into the unique properties of PDABiI5 and (PDA)2Bi2I10<middle dot>2H2O and their structure-property correlation, and offers promising avenues for the development of eco-friendly and functional optoelectronic devices. 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.