Tuning Optical and Electronic Properties in Low-Toxicity Organic–Inorganic Hybrid (CH3NH3)3Bi2I9 under High Pressure

Low-toxicity, air-stable methylammonium bismuth iodide (CH₃NH₃)₃Bi₂I₉ has been proposed as a candidate to replace lead-based perovskites as highly efficient light absorbers for photovoltaic devices. Here, we investigated the effect of pressure on the optoelectronic properties and crystal structure of (CH₃NH₃)₃Bi₂I₉ up to 65 GPa at room temperature. We achieved impressive photoluminescence enhancement and band gap narrowing over a moderate pressure range. Dramatic piezochromism from transparent red to opaque black was observed in a single crystal sample. A structural phase transition from hexagonal P6₃/ mmc to monoclinic P2₁ at 5.0 GPa and completely reversible amorphization at 29.1 GPa were determined by in situ synchrotron X-ray diffraction. Moreover, the dynamically disordered MA⁺ organic cations in the hexagonal phase became orientationally ordered upon entering into the monoclinic phase, followed by static disorder upon amorphization. The striking enhancement of conductivity and metallization under high pressure indicate wholly new electronic properties.