Reversible Thermochromic Bismuth Iodide Enabled by Self-Adjustment

Light-harvesting materials with dynamical management of light transmittance hold great promise in smart photovoltaics (PV). Here, a novel organic-inorganic halide, MVBi2I8 (MV = methyl viologen cation), exhibiting reversible thermochromism and constant bandgap of approximate to 1.65 eV over a wide temperature range of 77-453 K, is presented as a candidate. The compound consists of 0D (Bi4I16)(4-) clusters and MV2+ cations. And the adjacent (Bi4I16)(4-) clusters are in touch with each other, forming a 3D interaction for the inorganic part. The experimental and theoretical studies reveal that the ionic and covalent interactions in the crystal undergo a self-adjusting process in response to temperature changes. The self-adjustment and electron transfer between inorganic clusters and organic cations enable excellent reversible thermochromism and constant narrow bandgaps over a wide temperature range, which is expected for smart PV windows integrated with information displays and potentially other technologies.