Reversible Thermochromic Bismuth Iodide Enabled by Self‐Adjustment

Abstract Light‐harvesting materials with dynamical management of light transmittance hold great promise in smart photovoltaics (PV). Here, a novel organic–inorganic halide, MVBi 2 I 8 (MV = methyl viologen cation), exhibiting reversible thermochromism and constant bandgap of ≈1.65 eV over a wide temperature range of 77–453 K, is presented as a candidate. The compound consists of 0D (Bi 4 I 16 ) 4– clusters and MV 2+ cations. And the adjacent (Bi 4 I 16 ) 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.