The Narrowest Band Gap Ever Observed in Molecular Ferroelectrics: Hexane‐1,6‐diammonium Pentaiodobismuth(III)

Narrow band gaps and excellent ferroelectricity are intrinsically paradoxical in ferroelectrics as the leakage current caused by an increase in the number of thermally excited carriers will lead to a deterioration of ferroelectricity. A new molecular ferroelectric, hexane‐1,6‐diammonium pentaiodobismuth (HDA‐BiI₅), was now developed through band gap engineering of organic–inorganic hybrid materials. It features an intrinsic band gap of 1.89 eV, and thus represents the first molecular ferroelectric with a band gap of less than 2.0 eV. Simultaneously, low‐temperature solution processing was successfully applied to fabricate high‐quality ferroelectric thin films based on HDA‐BiI₅, for which high‐precision controllable domain flips were realized. Owing to its narrow band gap and excellent ferroelectricity, HDA‐BiI₅ can be considered as a milestone in the exploitation of molecular ferroelectrics, with promising applications in high‐density data storage and photovoltaic conversion.