Improper Narrow Bandgap Molecular Ferroelectrics Enable Light-Excited Pyroelectricity for Broadband Self-Powered Photoactivities

Narrow bandgap ferroelectrics are emerging as critical components for assembling high-performance optoelectronic devices with broadband spectral response, yet integrating narrow bandgap and robust ferroelectricity in a single-phase material system remains a huge challenge. Herein, we report a narrow bandgap improper molecular ferroelectric, (DMAPA)BiI₅ (1; DMAPA = dimethylaminopropylammonium), which has the band gap of 1.94 eV and spontaneous polarization (P_s) value of 1.38 µC cm^-2. It is notable that 1 exhibits unusual dielectric bistability near its Curie temperature (T_c) = 372 K, along with only quite small variation in dielectric constants. This characteristic of improper ferroelectricity endows 1 with large pyroelectric figures-of-merit. Strikingly, light-induced change of its electric P_s leads to ultraviolet-to-near-infrared pyroelectricity in a wide spectral region (266–980 nm), thus achieving broadband self-powered photoactivities. High-quality thin films of 1 fabricated via spin-coating process also exhibit excellent light-induced pyroelectric effects. The integration of photoactivities in narrow bandgap improper ferroelectrics offers a promising pathway toward scalable broadband optoelectronic device application.