Narrow‐Bandgap Mixed 0D/1D Bismuth Bromide Perovskite Hydrate Enabled by Aromatic Ditertiary Ammonium Spacer

Abstract Perovskite hydrates have attracted extensive attention in diverse fields with intriguing optoelectronic properties, while their structural distortions usually cause large band gaps, which yield inferior light absorption and diminished optoelectronic device performance. Herein, modulating the structural distortion by steric hindrance effects is reported. Specifically, an N,N,N′,N′‐tetramethyl‐1,4‐phenylenediammonium (TMPD)²⁺ is adopted to template regular BiBr₅ chains by interacting with bismuth bromide via weakening and reorienting H‐bonds, yielding a mixed 0D/1D (TMPD)₂Bi₂Br₁₀^.H₂O with a narrow bandgap of 1.89 eV. The 0D/1D (TMPD)₂Bi₂Br₁₀^.H₂O can be reversibly transformed into 0D (TMPD)₂Bi₂Br₁₀ (E_g = 2.55 eV) with good cycling behavior. The (TMPD)₂Bi₂Br₁₀^.H₂O turns out to be more stable than (TMPD)₂Bi₂Br₁₀, and a mechanism involving vacancy‐assisted bromide ion migration is proposed for understanding the formation of BiBr₅ chains. Density functional theory (DFT) calculations indicate that the (TMPD)²⁺ triggered hydration has a crucial impact on the downshift of the conduction band edge, thus decreasing the band gap. Moreover, the (TMPD)₂Bi₂Br₁₀^.H₂O single crystal exhibits promising photoconductive behavior under a high moisture atmosphere, showing superiority over the conventional CH₃NH₃PbI₃ perovskites that cannot survive moisture. This finding offers new insights into developing narrow band gap perovskite hydrates toward practical optoelectronic applications.