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 BiBr 5 chains by interacting with bismuth bromide via weakening and reorienting H‐bonds, yielding a mixed 0D/1D (TMPD) 2 Bi 2 Br 10 . H 2 O with a narrow bandgap of 1.89 eV. The 0D/1D (TMPD) 2 Bi 2 Br 10 . H 2 O can be reversibly transformed into 0D (TMPD) 2 Bi 2 Br 10 ( E g = 2.55 eV) with good cycling behavior. The (TMPD) 2 Bi 2 Br 10 . H 2 O turns out to be more stable than (TMPD) 2 Bi 2 Br 10 , and a mechanism involving vacancy‐assisted bromide ion migration is proposed for understanding the formation of BiBr 5 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) 2 Bi 2 Br 10 . H 2 O single crystal exhibits promising photoconductive behavior under a high moisture atmosphere, showing superiority over the conventional CH 3 NH 3 PbI 3 perovskites that cannot survive moisture. This finding offers new insights into developing narrow band gap perovskite hydrates toward practical optoelectronic applications.