Band Structure Optimized by Electron‐Acceptor Cations for Sensitive Perovskite Single Crystal Self‐Powered Photodetectors

Low‐dimensional perovskites afford improved stability against moisture, heat, and ionic migration. However, the low dimensionality typically results in a wide bandgap and strong electron–phonon coupling, which is undesirable for optoelectronic applications. Herein, semiconducting A‐site organic cation engineering by electron‐acceptor bipyridine (bpy) cations (2,2'‐bpy2+ and 4,4'‐bpy2+) is employed to optimize band structure in low‐dimensional perovskites. Benefiting from the merits of lower lowest unoccupied molecular orbital (LUMO) energy for 4,4'‐bpy2+ cation, the corresponding (4,4'‐bpy)PbI4 is endowed with a smaller bandgap (1.44 eV) than the (CH3NH3)PbI3 (1.57 eV) benchmark. Encouragingly, an intramolecular type II band alignment formation between inorganic Pb‐I octahedron anions and bpy2+ cations favors photogenerated electron–hole pairs separation. In addition, a shortening distance between inorganic Pb‐I octahedral chains in (4,4'‐bpy)PbI4 single crystal (SC) can effectively promote carrier transfer. As a result, a self‐powered photodetector based on (4,4'‐bpy)PbI4 SC exhibits 131 folds higher on/off ratio (3807) than the counterpart of (2,2'‐bpy)2Pb3I10 SC (29). The presented result provides an effective strategy for exporting novel organic cation‐based low‐dimensional perovskite SC for high‐performance optoelectronic devices. The engineering of A‐site electron‐acceptor cations in low‐dimensional perovskites is developed to regulate crystal configuration and corresponding electronic band structure. The excellent carrier generation, separation and transport capacities for (4,4'‐bpy)PbI4 single crystal endows a 618 folds higher photocurrent density and 131 times higher on/off ratio than those of (2,2'‐bpy)2Pb3I10 one.