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

Abstract 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.