Band Structure Optimized by Electron-Acceptor Cations for Sensitive Perovskite Single Crystal Self-Powered...

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.