Spacer Cation-Driven Band-Edge Reconstruction and Carrier Lifetime Enhancement in Two-Dimensional Ruddlesden–Popper Perovskites

Two-dimensional (2D) layered metal halide perovskites, characterized by heterojunctions between organic spacers and inorganic metal halide layers, allow for a customizable band alignment through spacer cation engineering. Enriching functional groups of spacer cations to extend π-conjugation is a practical strategy to reconfigure band edges, but photogenerated carrier dynamics with distinct band alignments remain unexplored. Using (ThMA)2PbI4 and (ThFA)2PbI4 (ThMA = 2-thiophenemethylammonium, ThFA = 2-thiopheneformamidinium) as representative systems, we demonstrate that the extended π-conjugation in the ThFA spacer reconfigures the conduction band edge states, resulting in a type-II band alignment, in contrast to the type-I alignment in (ThMA)2PbI4. Employing ab initio nonadiabatic molecular dynamics combined with time-dependent density functional theory, we show that the (ThFA)2PbI4 perovskite extends the charge carrier lifetime to over 5 times that of (ThMA)2PbI4, due to spatially separated electrons and holes, and fewer efficient recombination channels. The established composition–structure–property relationship provides a design strategy for improving the performance of 2D perovskite solar cells and optoelectronic devices.