General Post-annealing Method Enables High-Efficiency Two-Dimensional Perovskite Solar Cells.

Two-dimensional (2D) perovskites, with a formula of (RNH3)2MA n-1Pb nI3 n+1, have shown impressive photovoltaic device efficiency with improved stability. The operating mechanism of such photovoltaic devices is under debate and the scope of incorporated organic cations (RNH3+) is limited. We report a general post-annealing method to incorporate a variety of organic cations into 2D perovskites, which demonstrate significant device efficiencies (7-12%). A detailed investigation of the archetypical (C4H9NH3)2MA3Pb4I13 ( n = 4) reveals that such perovskites thin films contain multiple 2D phases (i.e., 2D quantum wells, n = 2, 3, 4, ...). These phases appear to be distributed with decreasing n values from the top to the bottom of the 2D perovskites thin film, enabling efficient energy transfer in the first 500 ps and possible charge transfer at longer time scale, thereby accounting for high device efficiencies. Our post-annealing method is compatible with ambient condition and only requires relatively low annealing temperature for a very short period of time, offering significant prospects for scalable manufacturing of 2D perovskites solar cells.