Phase homogeneity mediated charge-carrier balance in two-step-method halide perovskite photovoltaics†

The multi-scale chemical phase heterogeneity of halide perovskites seriously affects the physical functional integrity of semiconductors and the performance of photovoltaic devices. However, the phase homogeneity in two-step-method perovskites (TSPs) has rarely been investigated. Here, we elaborate on the severe heterogeneity of the FAMA-based TSP film and found that residual PbI₂ crystallites and Pb⁰ species accumulate at the top while a Cl/MA-rich interface is present at the bottom, impeding carrier nonequilibrium transport in the vertical direction. The homogeneity of the TSP film is reinforced by chemical tailoring with 4-methoxyphenethyl ammonium chloride, thus achieving superior structural stability and a charge carrier balance dynamic process. The target TSP p–i–n device achieves a recorded power conversion efficiency of 25.12% under 1-sun illumination (certified at 24.01%). This study uncovers the hidden physicochemical properties of the TSP film, guiding the understanding of microscopic homogeneity and functional integrity and the design of efficient two-step-method inverted perovskite solar cells.