Organic–inorganic lead halide perovskite solar cells have shown a rapid increase in power conversion efficiency (PCE) in the past decade. However, their performance still suffers from trap-assisted decline due to defects at the surface and grain boundaries in the polycrystalline active layer. In this study, a thin BAI layer was formed on the CH₃NH₃PbI₃ surface, which was then transformed into either a two-dimensional (2D) perovskite layer (BA₂PbI₄) or the organic salt itself by controlling the post-annealing process. A variety of characterization results show that the 2D perovskites could effectively reduce the trap-assisted charge recombination and increase the device stability. Therefore, a remarkably enhanced PCE of 20.6% was achieved in the modified inverted perovskite solar cells.