Constructing Vertically Oriented 2D Perovskite Passivation Layer by Surface Chemical Polishing for Efficient Carbon-Based Perovskite Solar Cells.

2D/3D perovskite heterojunctions can effectively passivate surface defects of perovskite films and improve device performance. However, the 2D perovskite formed on the complex surfaces of 3D perovskite is typically randomly oriented, limiting the effect of defect passivation and impeding charge transport. Achieving vertically oriented 2D perovskite on 3D perovskite remains a significant challenge. In this work, we propose a chemical polishing strategy to construct vertically oriented 2D perovskite. By a pre-transformation chemical polishing design, the impurities on the surface, including residual PbI2 and FAI, can be simultaneously removed. This achieves a "clean" surface environment on 3D perovskite, providing an ideal template for subsequent 2D perovskite formation. This strategy realizes the formation of vertically oriented 2D perovskites on the surface of 3D perovskites by the solution approach. The 2D/3D perovskite heterojunction constructed through this polishing route exhibits lower defect states and enhanced charge extraction capability. This strategy increases the efficiency of hole transport layer-free carbon-based perovskite solar cells to a new record of 21.36%. The operational stability of the device is also improved.