A universal reverse‐cool annealing strategy makes two‐dimensional Ruddlesden‐popper perovskite solar cells stable and highly efficient with Voc exceeding 1.2 V

Two‐dimensional Ruddlesden‐Popper (2D RP) layered metal‐halide perovskites have garnered increasing attention due to their favorable optoelectronic properties and enhanced stability in comparison to their three‐dimensional counterparts. Nevertheless, precise control over the crystal orientation of 2D RP perovskite films remains challenging, primarily due to the intricacies associated with the solvent evaporation process. In this study, we introduce a novel approach known as reverse‐cool annealing (RCA) for the fabrication of 2D RP perovskite films. This method involves a sequential annealing process at high and low temperatures for wet perovskite films. The resulting RCA‐based perovskite films show the smallest root‐mean‐square value of 23.1 nm, indicating a minimal surface roughness and a notably compact and smooth surface morphology. The low defect density in these 2D RP perovskite films with exceptional crystallinity suppresses non‐radiative recombination, leading to a minimal non‐radiative open‐circuit voltage loss of 149 mV. Moreover, the average charge lifetime in these films is extended to 56.3 ns, thanks to their preferential growth along the out‐of‐plane direction. Consequently, the leading 2D RP perovskite solar cell achieves an impressive power conversion efficiency of 17.8% and an open‐circuit voltage of 1.21 V. Additionally, the stability of the 2D RP perovskite solar cell, even without encapsulation, exhibits substantial improvement, retaining 97.4% of its initial efficiency after 1000 hours under a nitrogen environment. The RCA strategy presents a promising avenue for advancing the commercial prospects of 2D RP perovskite solar cells. High‐quality 2D RP perovskite film grown along the out‐of‐plane direction is fabricated by the reverse‐cool annealing strategy. 2D RP perovskite solar cells show high charge carrier extraction efficiency and small non‐radiative open‐circuit voltage loss. The champion efficiency of solar cells achieves 17.8%, which sustains 97.4% of the initial efficiency after 1000 h under the N2 environment.