Moisture Induced Secondary Crystal Growth Boosting the Efficiency of Hole Transport Layer‐Free Carbon‐Based Perovskite Solar Cells beyond 19.5%

Abstract Hole transport layer (HTL)‐free carbon‐based perovskite solar cells (C‐PSCs) show promising commercial application potential due to their attractive advantages of low cost and high stability. However, the power conversion efficiency of C‐PSCs is relatively low, mainly due to the poor crystalline quality of the C‐PSC applicable perovskite films and the energy level mismatch between the perovskite and carbon electrode. Herein, a moisture‐induced secondary crystal growth strategy to simultaneously improve the crystalline quality and optimize the energy level of perovskite film is proposed. The presence of moisture renders the surface of perovskite grains reactive by forming metastable intermediates. It is demonstrated that the commonly considered harmful intermediates can trigger secondary crystal growth. This secondary growth strategy results in improved crystallinity, larger grain size, and better morphology of the perovskite films, which reduce the density of defect states and also benefit the interface contact between the perovskite film and carbon electrode. Furthermore, the secondary growth modulates the surface composition of the film to achieve an optimized energy level alignment. As a result, this secondary growth strategy reduces the charge recombination loss and accelerates the charge transport process in C‐PSCs. Consequently, a new record efficiency of 19.52% is achieved for HTL‐free C‐PSCs.