High‐Efficiency and Stable Perovskite Solar Cells Enabled by Low‐Dimensional Perovskite Surface Modifiers

While interface modification based on organic spacer cations has been proven to be a viable strategy to boost the performance of 3D perovskite solar cells, the mechanisms behind the accomplished efficiency enhancement and the selection rule for organic spacers are yet to be clarified. Herein, based on representative four ammonium halide salts featuring different chain lengths as spacer cations for the 2D perovskite surface modifier, it is shown that choosing appropriately sized spacers can lead to synergetic mediation on interfacial passivation of point and structural defects and the crystallization of 2D perovskite components in protecting the buried 3D perovskite film. As a result, promoted charge transport and extraction with suppressed charge trapping and recombination are simultaneously realized in the 3D/2D perovskite solar cells. With the optimal spacer cation phenylpropylammonium iodide (PPAI), the resultant 3D/2D devices produce a power conversion efficiency of 22.57% with a fill factor exceeding 0.8. Favorably, the PPAI‐treated devices exhibit considerable gains in stability under various external stresses.