Overcoming strain‐induced vertical inhomogeneity in perovskite films for all‐perovskite tandem solar cells

Tandem solar cells offer a pathway beyond the Shockley–Queisser limit of single‐junction devices. Among these, all‐perovskite tandems are especially appealing for their low cost and facile fabrication. However, non‐radiative recombination at the interfaces between perovskite absorbers and charge‐transport layers continues to impede their translation from theoretical potential to experimental realization. Here, we develop a molecular‐design strategy for dual interface engineering of the perovskite photoactive layer, addressing the vertical inhomogeneity inherent to solution‐processed films. We demonstrate that the efficacy of surface modification hinges on matching the alkyl‐chain length of diammonium cations to the local lattice dimensions of each sub‐cell. By applying tailored alkyl diammonium salts to both the top and bottom interfaces, we achieve dramatic reductions in non‐radiative loss, lowered interfacial energy barriers, and suppressed vacancy formation. As a result, the power conversion efficiencies (PCEs) of single‐junction cells improved from 16.7% to 20.5% for the high‐bandgap sub‐cell and from 18.9% to 22.4% for the low‐bandgap sub‐cell. Integration into a monolithic tandem architecture yields a PCE of 27.5%, and the device retains 90% of its initial performance under maximum‐power‐point operation (AM 1.5G, 100 mW cm −2 ) at room temperature in ambient air for over 500 h. This work establishes a clear, structure‐guided paradigm for interface passivation in perovskite tandems, unlocking both high efficiency and operational durability.