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

Abstract

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.