Heterodimensional Interface for Improved Photovoltage and Stability in Narrow Bandgap Sn–Pb Perovskite Solar Cells via Vacancy‐Ordered Double Perovskite

Sn-Pb perovskite solar cells (PSCs) are considered promising candidates for the bottom subcell in all‑perovskite tandem solar cells, due to their low bandgap and potential to approach the Shockley-Queisser limit. However, their practical application is hindered by the facile oxidation of Sn²⁺ ions and high trap densities. Here, we introduce FA₂SnI₆ vacancy‑ordered double perovskite (FADP) with excellent stability under environmental conditions, to fabricate highly efficient and stable FA_0.7MA_0.3Sn_0.5Pb_0.5I₃ Sn-Pb PSCs. A heterodimensional interface is constructed between the Sn-Pb perovskite and the electron transport layer (ETL) by incorporating FADP, which effectively passivates interfacial defects and suppresses non‑radiative recombination, leading to a remarkable enhancement in open‑circuit voltage (V_OC). Furthermore, the n‑type nature of FADP induces favorable band alignment at the Sn-Pb perovskite/ETL interface, and its environmental robustness contributes directly to enhanced device stability. As a result, the champion Sn-Pb PSC achieved a power‑conversion efficiency (PCE) of 22.21% and maintained 91% of its initial PCE after 600 h of storage under nitrogen, highlighting the potential applicability of FADP.