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

ABSTRACT 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 Sn ²⁺ ions and high trap densities. Here, we introduce FA ₂ SnI ₆ vacancy‑ordered double perovskite (FADP) with excellent stability under environmental conditions, to fabricate highly efficient and stable FA _0.7 MA _0.3 Sn _0.5 Pb _0.5 I ₃ 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.