Efficient and Stable Tin–Lead Perovskite Photoconversion Devices Using Dual‐Functional Cathode Interlayer

Abstract Tin–lead halide perovskites (TLHPs) are promising photoactive materials for photovoltaics (PVs) due to reduced toxicity and broad light absorption. However, their inherent ionic vacancies facilitate inward metal diffusion, accelerating device degradation. Here, efficient, stable TLHP‐based PV and photoelectrochemical (PEC) devices are reported containing a chemically protective cathode interlayer—amine‐functionalized perylene diimide (PDINN). Solution‐processed PDINN effectively extract electrons and suppress inward‐metal diffusion by forming tridentate metal complexes with its nucleophilic sites. The PV device achieved an efficiency of 23.21% (>81% retention after 750 h at 60 °C and >90% retention after 3100 h at 23 ± 4 °C), and the first demonstration of TLHP‐based PEC devices exhibit a record‐high bias‐free solar hydrogen production rate (33.0 mA cm −2 ; ≈3.42 × 10 −6 kg s −1 m −2 ) when coupled with biomass oxidation, which is ≈1.7‐fold higher than the ultimate target set by the U.S. Department of Energy for one‐sun hydrogen production. These findings demonstrate the potential of TLHPs for efficient, stable photoconversion by the molecular design of the cathode interlayer.