Sodium Oxamate‐Assisted Dual Interface Modulation for High‐Efficient Hole Transport Layer‐Free Carbon‐Based Perovskite Solar Cells

The inferior interfaces at both the perovskite (PVK)/SnO2 electron transport layer (ETL) and PVK/carbon electrode (C), particularly defect states and energy level misalignment, remain primary bottlenecks. Here, a bifacial modification strategy based on sodium oxamate (SO) is proposed, revealing its effective modulation of the properties at both above interfaces. Comprehensive experimental and theoretical investigations reveal that SO effectively modulates both the SnO2/PVK and PVK/C interfaces. Density functional theory (DFT) calculations coupled with experimental evidence demonstrate that SO not only optimizes the electron mobility of SnO2 and synergistically aligns the energy level gradient across the SnO2/PVK/C stack, but also facilitates effective defect passivation at both interfaces. Specifically, the carbonyl groups of SO coordinates with undercoordinated Sn⁴⁺ ions at the SnO2 surface, passivating detrimental oxygen vacancies, while the amino groups of SO bind to undercoordinated Pb2+ ions at the PVK interfaces. Furthermore, the Na+ ions significantly suppress the migration of I- ions within the PVK lattice. Consequently, the champion power conversion efficiency (PCE) of HTL-Free C-PSCs with SO delivers is 18.03%. Moreover, the prolongs lifetime over 90% of the initial PCE after 1000 h in environmental conditions. This work introduces an effective bifacial interface engineering strategy for efficient and stable HTL-free C-PSCs.