Interfacial Manipulation of Electron Transport Layer via Viologen Surface Treatment for Highly Stable and Efficient Inorganic Perovskite Solar Cells

The stability and efficiency of inorganic perovskite solar cells (PSCs) remain limited owing to the presence of interfacial and bulk-related defects. To address this issue, a multifunctional defect-passivating interlayer can be introduced. In this study, 1,1′-bis(3-sulfonatopropyl)-viologen (BSP-Vi) was synthesized for depositing a multifunctional interlayer between a TiO₂ electron transport layer (ETL) and CsPbI₃ perovskite absorber. The sulfonate group in BSP-Vi effectively interacts with both oxygen vacancies on the TiO₂ surface and undercoordinated Pb²⁺ species in the perovskite, leading to substantial defect passivation in both the ETL and perovskite absorber. BSP-Vi induces a favorable shift in interfacial energy levels and facilitates the formation of a perovskite film with improved crystallinity and reduced defect density. Consequently, the optimized PSC incorporating 0.2 wt% BSP-Vi achieves a power conversion efficiency (PCE) of 16.93%, representing a marked increase from that of the control (PCE = 16.08%). The maximum power point tracking test demonstrates that the PSC with BSP-Vi-treated interlayer maintained 95% of the initial performance after 160 h of continuous operation. This study highlights the potential of introducing sulfonate-group-based materials at the ETL/perovskite interface as a promising route to simultaneously passivate defects in and enhance the efficiency and stability of inorganic perovskite photovoltaic devices.