The Role of Interfacial Energetics and Defects on the Efficiency of Tin Perovskite Solar Cells

Tin perovskites are emerging as a sustainable alternative to lead-based photovoltaics, yet their efficiency remains limited by energy-level mismatch and intrinsic instability from tin oxidation. Progress is further hindered by inconsistent electrical behavior, obscuring the true bottlenecks of device performance. Here, we deliver a comprehensive, layer-resolved analysis of FASnI3 solar cells, combining Kelvin probe and photoelectron yield spectroscopy to directly map the band structure and quantify interfacial losses. We reveal that the commonly overlooked Bathocuproine buffer layer plays a decisive role in boosting open-circuit voltage by forming a hybrid energy level with silver, enabling efficient electron extraction.Using time-resolved surface photovoltage, we uncover the ultrafast charge-transfer dynamics governing device operation. These insights expose a critical limitation in the conventional p-p-n, where severe recombination at the perovskite and hole transport interface suppresses performance. A transition to an n-p-p configuration significantly enhances charge extraction and minimizes recombination losses.By integrating these findings into a predictive digital twin, we establish a clear, experimentally validated roadmap toward tin perovskite solar cells exceeding 25% efficiency. This work provides both fundamental understanding and actionable design rules, accelerating the development of high-performance, lead-free photovoltaics.