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.