Synergistic Bulk and Interface Engineering of SnO2 via Ammonium Formate for Efficient and Stable Perovskite Solar Cells

Tin oxide (SnO 2 ) is a widely used electron transport layer (ETL) in perovskite solar cells (PSCs), yet its performance is often limited by surface defects, energy level mismatch, and insufficient crystallinity. While ammonium formate (AF) has emerged as a promising additive, the direct incorporation of salts into SnO 2 colloids typically induces severe agglomeration and colloidal instability, compromising film quality. In this study, we demonstrate a synergistic bulk and interface engineering strategy to overcome these limitations. We successfully stabilized the AF-doped SnO 2 precursor using ammonia (NH 3 ·H 2 O) to prevent agglomeration and employed a secondary AF surface modification to maximize interfacial passivation. This dual-passivation approach not only enhances the electron mobility of SnO 2 and aligns its energy levels with the perovskite but also facilitates the growth of high-quality perovskite films with reduced defect density. Consequently, the modified PSCs achieved a champion power conversion efficiency (PCE) of 24.54%, significantly higher than the 23.29% of the control device, along with excellent long-term storage stability.