Surface and interface engineering of SnO2-based electron transport layers for high-performance and flexible perovskite solar cells

Perovskite solar cells (PSCs) have emerged as a leading photovoltaic (PV) technology, achieving rapid progress in power conversion efficiency (PCE) and promising prospects for scalable, flexible applications. Within these devices, the electron transport layer (ETL) plays a decisive role in regulating charge extraction, interfacial recombination, and operational stability. Tin dioxide (SnO 2 ) has been widely adopted as an ETL owing to its low-temperature (LT) processability, suitable energy-level alignment, and favorable electronic properties, making it a strong alternative to conventional titanium dioxide (TiO 2 ). Despite these advantages, the performance of SnO 2 -based PSCs is often limited by intrinsic defects, surface hydroxyl groups, and interfacial mismatch, which lead to trap-assisted recombination and instability. Recent advances confirm that surface/interface engineering, via defect passivation, chemical modification, doping, and composite design, effectively tunes the morphology, crystallinity, electronic structure, and interfacial energetics of SnO 2 films. These strategies significantly enhance charge transport, suppress non-radiative losses, and improve device durability. This review provides a comprehensive and mechanism-oriented overview of recent progress in SnO 2 -based ETLs, covering key deposition methods, defect regulation strategies, and interfacial modification approaches across planar, inverted, and flexible PSC architectures. Emphasis is placed on understanding the structure, properties, and performance relationships governing device efficiency and stability. Finally, the remaining challenges and future perspectives are discussed to guide the rational design of high-performance and stable SnO 2 -based PSCs toward practical application.