Improved SnO₂ Electron Transport Layers Solution-Deposited at Near Room Temperature for Rigid or Flexible Perovskite Solar Cells with High Efficiencies

Electron transport layer (ETL) is a functional layer of great significance for boosting the power conversion efficiency (PCE) of perovskite solar cells (PSCs). To date, it is still a challenge to simultaneously reduce the surface defects and improve the crystallinity in ETLs during their low-temperature processing. Here, a novel strategy for the mediation of in situ regrowth of SnO₂ nanocrystal ETLs is reported: introduction of controlled trace amounts of surface absorbed water on the fluorinated tin oxide (FTO) or indium–tin oxide (ITO) surfaces of the substrates using ultraviolet ozone (UVO) pretreatment. The optimum amount of adsorbed water plays a key role in balancing the hydrolysis–condensation reactions during the structural evolution of SnO₂ thin films. This new approach results in a full-coverage SnO₂ ETL with a desirable morphology and crystallinity for superior optical and electrical properties, as compared to the control SnO₂ ETL without the UVO pretreatment. Finally, the rigid and flexible PSC devices based on the new SnO₂ ETLs yield high PCEs of up to 20.5% and 17.5%, respectively.