Ammonium iodide-incorporated SnO₂ obtains perovskite solar cells with over 24% efficiency

Tin dioxide (SnO2) as the most promising electron transport layer (ETL) has been widely used in high-efficiency perovskite solar cells (PSCs) due to its excellent optical/electronic properties, chemical stability, and low-temperature processing. However, the surface of SnO2 ETL contains defect sites, which result in energy losses in PSCs. In order to passivate the defects of SnO2 surface and together tune the electronic properties of SnO2 ETL for getting high-performance PSCs, we herein incorporate the low-cost material ammonium iodide (NH4I) into the SnO2. After the NH4I doping, the optimized photovoltaic power conversation efficiency is significantly enhanced (the highest efficiency can reach 24.4%), the hysteresis of device is largely suppressed to a negligible level, and the stability of device is also obviously improved. The origin of these enhancements is further disclosed by the positive effects of NH4I doping on both ETL and perovskite film: the surface morphology of ETL is effectively flatten, the energy level of ETL is suitably adjusted, the electron mobility of ETL and the perovskite grain size are clearly increased, the surface defects of ETL and the trap states in the perovskite film are greatly reduced, and the PbI2 residue in the perovskite layer is obviously diminished. The study here of incorporating cheap inorganic small molecule in the ETL provides an ingenious way to enhance the performance of the planar PSCs.