Facile Surface Engineering of Composite Electron Transport Layer for Highly Efficient Perovskite Solar Cells with a Fill Factor Exceeding 81%

Abstract After perovskite became a star material, research on perovskite solar cells (PSCs) became a hot topic and developed rapidly. Electron transport layers (ETLs) with high mobility, low‐defect state, and suitable energy level arrangement dominate the development of planar heterojunction PSCs. In this work, the K + and Cs + doped composite SnO 2 ETL ( x mg KC‐SnO 2 ) by a continuous spin‐coating of the x mg Cs + @KOH and SnO 2 aqueous solution shows a substantial improvement in carries extraction, transport and collection properties, leading to the increased fill factor (FF) from 72.48% to 81.00% of the planar heterojunction PSCs, and finally the champion device average efficiency is 21.62%. Moreover, this work improves the interface contacts between ETL/Perovskite, as it reduces energy disorder and simultaneously promotes electron transport, charge extraction at the interface, improve energy level alignment and quasi‐Fermi level splitting, and decreased resistance for charge transfer in the device. In conclusion, this simple and effective method of improving the FF value provides a certain shortcut to the performance of plane heterogeneous perovskite devices.