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Solution-processed Cu-doped SnO 2 as an effective electron transporting layer for high-performance planar perovskite solar cells are systematically studied. The energy-level of SnO 2 electron transporting layer and the crystallization of CsFAMA films exhibit dramatically different evolution trends with respect to the addition of copper. The replacement of Sn ions by copper ions in the materials result in an improvement in conductivity of Cu-doped SnO 2 , further improve the current density of the corresponding devices. A little bit of p-type CuO does a lot for SnO 2 electron transport layer and their solar cells are firstly demonstrated. [Display omitted] • The Cu-doped SnO 2 has been successfully achieved, which results in the enhanced conductivity and improved energy band position for the Cu: SnO 2 electron transport layer. • A better quality of perovskite film with enhanced crystallinity and enlarged grain size was obtained. • The doping concentration of 5 mol% was desirable for the fabricated CsFAMA solar cells with a champion PCE of 21.35%. • The PSCs device based on Cu: SnO 2 ETL delivers excellent stability under continuous light or thermal conditions. A high-quality electron transport layer (ETL) is indispensable for effective and stable planar perovskite solar cells (PSCs). Tin oxide (SnO 2) is considered the most promising alternative to TiO 2 as ETL due to its excellent optical and electrical properties. However, some of improvements need to be carried out for intrinsic SnO 2 , such as conductivity, energy loss, and lattice mismatch in the SnO 2 /perovskite interface. Herein, the Cu-doped SnO 2 (Cu: SnO 2) as ETL has been made through mixing the copper salt precursor solution with SnO 2 colloidal solution in room-temperature solution-processed. The Cu incorporated into SnO 2 increases the conductivity of ETL and uplifts the conduction band to obtain a more suitable band alignment in SnO 2 /perovskite interface for relieving the energy barrier in the charge transporting. Moreover, it further improves the quality of perovskite films with the enhanced crystallinity, enlarged grain size, which have an impact on light absorption and reduction of defect states for the perovskite layer. The power conversion efficiency (PCE) of the prepared planar PSCs with Cu:SnO 2 ETL exceeds 21%, compared with the SnO 2 -based PSCs (19.63%), and the long-term stability was improved. [ABSTRACT FROM AUTHOR]