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A novel SnO 2 -based catalyst doped with Cu and N for highly efficient photoelectrochemical reduction CO 2 to formate was synthesized by a two-step process of facile hydrothermal and pyrolysis methods. It is demonstrated that introducing Cu and N significantly enhances the catalytic activity for CO 2 reduction, which is attributed to the defect levels that narrow the band gap to accelerate the charge transfer. • Co-doping of (Cu, N) generated defect energy levels to improve catalysis performance. • The doping of (Cu, N) significantly increased oxygen vacancies in SnO 2 based catalysts. • The optimal doping amount of 7 mol% resulted in a maximum Faraday efficiency of 54.97%. Electrocatalytic and photocatalytic reductions of CO 2 into valuable chemicals or fuels have been considered a compelling strategy for environmental remediation and renewable energy conversion. In this field, SnO 2 based materials have been widely applied as a promising electrocatalysts in the conversion of CO 2 to formate, but the inferior catalytic efficiency is still the severe barrier for the application. In this study, a novel SnO 2 -based catalyst with Cu and N co-doping was designed for efficient and high-selective photoelectroreduction of CO 2 into formate. XRD, FT-IR and XPS analyses demonstrated that (Cu, N) was doped into the lattice of SnO 2 via replacing Sn and O, resulting the significant increase in the amount of oxygen deficiency from 27.18% to 36.72%. CV, LSV and EIS measurements proved that the (Cu, N) doping dramatically improved the photoelectrocatalytic performance of SnO 2 based electrocatalysts for CO 2 reduction. When the co-doping amount of (Cu, N) was 7 mol%, the Faradaic efficiency reached 54.97% with a lowest charge-transfer resistance and a fastest electron transfer rate. Furthermore, the DFT calculations revealed that the co-doping of Cu and N brought about the defect levels to narrow the band gap for the acceleration of the charge transfer, thereby improved the performances in electroreduction as well as photoreduction of CO 2. [ABSTRACT FROM AUTHOR]