In this study, highly efficient and flexible Na-doped kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells with an efficiency of over 8% were developed. A CZTSSe thin film was deposited on a flexible Mo foil using metal precursors with varying thickness of the NaF layers on the CZTSSe thin film. The sample maintained 92.2% of its performance after 1000 bending cycles with a bending radius of 12 mm. To investigate defect passivation effect of Na doping through the electrical properties of Na-passivated grain boundaries (GBs), the cells were examined by scanning probe microscopy. Downward band bending was observed at GBs in the Na-sufficient CZTSSe absorber layers; that is, Na enhanced the potential barrier at the CZTSSe GBs in the solar cells and the separation of carriers. The carrier separation increased under illuminated condition. Moreover, the surface photovoltage (SPV) of the cells was measured at laser wavelengths of 405, 532, and 640 nm using photo-assisted Kelvin probe force microscopy to investigate the photogenerated carrier transport in Na-passivated CZTSSe solar cells. The Na content strongly affected the photo-induced changes in the carrier behavior. A cell with a sufficiently Na-doped CZTSSe absorber layer exhibited a large SPV with a maximum value of 46 meV under illumination, whereas Na-deficient samples showed a lower SPV, indicating that Na passivates defects that act as recombination sites. A photo-induced defect passivation effect was observed in a cell with optimal Na content under 405 nm wavelength illuminated condition, which resulted in charge accumulation at the absorber surface.