Initially, the influence of ηinj can be disregarded, given the identical deposition of Pt cocatalyst on both Sb2Se3-Control and Sb2Se3-Sn (0.5) photocathodes. Thus, Jabs and ηsep emerge as the primary determinants of PEC performance. Direct measurement of Jabs for undoped and Sndoped Sb2Se3 is challenging due to the opaque Mo substrate. Therefore, we combined ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) and thickness data of Sb2Se3-Control and Sb2Se3-Sn (0.5) photocathodes to deduce the value of Jabs. The UV-Vis DRS spectrum of Sb2Se3-Control and Sb2Se3-Sn (0.5) photocathodes with the structure of Mo/Sb2Se3/TiO2 exhibits similar results, as seen in Figure . Based on the reflectance and identical thickness for Sb2Se3-Control and Sb2Se3-Sn (0.5) photocathodes (Figure ), we can reasonably assume that their light absorption is roughly equal. The aforementioned analysis strongly suggests that the improvement in the PEC performance for the Sb2Se3-Sn (0.5) photocathode mainly originates from an increase in ηsep.