To gain insight into the PEC performance enhancement in the Sb2Se3-Sn (0.5) photocathode, incident photon-to-current conversion efficiency (IPCE) measurements were performed at 0 VRHE in 1 M H2SO4 electrolyte under 10% white light illumination, as seen in Figure . Both photocathodes show the same photo-response onset (1050 nm), indicating the unchanged band gap with the Sn 2+ doping treatment. The Sb2Se3-Sn (0.5) photocathode exhibits improved photoconversion efficiency across abroad wavelength range from 400 to 800 nm and reaches 80% of IPCE at 400 nm. In contrast, the IPCE value of the Sb2Se3-Control photocathode is below 67% in the whole wavelength range. The IPCE values of both Sb2Se3 photocathodes gradually decrease as the wavelength increases. Moreover, the IPCE values of both Sb2Se3 photocathodes at longer wavelengths (over 850 nm) are identical. This finding potentially implies reduced recombination near the Sb2Se3/TiO2 junction of the Sn-doped Sb2Se3 photocathode, as indicated by the higher IPCE in the blue region. Furthermore, the consistent IPCE in the longer wavelength region is likely attributed to the thin Sb2Se3 films, which are unable to fully absorb all incident radiation (around 270 nm thickness, Figure ). To fully absorb the photons at higher wavelengths, λ > 800 nm, an 800 nm thick Sb2Se3 thin film is required. Integrating the IPCE values with the AM 1.5 G spectrum yields a current density of 18.4 mA cm -2 and 20.6 mA cm -2 for the Sb2Se3-Control and Sb2Se3-Sn (0.5) photocathodes, respectively. Both values are slightly higher than the Jph obtained from the LSV measurements (Figure ) tested under 100% illumination of simulated solar light. The slight discrepancy in the integrated current is due to the light intensity dependence of the photocurrent. The Jph of the Sb2Se3-Control and Sb2Se3-Sn (0.5) photocathodes at 0 VRHE under different light intensities are depicted in Figure . Both Sb2Se3 photocathodes exhibit increased recombination and therefore more losses at higher light intensity, which explains the higher integrated photocurrent obtained from IPCE measurement under 10% white light illumination. IPCE measurement was also carried out at 0.15 VRHE. A considerable enhancement of the IPCE values in the wavelength range of 400-1050 nm was observed in the Sb2Se3-Sn (0.5) photocathode (Figure ). The IPCE value for the Sb2Se3-Sn (0.5) photocathode reaches 65% at the 400 nm excitation wavelength, which is twice as high as that of the Sb2Se3-Control photocathode. This improvement indicates the increased light harvesting efficiency near the front surface of the Sb2Se3 absorber. In addition, the FF values display a rise from 21.7 ± 2.4% (Sb2Se3-Control) to 27.9 ± 1.9% (Sb2Se3-Sn (0.5)). It is unequivocal from these observations that the Sb2Se3-Sn (0.5) photocathodes outperform their Sb2Se3-Control counterparts. Furthermore, we performed the PEC stability test for the Sb2Se3-Sn (0.5) photocathode at a constant potential of 0.1 VRHE under illumination (Figure ). The Jph of the Sb2Se3-Sn (0.5) photocathode steadily declined to 10.9 mA cm -2 after 1.7 hours of illumination, which is approximately 77% of its initial value of 14.1 mA cm -2 , which we attribute to the detachment of the Pt catalyst. After re-depositing the Pt multiple times, the Jph recovers its initial value and then gradually decreases again. As it is not the main focus of this work, we did not attempt to further improve the stability of the devices, which likely lose performance due to an unoptimized TiO2/co-catalyst interface.