To elucidate the origin of the enhanced ηsep observed in Sb2Se3-Sn (0.5) photocathode, scanning electron microscopy (SEM) and X-ray diffraction (XRD) were first utilized to explore the morphology and orientation of undoped and Sn-doped Sb2Se3 films. Figure displays the top-view and cross-sectional SEM images of undoped and Sn-doped Sb2Se3 films, clearly revealing the formation of compact, pinhole-free layers. From the cross-sectional SEM images, all Sb2Se3 films are stacked layer by layer, parallel to the Mo substrate. Observations from the high magnification top-view SEM images (Figure ) show small particles aggregating on the surfaces of the Sb2Se3-Sn (1.0) and Sb2Se3-Sn (1.5) films, which could be related to the noticeable decrease in PEC performance. To evaluate this assumption, the surface composition was examined using Raman spectra (see below).