Photovoltaic (PV) and photoelectrochemical (PEC) cells based on Sb2Se3 have seen a rapid rise in interest in recent years and significant progress has been made in their power conversion efficiencies, which is primarily due to its high absorption coefficient and excellent optoelectronic properties. However, a major challenge for Sb2Se3 is the low photovoltage obtained from such devices. Originally it was thought that Sb2Se3 would exhibit low defect concentration as it is a binary compound (better stoichiometric control) with benign grain boundaries. However, recent computational studies have shown the complex defect chemistry of Sb2Se3 and the formation of deep defect states that are detrimental to PV/PEC performance. Cation-anion anti-site defects have been shown to have a low formation energy and are energetically positioned close to the mid gap, serving as deep recombination centers. Therefore, the understanding and the suppression/passivation of such deep defects is crucial for the further development of Sb2Se3 as a PV/PEC material. Several strategies have been presented in the literature such as Te incorporation for defect passivation, sulfurization treatment for improved photovoltage and gradient sulfur doping in Sb2Se3. More fundamental insights into the basic optoelectronic properties for defect passivation/suppression for Sb2Se3, however, have not yet been reported.