One of the promising methods to solve the current energyand environmentissues is photoelectrochemical (PEC) water splitting and CO2 reduction reaction. Chalcopyrite copper indium gallium sulfide (CIGS)and CuIn0.3Ga0.7S2 nanocrystal thinfilms have been considered photovoltaic materials, and here, we demonstratedtheir potential to reduce CO2 into CO. A modified solutionprocess affords to decrease the number of surface ligands, which triggersan increase in the crystal size and an improved reproducibility inperformance and output current. After depositing gold nanoparticleson the surface of CIGS, the Faradic efficiency related to the CO2-to-CO conversion can reach 12.93% in aqueous electrolytesand 25% in nonaqueous electrolytes. Further investigation points outthat in the range of applied potential between -0.1 and -0.2V vs a reversible hydrogen electrode (RHE), the CIGSphotocathode appears to display a stable photocurrent density for1 h, but when applying a higher applied bias, such as of -0.3V vs RHE, the degradation of the current is significant.Additionally, we found that coupling catalysts to the CIGS increasesthe selectivity toward CO and minimizes the competitive hydrogen evolutionreaction.