Modification of surface conduction band minimum in Cu(In1-x,Gax)Se2 photoelectrodes under CO2-saturated conditions

• The effect of surface conduction band minimum control on CO 2 reduction was studied. • The variation in band structure was linked to photoelectrochemical performance. • Increases in Ga ratio increased the Cu(In,Ga)Se 2 photoelectrode photocurrent. • The onset potential was increased for Cu(In,Ga)Se 2 with CsF-treated at 500°C. In this study, two strategies to modulate the conduction band minimum (CBM) of Cu(In,Ga)Se 2 (CIGS) photoelectrodes were investigated to enhance their photoelectrochemical (PEC) performance for CO 2 reduction. First, the surface Ga composition of CIGS film was systematically varied, demonstrating that increasing the Ga content elevates the CBM above the CO 2 reduction potential, thereby enhancing the driving force for reduction reactions and increasing photocurrent density. Second, the effect of CsF post-deposition treatment (CsF-PDT) was evaluated, revealing that CsF-PDT modifies the band structure through the formation of Cs-containing compounds (e.g., CsInSe₂), which reduce grain boundary recombination and improve carrier lifetime, as evidenced by time-resolved photoluminescence. Electrochemical impedance spectroscopy confirmed that the interfacial charge transfer resistance was reduced after CsF-PDT. These combined modifications increased the photocurrent density and onset potential, improving PEC performance. This work highlights the importance of CBM tuning and interface engineering in CIGS photoelectrodes, and will provide insights for future optimizations in solar-driven CO 2 reduction efficiency.