From solar cells to solar fuels: assessment of Cu(In,Ga)Se₂ absorbers as photocathode for CO₂ reduction

This study explores the potential of Cu(In,Ga)Se₂ (CIGS) absorbers, commonly used in high-efficiency solar cells, as photocathodes to improve solar-to-fuel conversion efficiency. CIGS’s exceptional light absorption, tunable band gaps, and stability make it an ideal candidate for this application. We examined the performance of bare CIGS and CIGS|CdS pn junctions in photoelectrochemical (PEC) CO₂ reduction, emphasizing the crucial role of interface engineering. Additionally, we assessed the impact of additional ultra-thin functional oxide layers (TiO₂, NiO, Al₂O₃, and SnO₂) and inorganic nanostructured co-catalysts (ZnO and Cu₂O). Our findings reveal that while bare CIGS can reduce CO₂ to CO, the introduction of a pn junction significantly enhances current density and selectivity. A well-optimized photoelectrode based on a CIGS|CdS pn junction can achieve remarkable performance despite its instability, attaining up to 98.5% CO₂ reduction selectivity and photocurrent densities of approximately 8 mA cm^-2 at -1.3 V vs SCE under 1-sun illumination. However, protective layers, while improving stability, often led to decreased photocurrent. These insights highlight that combining CIGS absorber layers with appropriate charge transport and catalytic layers is essential for developing efficient and stable PEC systems for CO₂ reduction.