A robust photovoltaic (PV) is essential for long-term redox biotransformations in biocatalytic photoelectrochemical (PEC) platforms. Here, we report a single Cu(In,Ga)Se2 (CIGS) solar cell for unbiased photobiocatalytic reduction reactions. The photoanode/CIGS/cathode tandem assembly drives cofactor-dependent biocatalytic CO2 reduction under visible light. Our scalable PEC-PV tandem device achieves the longest reaction time of 72 h and the highest ever recorded turnover frequency and total turnover number of the cofactor of 0.236 h−1 and 11.2, respectively, for biocatalytic PEC production of formate through cofactor regeneration. This benchmark performance is attributed to the excellent PEC stability of the CIGS component; the substitution of CIGS with a perovskite solar cell (PSC) results in unstable generation of photocurrent and a lower concentration of formate under high-humidity environments because of the water-induced degradation of PSC. This work demonstrates the propriety of CIGS in robust PEC-PV tandems for artificial photosynthesis.