By introducing ZnS between Cu(In, Ga)(S,Se)2 (CIGS) and the CdS, we greatly improved the photoelectrochemical (PEC) performance of the CIGS photocathode for hydrogen evolution. Chemical and structural analysis reveals that the enhanced performance is due to additional band bending driven by in-diffusion of Zn into the CIGS and suppression of nonradiative recombination. The improved onset potential of CIGS photocathode was exploited by building a tandem device with a perovskite absorber for bias-free water splitting. A PEC device with a solar-to-hydrogen conversion efficiency exceeding 9% (the highest among PEC cells including a CIGS photocathode) with a stable operation of 6.5 h is demonstrated.