Surface modification of a Cu(In,Ga)(S,Se)2 (CIGSSe) absorber layer is commonly required to obtain high performance CIGSSe photocathodes. However, surface modifications can cause disadvantages such as optical loss, low stability, the use of toxic substances and an increase in complexity. In this work, we demonstrate that a double-graded bandgap structure (top-high, middle-low and bottom-high bandgaps) can achieve high performance in bare CIGSSe photocathodes without any surface modifications via a hetero-materials overlayer that have been fabricated in a cost-effective solution process. We used two kinds of CIGSSe film produced by different precursor solutions consisting of different solvents and binder materials, and both revealed a double-graded bandgap structure composed of an S-rich top layer, Ga- and S-poor middle layer and S- and Ga-rich bottom layer. The bare CIGSSe photocathode without surface modification exhibited a high photoelectrochemical activity of ∼6 mA·cm-2 at 0 V vs. RHE and ∼22 mA·cm-2 at -0.27 V vs. RHE, depending on the solution properties used in the CIGSSe film preparation. The incorporation of a Pt catalyst was found to further increase their PEC activity to ∼26 mA·cm-2 at -0.16 V vs. RHE.
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