Abstract
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.
Highlights
Solar hydrogen production is attracting attention as a fuel supply system due to its carbon free, environmentally friendly and sustainable characteristics
A CIGSSe photocathode without n-type semiconductor material surface modification suffers from low PEC activity, presumably due to its low e−/h+ pair separation efficiency and e−/h+ recombination by holes diffused along the semiconductor/electrolyte interface
The structures of the two films were very similar, the upper part of the polyvinyl acetate (PVA)-CIGSSe film showed slightly larger grain size and a flatter surface than the ethyl cellulose (EC)-CIGSSe film, which was confirmed by cross-sectional and surface images (Fig. 1a–d)
Summary
Solar hydrogen production is attracting attention as a fuel supply system due to its carbon free, environmentally friendly and sustainable characteristics. To increase the separation ability, surface modifications with n-type hetero-materials are often applied to form the p-n junction at the interface with CIGSSe9–19. Diffusion of holes on the solid-electrolyte interface could be blocked as a result of CdS’s deeper (than CIGSSe’s) valence band maximum (VBM)[10,12] Due to these affirmative merits of surface modification with hetero-materials, the CdS/CIGSSe junction has long been successfully applied in thin film solar cell technologies. In the case of photocathode application of CIGSSe film for liquid-solid interfaces, the usage of n-type semiconductors for the surface modification is not highly necessary, unlike in solar cells. A CIGSSe photocathode without n-type semiconductor material surface modification suffers from low PEC activity, presumably due to its low e−/h+ pair separation efficiency and e−/h+ recombination by holes diffused along the semiconductor/electrolyte interface. Little research has been done on the role of the compositional gradient in photocathode applications
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