Cu_xS is a promising semiconductor for a range of optoelectronics applications, particularly hydrogen generation by photoelectrochemical (PEC) water splitting under sunlight, due to its low band gap and earth abundance. However, it has been little studied for PEC applications. Pulsed Laser Deposition (PLD) is a useful technique for depositing films of such sulfide materials, due to the ability to tune several deposition parameters thus providing control of the film structure and morphology [1].

In this study, Cu_xS thin films were synthesized by PLD using two different substrate temperatures (room temperature and 500°C) and both under vacuum and in N₂ background pressure. Photocurrent densities obtained under visible light for films deposited at 500°C were up to an order of magnitude higher compared to films deposited at room temperature. This increase in photocurrent could be attributed to increased surface roughness, giving increased contact area with the electrolyte, as well as changes in the crystal phases present, resulting in a reduced band gap.

However, stability remains a limitation of Cu_xS for water splitting applications. This can be mitigated by combining Cu_xS in thin film structures with other, more stable semiconductors. In this work, Cu_xS was combined with ZnS in a thin film mixture. It was found that these Cu_xS-ZnS mixtures produced higher photocurrents under visible light than films of either ZnS or Cu_xS on their own, attributed to reduced band gap, as well as decreased recombination losses due to physical separation of valence and conduction band states and hence electrons and holes, i.e. localization of states to interface regions vs. within each compound. Significantly, the mixed thin films had better stability than films of Cu_xS on its own.

[1] F. Kurnia et al., Solar Energy Materials and Solar Cells, 2016, 153, 179-185.