Progress in new semiconductor materials classes for solar photoelectrolysis

For several decades, the main body of research in photoelectrochemical (PEC) hydrogen production has centered on a small number of semiconductor materials classes, including stable but inefficient metal-oxides, as well as some more efficient materials such as III–V compounds which suffer from high cost and poor stability. While demonstrating some limited success in meeting the rigorous PEC demands in terms of bandgap, optical absorption, band-edge alignment, surface energetics, surface kinetics, stability, manufacturability and cost, none of the ‘traditional’ PEC semiconductors are adequate for application in water-splitting devices with high performance (greater than 15% solar-to-hydrogen conversion) and long durability (greater than 200 h life). As a result, it is widely held that new semiconductor classes and configurations need to be identified and developed specifically for practical implementation of solar water-splitting. Examples include ternary and quaternary metal-oxide compounds, as well as non-oxide semiconductor materials, such as silicon-carbide and the copper-chalcopyrites. This paper describes recent progress at the University of Hawaii to develop improved semiconductor absorbers and interfaces for solar photoelectrolysis based on polycrystalline tungsten trioxide and polycrystalline copper-gallium-diselenide. Specific advantages and disadvantages of both materials classes in terms of meeting long-term PEC hydrogen production goals are detailed.