Transparent Ta₃ N₅ Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta₃ N₅ photoanodes were fabricated on n-type GaN/sapphire substrates to promote O₂ evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm^-2 was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta₃ N₅ to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe₂ (CIS), which absorbs up to 1100 nm. A stand-alone tandem cell with a serially-connected dual-CIS unit terminated with a Pt/Ni electrode was thus constructed for H₂ evolution. This tandem cell exhibited a solar-to-hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.