Transparent Ta 3 N 5 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 Ta 3 N 5 photoanodes were fabricated on n‐type GaN/sapphire substrates to promote O 2 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 Ta 3 N 5 to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe 2 (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 H 2 evolution. This tandem cell exhibited a solar‐to‐hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.