Junction Electrochemistry of InGaN Anode Materials in Solar Energy Conversion

Abstract

The indium gallium nitride (InGaN) ternary alloy system offers an ideal semiconductor basis as a light absorber for driving chemical reactions such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It is a direct bandgap semiconductor whose energy can be tuned for optimal solar absorption and overcoming kinetic overpotential losses, it can be grown epitaxially as a single crystal for high material quality, and it exhibits relatively high corrosion resistance in aqueous solutions for a III-V material. In this work, we report on the design, synthesis, and photoelectrochemical studies of InGaN in an anode configuration with a goal of using this material as a light absorber basis for driving solar fuels reactions such as the OER and the HER. Studies are shown reporting on how reactive vapor synthesis of GaN and InN with different structuring affects the light conversion properties and band diagram electrostatics in an electrochemical junction with ferrocene and cobaltocene redox pairs. This structure is extended to thin films of GaN and InGaN grown by molecular beam epitaxy for high efficiency light conversion, with the resulting electrochemical junction also made with ferrocene and cobaltocene. The effects of ternary alloying, doping, and the two different c-plane polarities (III-polar and V-polar) on the junction are investigated using the same electrochemical junction to evaluate the light conversion properties in relation to the electrostatics; specifically, tuning the bandgap energy for optimal light absorption, determining the c-plane polarity configuration for the electric fields to have the correct band alignment, and incorporating silicon doping to create a conductive substrate. The combination of these engineering techniques extends the depletion region of the active layer for high efficiency light absorption as similarly demonstrated for epitaxial phosphide films [1], and is utilized toward the goal of high performance solar energy conversion. [1] V. Parameshwaran, X. Xu, and B. Clemens, Journal of the Electrochemical Society, vol. 163, no. 8, pg. H714-H721 (2016).