Gallium Nitride and Indium Gallium Nitride Based Photoanodes in Photoelectrochemical Cells

Clinger, John D.
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University of Delaware
Photoelectrochemical (PEC) cells are integrated electrolyzers that split water into hydrogen and oxygen, using energy from the sun to create an energy storage medium that does not release undesirable emissions. New materials for both the photoanode and cathode of the device are needed to reduce cost as well as increase efficiency. An effective photoanode must be non toxic, have the ability to split water, absorb most of the solar spectrum, and demonstrate stability in aqueous solutions. Gallium nitride is stable and non toxic and has the ability to split water, but can only absorb a small fraction of the solar spectrum due to having a bandgap of 3.2eV. This thesis focuses on introducing indium into the gallium nitride, lowering the bandgap of the photoanode, while maintaining the desirable characteristics inherent with GaN. GaN, InGaN and GaN/InGaN samples were grown using Metal-Organic Chemical Vapor Deposition (MOCVD) and were electrochemically tested to determine stability as well as performance, through the use of cyclic voltammetry, linear scan voltammetry, and incident photon conversion efficiency measurements. GaN sample behavior resembles that of titanium dioxide, another commonly used material for photoanodes with a similar wide bandgap, primarily producing photocurrent in the ultraviolet (UV) range of the spectrum. InGaN samples with compositions of In.₁₀ Ga.₉₀ N and In.₁₅ Ga.₈₅ N show an increase in photocurrent in the visible spectrum, with n-type doped samples generating more photocurrent than p-type samples of similar composition due to higher resistivity in the p-type samples. GaN/InGaN devices show poor performance, due to recombination at the junction formed between the GaN and InGaN layers at the surface. Further study is required to investigate using higher compositions of indium that have a band gap closer to the theoretical minimum energy needed to split water, 2.0eV.