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InxGa1-xN is a promising material for flexible and efficient water-splitting photoelectrodes since the bandgap is tunable by modifying the indium content. We investigate the potential of an InxGa1-xN/Si tandem used as a water-splitting photoelectrode. We predict a maximum theoretical photogeneration efficiency of 27 % for InxGa1-xN/Si tandem photoelectrodes by computing electromagnetic wave propagation and absorption. This maximum is obtained for an indium content between 50 % and 60 % (i.e. a bandgap between 1.4 eV and 1.2 eV, respectively) and a film thickness between 280 nm and 560 nm. We then experimentally assess InxGa1-xN photoanodes with indium content varying between 9.5 % and 41.4 %. A Mott-Schottky analysis indicates doping concentrations (which effectively represent defect density, given there was no intentional doping) above 8.1·1020 cm-3 (with a maximum doping concentration of 1.9·1022 cm-3 for an indium content of 9.5 %) and flatband potentials between –0.33 VRHE for x=9.5 % and -0.06 VRHE for x=33.3 %. Photocurrent-voltage curves of InxGa1-xN photoanodes are measured in 1 M H2SO4 and 1 M Na2SO4, and incident photon-to-current efficiency spectra in 1 M Na2SO4. The incident photon-tocurrent efficiency spectra are used to computationally determine the diffusion length, the diffusion optical number, as well as surface recombination and transfer currents. A maximum diffusion length of 262 nm is obtained for an indium content of 23.5 %, in part resulting from the relatively low doping concentration (9.8·1020 cm-3 at x=23.5 %). Nevertheless, the relatively high surface roughness (RMS of 7.2 nm) and low flatband potential (-0.1 VRHE) at x=23.5 % cause high surface recombination and affect negatively the overall photoelectrode performance. Thus, the performance of InxGa1-xN photoelectrodes appears to be a tradeoff between surface recombination (affected by surface roughness and flatband potential) and diffusion length (affected by doping concentration/defect density). Performance improvements of the InxGa1-xN photoanodes are most likely achieved through modification of the doping concentration (defect density) and reduction of the surface recombination (e.g. by the deposition of a passivation layer and co-catalysts). Investigations of the ability to reach high performance by nanostructuring indicate that reasonable improvements through nanostructuring might be very challenging.
Kevin Sivula, Charles Roger Jean Lhermitte, Nukorn Plainpan, Annalisa Polo, Ivan Grigioni
Mohammad Khaja Nazeeruddin, Jianxing Xia, Ruiyuan Hu