Theoretical maximum photogeneration efficiency and performance characterization of InxGa1-xN/Si tandem water-splitting photoelectrodes

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.