Abstract
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 the 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 1M H2SO4 and 1M Na2SO4, and the incident photon-to-current efficiency spectra in 1M Na2SO4. The incident photon-to-current 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). The 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). The investigations of the ability to reach high performance by nanostructuring indicate that reasonable improvements through nanostructuring might be very challenging.
Highlights
InxGa1−xN layers of high structural and optical quality grown on Si have been successfully synthesized in the last few years,1,2 opening a new perspective for inexpensive and efficient solar harvesting devices
The calculated photogeneration efficiencies for varying InxGa1−xN film thicknesses and bandgaps are depicted in Fig. 4
Despite the high theoretical efficiency of InxGa1−xN/Si tandem photoelectrodes, our fabricated InxGa1−xN photoelectrodes led to a maximum photocurrent density of only 1.9 mA cm−2 at 1.23 VRHE for x = 41.4% and when using a hole scavenger (1M Na2SO4) electrolyte
Summary
InxGa1−xN layers of high structural and optical quality grown on Si have been successfully synthesized in the last few years, opening a new perspective for inexpensive and efficient solar harvesting devices. Notwithstanding the high theoretical efficiency of InxGa1−xN for solar hydrogen production, previous attempts to fabricate InxGa1−xN water-splitting photoelectrodes have led to very poor performance with photocurrents below 0.1 mA cm−2 under AM1.5G irradiation, i.e., a performance even inferior to pure n-GaN with a bandgap of 3.4 eV.. Notwithstanding the high theoretical efficiency of InxGa1−xN for solar hydrogen production, previous attempts to fabricate InxGa1−xN water-splitting photoelectrodes have led to very poor performance with photocurrents below 0.1 mA cm−2 under AM1.5G irradiation, i.e., a performance even inferior to pure n-GaN with a bandgap of 3.4 eV.6 This low performance was attributed to the low crystalline quality of InxGa1−xN without further investigating the reasons for the gap between theoretically predicted and experimentally observed efficiencies. These simulations predicted 28.9% efficiency and showed that the thickness and the doping concentration of the graded region substantially affected the performance. The modeling of InGaN/Si tandem solar cells (ignoring space charge recombination) predicted a theoretical maximum efficiency of 31%.8 These models were based on InxGa1−xN solar cells and not on InxGa1−xN water-splitting photoelectrodes, which are somewhat different physical systems due to the presence of the semiconductor–electrolyte interface. Experimental and computational studies of InxGa1−xN/Si tandem water-splitting photoelectrodes, to identify and quantify their main losses, are yet to be completed
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