(Invited) Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis

Abstract

Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the-art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water splitting due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) recombination. In order to circumvent these limitations considerable research efforts have been directed at creating more complex architectures involving e.g. tandem cells, nanostructures, and buried junctions to correctly position the bands and provide enough driving force to surmount overpotentials and the 1.23 eV energetic barrier of the water splitting reactions. Therefore, developing novel III-V photoabsorbers with adequate band energetics is key to conceiving simpler, yet efficient single semiconductor PEC cells for production of solar fuels at a competitive cost. First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb narrows the band gap in Ga(Sbx)N1-xand changes the electronic band gap from indirect to direct in GaSbP. Theoretical computations predict that, with band gaps in the order of 2 eV, these materials straddle the potential window for water oxidation and proton reduction in acidic solution. Single crystalline films of these two materials have been deposited by metal organic chemical vapor deposition and halide vapor phase epitaxy, in a wide range of Sb incorporation without phase segregation. Experimental results corroborate the significant band gap reduction in GaSbN from 3.4 to 1.5 eV (Fig 1a) and suggest the conversion of excitonic transitions in the electronic band structure of GaSbP (Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data and low temperature photoluminescence spectroscopy. Electrodes comprising Ga(Sbx)N1-x and Ga(Sbx)P1-x have been benchmarked employing 2- and 3-electrode standard methods for assessment of their characteristic attributes, i.e. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situfluorescence probing. New insights into these materials raise questions regarding the native conductivity type of GaP alloys and the effect of incorporation or defects in the extent of band bending at a solid liquid electrolyte interface and work function of GaSbN. This presentation will highlight recent advances in the understanding of the inter-relationship of processing/synthesis, material structure and photoelectrochemical properties of this new class of materials. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).