Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis
Authors: Garcia Alejandro Martinez , Spurgeon Joshua M , Young James L , Deutsch Todd G , Sunkara Mahendra Kumar 
Published: February 2020

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 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-x and 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 reduction of the indirect band gap of GaP when Sb is incorporated(Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data, low temperature photoluminescence spectroscopy, and photocurrent 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 situ fluorescence probing. New insights into these materials regarding the donor concentration in GaP alloys, and the effect of incorporation help in fundamentally understanding the thermodynamics at a solid liquid electrolyte interface and work function of GaSbP to realize the full potential of these novel III-V alloys.
This poster 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).
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