A hybrid multijunction photoelectrode for hydrogen production fabricated with amorphous silicon/germanium and iron oxide thin films

A multijunction photoelectrode for hydrogen production is being developed at the University of Hawaii (UH) which incorporates an Fe2O3/electrolyte photoelectrochemical top junction with two underlying amorphous silicon/germanium (a-Si:Ge) solid-state junctions. The device structure is fabricated onto stainless-steel foil, which is coated on the back surface with a thin film of nickel-molybdenum hydrogen catalyst, also developed at UH. The Fe2O3 films for this photoelectrode were initially based on a pyrolytically deposited material developed at Duquesne University which has demonstrated rectifying behavior in alkaline electrolyte with a broad absorption spectra in the 300-500 nm range, and photo-generated currents exceeding 8 mA/cm(2) for 1 sun illumination. An investigation into the use of reactively sputtered films, which can be synthesized at lower temperatures than the spray-pyrolytic material, has also been initiated. The a-Si:Ge tandem "nipnip" structure for the initial photoelectrode design is based on the bottom and middle layers of the high-efficiency triple junction photovoltaic devices developed by the University of Toledo. These monolithically stacked solid-state structures, which absorb strongly in the 500-700 and 600-900 nm ranges provide the supplementary voltage bias needed to sustain the hydrogen and oxygen evolution reactions, respectively, occurring simultaneously at the catalyzed back surface and the Fe2O3 front surface, respectively. Performance and stability results from initial tests of fabricated prototype photoelectrodes in potassium hydroxide under simulated AM1.5 illumination are presented. Also, efforts to optimize the device design using bandgap and thickness tailoring for improved optics and current matching are described. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.