In the context of photoelectrochemical water-splitting, tungsten oxide films have been fabricated at low processing temperatures (<250∘C) by reactive sputtering from tungsten targets in an argon/oxygen ambient. The films have a dense, compact morphology and show columnar growth. Amorphous and highly polycrystalline films can be produced depending on the deposition conditions; polycrystalline phases appear only at higher temperatures and under certain sputter target conditions. Large crystallites proved beneficial to photoelectrochemical performance. A maximum photocurrent of 2.7mA/cm2 (at 1.6V vs SCE) was observed in 0.33M H3PO4 under AM 1.5 Global illumination, exceeding published results for material fabricated at higher temperatures (in the 400–600∘C range). Doping of sputtered tungsten oxide films with nitrogen results in a red-shifted absorption edge, but so far not in increased photocurrents. The maximum photocurrent of a nitrogen-doped sample was measured at 2.3mA/cm2 (at 1.6V vs SCE). A multi-junction photoanode based on the best available sputtered WO3 film and an amorphous silicon photovoltaic device is projected to operate at 2.2% solar-to-hydrogen efficiency.

Tungsten trioxide (WO3) thin films are being developed for use in photoelectrochemical devices for solar water splitting, however the material's high bandgap limits device performance. Prompted by success with other metal oxides, bandgap modification of WO3 films through doping techniques is proposed. In this work, nitrogen doping was used to modify structural, optical, and electronic properties of the material. In a straightforward extension of the reactive-sputtering process, incorporating argon, oxygen, and nitrogen mixtures in the gas ambient, films were produced with measured bandgaps ranging from 3.0 down to 2.2 eV. Details of the modified films are discussed. (c) 2005 The Electrochemical Society.

The multijunction hybrid photoelectrode for hydrogen production developed at UH incorporates a metal-oxide photoelectrochemical top junction which is deposited onto an underlying solid state junction which generates additional voltage bias for efficient water splitting. Initial attempts to fabricate hybrid photoelectrodes using nano-structured iron-oxide films deposited by spray pyrolysis were largely unsuccessful because the pyrolysis temperatures of approximately 400°C were high enough to cause significant damage to the underlying amorphous-silicon-based solid state junctions. This paper describes the development of low temperature (<200°C) reactively sputtered metal oxide films with properties specifically optimized for use in hybrid photoelectrode applications. The primary materials investigated include iron oxide, for use in alkaline photoelectrolysis, and tungsten trioxide, for use in acid photoelectrolysis. To date, the sputtered tungsten trioxide materials have demonstrated higher levels of photo-generated current compared with sputtered iron-oxide films. Initial hybrid photoelectrode devices fabricated using tungsten-trioxide films sputter deposited onto tandem amorphous-silicon junctions are described which exhibited stable solar-to-hydrogen conversion in acidic media at efficiencies up to 1% in outdoor tests. Plans to enhance efficiency by further oxide film optimizations, for example using surface texturing and possible doping, are also discussed.

The development of a low-temperature reactive-sputtering process for producing high-quality hematite films was explored. Iron oxide thin films were sputter-deposited under different conditions of oxygen partial pressure and at substrate temperatures up to 200 °C, and characterized for comparison. Although all sample films exhibited structural and compositional characteristics consistent with polycrystalline rhombohedral Fe2O3 (hematite), it was found that the detailed grain features and morphology, as well as the optical and electrical properties, were significantly altered by process parameter variations. Average grain size increased with deposition temperature and decreased with oxygen partial pressure. Measured film conductivities ranged from 10−9 S/cm in 100 °C samples up to 5×10−4 S/cm in the 200 °C samples. Highest conductivities were found in the sample films exhibiting the largest grains, which is attributed to the effects of mobility enhancement through reduced inter-grain scattering.