Progress in sputtered tungsten trioxide for photoelectrode applications

In the context of photoelectrochemical water-splitting, tungsten oxide films have been fabricated at low processing temperatures (< 250 degrees 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 2.7 mA/cm(2) (at 1.6V vs SCE) was observed in 0.33 M H3PO4 under AM 1.5 Global illumination, exceeding published results for material fabricated at higher temperatures (in the 400-600 degrees 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 2.3 mA/cm(2) (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. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.