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∘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 1.6 V vs SCE) was observed in 0.33MH3PO4H3PO4 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 1.6 V vs SCE). A multi-junction photoanode based on the best available sputtered WO3WO3 film and an amorphous silicon photovoltaic device is projected to operate at 2.2% solar-to-hydrogen efficiency.