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For more than a decade, the Hawaii Natural Energy Institute has conducted research on photoelectrochemical (PEC)
technologies and achieved major milestones, including the fabrication of high-performance photoactive thin film
materials and the development of innovative device integrations (hybrid-photo-electrode). In this paper, we focus our
discussion on tungsten oxide-based materials, one of our two principal topics of research in this field. After a description
of pure WO3 physical, chemical and energetic properties we present our latest results on tungsten oxide PEC properties
improvement. In our general approach, each component of the PEC electrode is addressed, from the absorber (bulk) to
the surface energetics (near-surface) and catalysis (surface). Recently, progresses have been made on surface treatment
for catalytic purposes as well as on PEC materials integration. In the case of catalytic treatment, our studies show that
reactive sputtering technique is suitable to form high quality RuO2 thin films and nanoparticles. Tests conducted on
RuO2 thin films pointed out an oxygen evolution reaction potential as low as 0.2 V. When used as an anode in 2-
electrode configuration, RuO2 thin films lead to a photocurrent onset potential reduction as low as 500 mV for p-type
PEC materials (CGSe2 and a-SiC, so far tested) when compared to platinum. In the case of RuO2 nanoparticles, a
photocurrent density increase of approx. 20% was observed on treated tungsten oxide films. Finally, we present a new
integration scheme to increase photocurrent density using highly textured substrates (HTS). In our approach, HTS were
obtained by anisotropic etching of [100] silicon substrates in KOH solution. Initial results indicated a very good
coverage of WO3 onto the silicon pyramids and a photocurrent doubling is observed when compared to WO3 deposited
on flat silicon substrates.
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