Brian Cole

Low-Temperature Reactively Sputtered Tungsten Oxide Films for Solar-Powered Water Splitting Applications

Tungsten oxide semiconductor films for photoelectrochemical water-splitting applications, reactively sputtered at low temperatures (<300°C), have demonstrated photocurrents of 3.4 mA/cm 2 under AM 1.5 illumination in 0.33M H 3 PO 4 , exceeding published results for material pyrolytically fabricated at higher temperature, and approaching the theoretical maximum for tungsten trioxide based on optical absorption limits. The microstructural form of the reactively sputtered films, which is sensitive to the sputter process parameters, critically affects the photoconversion efficiency. It has also been determined that the sputtered WO 3 films form true compact photoelectrodes in solution, with active charge generation and separation over the materials optical width.

Improved current collection in WO 3 :Mo/WO 3 bilayer photoelectrodes

We report on the incorporation of molybdenum into tungsten oxide by co-sputtering and its effect on solar-powered photoelectrochemical (PEC) water splitting. Our study shows that Mo incorporation in the bulk of the film (WO 3 :Mo) results in poor PEC performance when compared with pure WO 3 , most likely due to defects that trap photo-generated charge carriers. However, when a WO 3 :Mo/WO 3 bilayer electrode is used, a 20% increase of the photocurrent density at 1.6 V versus saturated calomel reference electrode is observed compared with pure WO 3 . Morphological and microstructural analysis of the WO 3 :Mo/WO 3 bilayer structure reveals that it is formed by coherent growth of the WO 3 :Mo top layer on the WO 3 bottom layer. This effect allows an optimization of the electronic surface structure of the electrode while maintaining good crystallographic properties in the bulk.

a-SiC:H films used as photoelectrodes in a hybrid, thin-film silicon photoelectrochemical (PEC) cell for progress toward 10% solar-to hydrogen efficiency

In this paper we describe the fabrication of amorphous SiC:H materials and using them as photoelectrodes in photoelectrochemical cells (PEC). With the increase of CH4 flow (in SiH4 gas mixture) during growth, the bandgap, Eg, increases from ~ 1.8eV to ~2.0eV, while the photoconductivity decreases from ~10-5 S/cm to ~10-8 S/cm. These high-quality a-SiC:H materials with Eg of 2.0eV included into a solar cell configuration led to a conversion efficiency,η~7% on textured Asahi U type SnO2 coated substrates, with the i-layer thickness of ~300nm. For a reduced i-layer thickness of ~100 nm, a current density, Jsc ~8.45mA/cm2 has been achieved, Immersing the a-SiC:H(p)/a-SiC:H(i) structure in 0.33M H3PO4 electrolytes, produced a photocurrent of ~7mA/cm2. With a further optimization we expect that the photocurrent could exceed 9mA/cm2. With the use of this configuration substrate/silicon tandem device (a-Si/a-Si or a- Si/nc-Si)/a-SiC:H(p)/a-SiC:H(i), it may therefore be possible to increase the solar-to-hydrogen (STH) efficiencies to beyond 10%.

Copper gallium diselenide photocathodes for solar photoelectrolysis

Copper chalcopyrite films exhibit properties suitable for solar energy conversion processes such as direct bandgap, and excellent carrier transport. To explore the possibilities of solar-powered hydrogen production by photoelectrolysis using these materials, we have synthesized p-type polycrystalline CuGaSe2 films by vacuum co-evaporation of the elemental constituents, and performed physical and electrochemical characterizations of the resulting films and electrodes. Based on CuGaSe2 material with 1.65 eV bandgap, a 2.2 micron thick electrode exhibited an outdoor 1-sun photocurrent of 16 mA/cm2, while a 0.9 micron thin device still produced 12.6 mA/cm2 in conjunction with vigorous gas evolution. Flatband potential measurements and bias voltage requirements for saturation photocurrents indicate a valence band position to high for practical device implementation. Future photoelectrolysis devices may be based on copper chalcopyrites with lower valence band maximum in conjunction with a suitable auxiliary junction.

Influence of Nitrogen Doping on Tungsten Oxide Thin Films for Photoelectrochemical Water Splitting

Thin films of tungsten oxide were investigated for use as a top junction in a hybrid photoelectrode. To increase the solar to hydrogen efficiency, the tungsten oxide requires a bandgap reduction to the range of 2.2 to 2.4 eV. Nitrogen doping of WO 3 films was employed to reduce the bandgap via valence band modification. For low levels of doping, the bandgap was observed to increase, an effect attributed to decreased size of the polycrystals in the film. The photoelectrochemical efficiency was found to decrease from 80% for pure WO 3 to 56% for films deposited at a nitrogen partial pressure of 1.5 mTorr. For even higher doping levels (to 5 mTorr N 2 ), the bandgap was shown to decrease to a value of ∼1.9 eV, but the structural data indicates that significant disorder had been introduced. This disorder is consistent with recurrent dislocations, an effect that is common for the tungsten oxide material.