The Importance of Oxygen Vacancies in Nanocrystalline WO3–x Thin Films Prepared by DC Magnetron Sputtering for Achieving High Photoelectrochemical Efficiency

Abstract

The photoelectrochemical properties of tungsten oxide thin films with different stoichiometry (WO3–x) and thickness were investigated. The films were sputtered in O2/Ar gas (ratio 0.43) on glass substrates coated with fluorine-doped tin dioxide at two sputter pressures, Ptot = 10 and 30 mTorr, yielding O/W ratios of the films, averaged over three samples, of 2.995 and 2.999 (x ∼ 0.005 and x ∼ 0.001), respectively. The films were characterized by X-ray diffraction, scanning electron microscopy, and spectrophotometry. The 10 mTorr samples showed large absorption in the near-infrared (NIR) range, whereas the 30 mTorr samples had a small absorption in this region. The concentration of oxygen vacancy band gap states was estimated from cyclic voltammetry and was found to correlate with the optical absorption in the NIR region. The incident photon to current efficiency for illumination from the electrolyte side (IPCEEE) and substrate electrode side (IPCESE) showed higher efficiency for the more stoichiometric films, indicating that oxygen vacancies in the band gap act as recombination centers. Surprisingly high values of IPCEEE and IPCESE were found, and it was concluded that efficient charge separation and transport take place almost throughout the entire film even for film electrodes as thick as 2 μm. Analysis of the spectral distribution of the photoresponse (action spectra) using an extended Gärtner–Butler model to calculate the IPCE for front-side and back-side illumination was performed and showed that the diffusion length is large, of the order of the depletion layer thickness.