Enhanced energy conversion efficiency of TiO2 electrode modified with WO3 in dye-sensitized solar cells

Nanocrystalline TiO2 films modified with WO3 were fabricated by depositing TiO2 suspension containing small amounts of ammonium tungstate solution. The films were characterized with X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), photoluminescence (PL) spectrum, Brunauer–Emmett–Teller (BET) surface area, zeta potential and transient photovoltage measurements. The results indicated that the modification of WO3 extremely reduced the surface states of TiO2 and suppressed charge recombination significantly. Compared with unmodified TiO2 film, the suppression of the interfacial charge recombination and the increased driving force of electron injection contributed to the improved photovoltaic performance of TiO2 films modified with WO3. The power conversion efficiency increased as a function of WO3 content when its concentration was less than 0.5 wt.%. The highest conversion efficiency was obtained with TiO2 films modified with 0.5 wt.% WO3, with an increase of 18% improvement in the conversion efficiency than the blank. However, further increasing amount of WO3 resulted in a decrease in the photocurrent thus reducing the overall conversion efficiency. The retardation of electron injection by the thicker overlayer of WO3 together with the decrease of the dye loading was responsible for the loss of the efficiency for cells modified with higher concentrations of WO3.