Surface-oxidized tungsten for energy-storable dye-sensitized solar cells

Tungsten oxide electrodes were investigated as charge-storage materials for energy-storable dye-sensitized solar cells (ES-DSSCs). The electrochemical and structural properties of the surface-oxidized tungsten (so-WO3 − x) and monoclinic nanocrystalline WO3 (nc-WO3) were studied on the difference of the charge–discharge properties. Although, the electromotive force (EMF) curve of the so-WO3 − x was associated with structural change, the so-WO3 − x did not show the significant structural change indicated by X-ray diffraction (XRD) patterns. On the other hand, the nc-WO3 showed crystal transformation from monoclinic phase to tetragonal phase. The Li+ diffusion coefficients of the so-WO3 − x with different Li+ content ratios obtained by the galvanostatic intermittent titration technique (GITT) did not fall down up to 0.3 of Li/W ratio, whereas the diffusion coefficients of nc-WO3 decreased about two orders of magnitude in the vicinity of phase transitions. The different electrochemical properties could be explained by the less structural change of so-WO3 − x compared with the nc-WO3. The large-sized ES-DSSCs with the so-WO3 − x were fabricated for the first time, and their photocharge–discharge performances were studied.