Comparison of Dye-Sensitized Rutile- and Anatase-Based TiO2 Solar Cells

Crack-free nanocrystalline rutile TiO₂ films with thicknesses of up to 12μm were prepared and characterized in connection with their application to dye-sensitized solar cells. The photoelectrochemical properties of the rutile-based solar cell are compared with those of the anatase-based cell. Scanning electron microscopy (SEM) shows that the rutile films consist of homogeneously distributed rod-shaped particles with an average dimension of 20 × 80 nm. Both the thickness and the morphology of the rutile films have a strong influence on the photoelectrochemical properties of the solar cells. Measurements of the incident monochromatic photon-to-current conversion efficiency (IPCE) as a function of film thickness imply that a significant fraction of light in the spectral region below 600 nm is absorbed in the first few microns of the dye-covered films due to strong light absorption by the dye. At longer wavelengths, where the dye absorbs weakly, the IPCE increases in direct proportion to the film thickness, suggesting that the electron-injection rate throughout the cell approaches homogeneity. The open-circuit photovoltage (V_∝) shows only a small change with film thickness, which is attributed to the compensating effect associated with the dependence of the number of dye molecules and recombination centers on the surface area. For the same film thickness, the photocurrent-voltage responses of the dye-sensitized rutile and anatase films at one-sun light intensity are remarkably close. Their V_∝ is essentially the same, whereas the short-circuit photocurrent of the rutile-based cell is only about 30% lower than that of the anatase-based cell. The lower photocurrent of the rutile film correlates with a lesser amount of adsorbed dye, owing to a smaller surface area per unit volume compared with that of the anatase film. Intensity-modulated photocurrent spectroscopy and SEM studies indicate that electron transport is slower in the rutile layer than in the anatase layer due to differences in the extent of interparticle connectivity associated with the particle packing density.