Comparison of dye-sensitized rutile- and anatase-based TiO2 solar cells

Crack-free nanocrystalline rutile TiO2 films with thicknesses of up to 12 m u m were prepared and characterized in connection with their application to dye-sensitized solar cells. The photoelectrochemical properties of the rut ile-based solar cell are compared with those of the anatase-based cell. Sca nning electron microscopy (SEM) shows that the rutile films consist of homo geneously distributed rod-shaped particles with an average dimension of 20 x 80 nm. Both the thickness and the morphology of the rutile films have a s trong influence on the photoelectrochemical properties of the solar cells. Measurements of the incident monochromatic photon-to-current conversion eff iciency (IPCE) as a function of film thickness imply that a significant fra ction 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 increas es in direct proportion to the film thickness, suggesting that the electron -injection rate throughout the cell approaches homogeneity. The open-circui t photovoltage (V-oc) 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 remarkab ly close. Their V-oc is essentially the same, whereas the short-circuit pho tocurrent 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 w ith a lesser amount of adsorbed dye, owing to a smaller surface area per un it volume compared with that of the anatase film. Intensity-modulated photo current spectroscopy and SEM studies indicate that electron transport is sl ower in the rutile layer than in the anatase layer due to differences in th e extent of interparticle connectivity associated with the particle packing density.