Dye-Sensitized TiO2 Solar Cells: Structural and Photoelectrochemical Characterization of Nanocrystalline Electrodes Formed from the Hydrolysis of TiCl4

The structure and photoelectrochemical properties of TiO₂ films deposited onto SnO₂ conducting glass from the ambient hydrolysis of TiCl₄ and annealed at temperatures ranging from 100 to 500 °C were studied by Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), intensity-modulated photovoltage spectroscopy (IMVS), and intensity-modulated photocurrent spectroscopy (IMPS) measurements. Analysis of the XRD and Raman spectra shows that TiCl₄-produced TiO₂ films have the rutile structure, regardless of annealing temperature. The TEM reveals that the rutile TiO₂ films consist of rod-shaped particles that grow with increasing annealing temperature. The AM-1.5 short-circuit photocurrent J_sc and open-circuit photovoltage V_oc of Ru[LL′(NCS)₂]-sensitized (L = 2,2′-bypyridyl-4,4′-dicarboxylic acid, L′ = 2,2′-bipyridyl-4,4-ditetrabutylammoniumcarboxylate) 4.5 μm thick rutile films increase significantly with annealing temperature, from 1.1 mA/cm² and 602 mV at 100 °C to 8.7 mA/cm² and 670 mV at 500 °C. Studies of the incident photon-to-current conversion efficiency (IPCE), the photocurrent-voltage characteristics, the optical appearance, the water content, and the particle size of the films indicate that the increase of both J_sc and V_oc with annealing temperature is due, in part, to increased dye adsorption resulting from the evaporation of surface water and the improved light-scattering properties of the film associated with the growth of rutile particles. IMVS and IMPS measurements indicate that variations of the charge-collection efficiency of the cell, which increases from 86% for the 300 °C annealed samples to above 99% for the 500 °C annealed samples, have only a minor effect on J_sc. Analysis of the time constants at open circuit and short circuit for a given electron injection current suggests that the ratio of free-to-trapped electrons at short circuit decreases and the diffusion coefficient of free electrons increases with annealing temperature. Raman and XRD measurements and other observations indicate that treating transparent nanocrystalline anatase TiO₂ electrodes with TiCl₄ produces a translucent overlayer of rutile TiO₂. The increased film thickness and light-scattering characteristics of the rutile overlayer may explain, in part, the improved IPCE observed for dye-sensitized TiCl₄-treated nanocrystalline anatase TiO₂ electrodes.