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 o nto SnO2 conducting glass from the ambient hydrolysis of TiCl4 and annealed at temperatures ranging from 100 to 500 degrees C were studied by Raman sp ectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) , intensity-modulated photovoltage spectroscopy (IMVS), and intensity-modul ated photocurrent spectroscopy (IMPS) measurements. Analysis of the XRD and Raman spectra shows that TiCl4-produced TiO2 films have the rutile structu re, regardless of annealing temperature. The TEM reveals that the rutile Ti O2 films consist of rod-shaped particles that grow with increasing annealin g 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'-dic arboxylic acid, L' = 2,2'-bipyridyl-4,4-ditetrabutylammoniumcarboxylate) 4. 5 mu m thick rutile films increase significantly with annealing temperature , from 1.1 mA/cm(2) and 602 mV at 100 degrees C to 8.7 mA/cm(2) and 670 mV at 500 degrees C, Studies of the incident photon-to-current conversion effi ciency (IPCE), the photocurrent-voltage characteristics, the optical appear ance, the water content, and the particle size of the films indicate that t he increase of both J(sc) and V-oc with annealing temperature is due, in pa rt, to increased dye adsorption resulting fr om the evaporation of surface water and the improved light-scattering properties of the film associated w ith the growth of rutile particles. IMVS and IMPS measurements indicate tha t variations of the charge-collection efficiency of the cell, which increas es from 86% for the 300 degrees C annealed samples to above 99% for the 500 degrees 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 s hort circuit decreases and the diffusion coefficient of free electrons incr eases with annealing temperature. Raman and XRD measurements and other obse rvations indicate that treating transparent nanocrystalline anatase TiO2 el ectrodes with TiCl4 produces a translucent overlayer of rutile TiO2. The in creased film thickness and light-scattering characteristics of the rutile o verlayer may explain, in part, the improved IPCE observed for dye-sensitize d TiCl4-treated nanocrystalline anatase TiO2 electrodes.