The charge-recombination kinetics and band edge movement in dye-sensitized nanocrystalline TiO2 solar cells are investigated by intensity modulated photovoltage spectroscopy (IMVS). A theoretical model of IMVS for dye-sensitized nanocrystalline semiconductor electrodes is developed, and analytical expressions for the frequency dependence of the photovoltage response at open circuit are derived. The model considers charge trapping/detrapping and electron transfer from the conduction band and surface states of the semiconductor to redox species at the solid/solution interface. IMVS is shown to be valuable in elucidating the contributions of band edge shift and recombination kinetics to changes of the open-circuit photovoltage (V oc) resulting from surface modifications of the semiconductor. IMVS measurements indicate that surface treatment of [RuL2(NCS)2] (L = 2,2‘-bipyridyl-4,4‘-dicarboxylic acid)-sensitized TiO2 electrodes with 4-tert-butylpyridine or ammonia leads to a significant band edge shift concomitant with a more negative V oc. Surface-modified dye-covered TiO2 electrodes exhibit a much higher photovoltage, for a given concentration of accumulated photogenerated electrons, than the unmodified dye-covered electrode. The accumulated charge in the TiO2 electrode is not sufficient to induce a major potential drop across the Helmholtz layer and cannot thus explain the observed photovoltage. The surface charge density is also not sufficient to support an accumulation layer strong enough to have a major influence on the photovoltage. The movement of the Fermi level of the TiO2 electrode, arising from the accumulation of photogenerated electrons in the conduction band, accounts for the observed V oc. The second-order nature of the recombination reaction with respect to I3 - concentration is confirmed. Furthermore, the IMVS study indicates that recombination at the nanocrystallite/redox electrolyte interface occurs predominantly via trapped electrons in surface states.

Charge recombination between dye-sensitized nanocrystalline TiO2 electrodes and the I3 -/I- couple in nonaqueous solution is described. The sensitizer was [RuL2(NCS)2] (L = 2,2‘-bipyridyl-4,4‘-dicarboxylic acid). An apparent inequality between the dark current and the recombination current is ascribed to a voltage shift caused by a potential drop at the SnO2/TiO2 interface, ohmic losses in the SnO2 and TiO2, and an overpotential for the redox reaction at the Pt counter electrode. Treating the dye-coated TiO2 electrodes with pyridine derivatives (4-tert-butylpyridine, 2-vinylpyridine, or poly(2-vinylpyridine)) improves significantly both the open-circuit photovoltage V oc (from 0.57 to 0.73 V) and the cell conversion efficiency (from 5.8 to 7.5%) at a radiant power of 100 mW/cm2 (AM 1.5) with respect to the untreated electrode. An analytical expression relating V oc to the interfacial recombination kinetics is derived, and its limitations are discussed. Analysis of V oc vs radiant power data with this expression indicates that the pyridine compounds may lower the back-electron-transfer rate constant by 1−2 orders of magnitude. The pyridines are found to have no significant effect on the recombination mechanism and kinetics of electron injection from excited dye molecules to TiO2. Studies of the dye-covered electrodes show that the rate of recombination is second order in I3 - concentration, which is attributed to the dismutation reaction 2I2 - → I3 - + I- with I2 as the electron acceptor in the back-reaction. Mass-transport theory is applied to understand the dependence of the short-circuit photocurrent on the radiant power at low I3 - concentration and to calculate the diffusion coefficient of I3 - ions (7.6 × 10-6 cm2/s) in the porous TiO2 structure. The dependence of other cell parameters on the I3 - concentration is also investigated.

Intensity modulated photovoltage spectroscopy (IMVS) and intensity modulated photocurrent spectroscopy (IMPS) are used to evaluate the charge-collection efficiency of dye-sensitized nanocrystalline TiO2 solar cells. The charge-collection efficiency of the photoinjected electrons from dye sensitization is estimated from the respective time constants for charge recombination at open circuit τoc and the combined processes of charge collection and charge recombination at short circuit τsc obtained by IMVS and IMPS measurements. Three models are developed for relating the charge-collection efficiency to τoc/τsc. The first model determines the charge-collection efficiency from τoc/τsc without considering the underlying physical processes measured by IMVS and IMPS. The second model obtains τoc/τsc by simulating the frequency response of IMVS and IMPS from the time-dependent continuity equation for simplified conditions. The third model determines the time constants for IMVS and IMPS from electron-concentration profiles calculated for constant light intensity and more realistic conditions. To obtain a realistic steady-state electron concentration profile, a nonlinear dependence of the rate of recombination on the electron concentration in the TiO2 film is considered. Furthermore, the continuity equation is modified to account for charge trapping and detrapping. For the first time, expressions are derived for calculating the time constants from the steady-state electron concentration profile. The validity of this method is demonstrated for the second model from which the exact IMPS and IMVS responses are calculated. The three models are compared with each other. A simple expression is derived for calculating the charge-collection efficiency from the measured values of τoc/τsc and the light intensity dependence of τoc.

The structure and photoelectrochemical properties of TiO 2 films deposited onto SnO 2 conducting glass from the ambient hydrolysis of TiCl 4 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 TiCl 4 -produced TiO 2 films have the rutile structure, regardless of annealing temperature. The TEM reveals that the rutile TiO 2 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/cm 2 and 602 mV at 100 °C to 8.7 mA/cm 2 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 TiO 2 electrodes with TiCl 4 produces a translucent overlayer of rutile TiO 2 . The increased film thickness and light-scattering characteristics of the rutile overlayer may explain, in part, the improved IPCE observed for dye-sensitized TiCl 4 -treated nanocrystalline anatase TiO 2 electrodes.

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