Illumination intensity dependence of the photovoltage in nanostructured TiO2 dye-sensitized solar cells

The open-circuit voltage (V-oc) dependence on the illumination intensity (Phi(0)) under steady-state conditions in both bare and coated (blocked) nanostructured TiO2 dye-sensitized solar cells (DSSCs) is analyzed. This analysis is based on a recently reported model [Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 8774] which describes the rate of interfacial electron transfer from the conduction band of TiO2 to acceptor electrolyte levels (recombination). The model involves two possible mechanisms: (1) direct, isoenergetic electron injection from the conduction band and (2) a two-step process involving inelastic electron trapping by band-gap surface states and subsequent isoenergetic transfer of trapped electrons to electrolyte levels. By considering the variation of V-oc over a wide range of illumination intensities (10(10) < Phi(0) < 10(16) cm(-2) s(-1)), three major regions with different values of dV(oc)/d Phi(0) can be distinguished and interpreted. At the lower illumination intensities, recombination mainly involves localized band-gap, deep traps at about 0.6 eV below the conduction band edge; at intermediate photon fluxes, recombination is apparently controlled by a tail of shallow traps, while, for high enough Phi(0) values, conduction band states control the recombination process. The high Phi(0) region is characterized by a slope of dV(oc)/d log Phi(0) congruent to 60 mV, which indicates a recombination of first order in the free electron concentration. The study, which was extended to different solar cells, shows that the energy of the deep traps seems to be an intrinsic property of the nanostructured TiO2 material, while their concentration and also the density (Theta(t) approximate to 10(18)-10(19) cm(-1)) and distribution of shallow traps, which strongly affects the shape of the V-oc vs Phi(0) curves, change from sample to sample and are quite sensitive to the electrode preparation. The influence of the back-reaction of electrons from the fluorine-doped tin oxide (FTO) conducting glass substrate with electrolyte tri-iodide ions on the V-oc vs Phi(0) dependence characteristic of the DSSC is analyzed. It is concluded that this back-reaction route can be neglected, even at low light intensities, when its rate (exchange current density, j(0)), which can vary over 4 orders of magnitude depending on the type of FTO used, is low enough (j(0) <= 10(-8)A cm(-2)). The comparison of V-oc vs Phi(0) measurements corresponding to different DSSCs with and without blocking of the FTO-electrolyte contact supports this conclusion.