Dye-sensitized solar cells (DSCs) based on electrodeposited mesoporous ZnO films present a low-temperature alternative to conventional DSCs built from high-temperature treated nanoparticulate TiO2 films. Using a liquid iodide/triiodide electrolyte and the indoline dye D149 as photosensitizer, the highest efficiency reported for such cells is 5.6%, which is slightly less than half of that reached by titania-based systems. In the present work, the role of the dye layer at the interface between ZnO and electrolyte in determining cell performance was investigated. Optical spectroscopy and time- and frequency-resolved photoelectrochemical methods involving small or large electrical or illumination perturbations were employed to study light harvesting, charge separation, charge transport, and recombination in sandwich-type solar cells with systematically varied dye loadings, dye combinations, or dye/coadsorbate combinations. On the basis of analytical models considering an exponential distribution of band gap states in the semiconductor and nonlinear recombination to the electrolyte (beta-recombination model), strategies to quantitatively detangle the microscopic factors that determine global device characteristics were introduced. It was shown that the relatively low fill factor in cells based on electrodeposited ZnO/D149 is strongly determined by D149 aggregates accelerating recombination at intermediate cell voltages. The beneficial effect of the coadsorbate cholic acid (CA) on device performance resulted from a suppression of the dye-related recombination by limiting the extent of D149 aggregation. Addressing the low short-circuit photocurrent densities of ZnO/D149-based DSCs with respect to standard TiO2-based cells, an extension of the spectral light harvesting efficiency was achieved by co-sensitization of electrodeposited or screenprinted nanoparticulate ZnO with D149, the indoline dye D131, and a red-absorbing sensitizer, which was either the squaraine dye SQ2 or the partially sulfonated zinc(II) phthalocyanine S1.15PcZn. The beneficial effect of panchromatic light harvesting was counteracted by significant voltage losses due to undesired dye/dye and dye/semiconductor interactions in the presence of the red absorbers, including downward shifts of the ZnO conduction band edge and formation of recombination-promoting surface trap states. On the basis of the specific properties of ZnO-based DSCs determined in this work, guidelines for the choice of alternative sensitizers and red-absorbing co-sensitizers were discussed.
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