Comparison of Dye-Sensitized ZnO and TiO2 Solar Cells: Studies of Charge Transport and Carrier Lifetime

Nanocryst. particles of ZnO and TiO2 of approx. equal size (∼15 nm) were used to prep. mesoporous electrodes for dye-sensitized solar cells. Electron transport in the solar cells was studied using intensity-modulated photocurrent spectroscopy and revealed similar results for ZnO and TiO2. Apparent activation energies for electron transport in nanostructured ZnO of ≤0.1 eV were calcd. from the temp. dependence of transport times under short-circuit conditions. The lifetime of electrons in the nanostructured semiconductors was evaluated from open-circuit voltage decay and intensity-modulated photovoltage spectroscopy. Significantly longer lifetimes were obtained with ZnO. Despite the reduced recombination, ZnO-based solar cells performed worse than TiO2 cells, which was attributed to a lower electron injection efficiency from excited dye mols. and/or a lower dye regeneration efficiency. The internal voltage in the nanostructured ZnO film under short-circuit conditions was ∼0.23 V lower than the open-circuit potential at the same light intensity. Results may be explained using a multiple trapping model, but as electrons are usually only shallowly trapped in ZnO, an alternative view is presented. If there is significant doping of the ZnO, resulting band bending in the nanocrystals will form energy barriers for electron transport and recombination that can explain the obsd. properties.