Effect of solvent and additives on the open-circuit voltage of ZnO-based dye-sensitized solar cells: a combined theoretical and experimental study

We have investigated the role of electrolyte composition, in terms of solvent and additive, on the open-circuit voltage (Voc) of ZnO-based dye-sensitized solar cells (DSSCs) using a combined experimental and theoretical approach. Calculations based on density functional theory (DFT) have been performed in order to describe the geometries and adsorption energies of various adsorbed solvents (nitromethane, acetonitrile and dimethylformamide) and p-tert-butylpyridine (TBP) (modeled by methylpyridine) on the ZnO (100) surface using a periodic approach. The densities of states (DOS) have been calculated and the energy position of the conduction band edge (CBE) has been evaluated for the different molecules adsorbed. The effect of the electrolyte composition on the standard redox potential of the iodide/triiodide redox couple has been experimentally determined. These two data values (CBE and standard redox potential) allowed us to determine the dependence of Voc on the electrolyte composition. The variations determined using this method were in good agreement with the measured Voc for cells made of electrodeposited ZnO films sensitized using D149 (indoline) dye. As in the case of TiO2-based cells, a correlation of Voc with the donor number of the adsorbed species was found. The present study clearly points out that both the CBE energy and the redox potential variation are important for explaining the experimentally observed changes in the Voc of DSSCs.