Influences of cation charge density on the photovoltaic performance of dye-sensitized solar cells: lithium, sodium, potassium, and dimethylimidazolium.

We investigate the dependence of the photovoltaic performance of dye-sensitized solar cells on the cations with different charge densities, such as lithium (Li(+)), sodium (Na(+)), potassium (K(+)), and dimethylimidazolium (DMI(+)). The efficiencies of light harvesting, electron injection and charge collection were evaluated to clarify the influence of cation selection on photocurrent generation. It is found that the short-circuit photocurrents of DSCs gradually diminish with decreasing cation charge densities, partially owing to reduced electron injection rates which are intimately related to the reaction Gibbs free energies. Further experiments indicate that the upward movement of conduction band edge results in decreased reaction Gibbs free energy of electron injection from Li(+) to DMI(+). At an irradiation of 100 mW cm(-2) AM1.5 sunlight, the open-circuit photovoltage and the fill factor of a typical dye-sensitized solar cell increase in the order of Li(+) < Na(+) < K(+) < DMI(+). Analyses of impedance data reveal that the increase of cell photovoltage mainly correlates with the upward shift of the conduction band edge induced by the adsorption of low-charge-density cations on the surface of titania nanocrystals. A J-V model was proposed to understand the improvement of the fill factor. It is found that the increase of the fill factor stems from the decrease of recombination current density under the equilibrium state in the dark by fitting the J-V data with the model.