Impact of High Charge-Collection Efficiencies and Dark Energy-Loss Processes on Transport, Recombination, and Photovoltaic Properties of Dye-Sensitized Solar Cells

We report on the relationships between the energy-loss processes in the dark, charge-collection efficiency and photocurrent density-voltage characteristics of dye-sensitized solar cells (DSSCs). The charge-collection efficiencies (ν_cc) of the DSSCs with different electrolytes were close to 100%. Despite the high ν_cc values, the DSSCs showed significant loss of photocurrent density and power density resulting from dark processes associated with the exchange current density J₀ at the TiO₂/electrolyte interface, series resistance of the cells, and diode ideality factor. Even in DSSCs with high ν_cc values, making recombination slower and transport faster in the dark would reduce these losses. The opposing effects of J₀ and light absorption properties of DSSCs were found to determine the optimum film thickness (L_opt) for the highest output power density. These effects also explain why L_opt is generally much less than the electron diffusion length.