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 (eta(cc)) of the DSSCs with different electrolytes were close to 100%. Despite the high eta(cc) values, the DSSCs showed significant loss of photocurrent density and power density resulting from dark processes associated with the exchange current density J(0) at the TiO2/electrolyte interface, series resistance of the cells, and diode ideality factor. Even in DSSCs with high eta(cc) values, making recombination slower and transport faster in the dark would reduce these losses. The opposing effects of J(0) 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.