Charge-Transfer Processes in Dye-Sensitized NiO Solar Cells

We fabricated dye-adsorbed NiO solar cells (pDSCs) with six different metal-free organic dyes having various ground-state oxidation potentials. Under monochromatic light irradiation, all dyes showed cathodic current at wavelengths where the dyes absorb, suggesting hole injection occurred from the adsorbed dyes to the valence band of NiO. Absorbed photon-to-current conversion efficiency (APCE) tends to increase with the increase of energy difference (Delta E) between the valence band edge of NiO and the ground state of the dyes. The maximum APCE of 30% was obtained with 0.6 eV of the Delta E. The apparent hole diffusion coefficient in the NiO electrode was nearly independent from light intensity, and the values were estimated to be 4 x 10(-8) cm(2)/s. On the other hand, hole lifetime depends on light intensity, ranging from 3 X 10(-2) to 1 x 10(0) s. Investigation of the anchoring site of the dyes and the results of molecular orbital calculations suggested that electron injection from the dye to the valence band of NiO, occurring just after the hole injection, is the major factor of the relatively low efficiency even with the case of large Delta E.