Reduced Graphene Oxide-NiO Photocathodes for p-Type Dye-Sensitized Solar Cells

Reduced graphene oxide (rGO) has been prepared from commercial graphene oxide by a thermal reduction method, and NiO/rGO photocathodes were obtained by a mixed NiCl2/rGO sol-gel process by using different amounts of rGO material. A detailed characterization of NiO/rGO films was carried out by SEM, Raman, and surface analysis. BET analysis revealed that the optimum surface area of the films was between 0.0 and 0.3 mg of rGO, while the surface area was lower with increasing graphene content. Despite the lower surface area, in the presence of the higher amount of rGO material, a higher dye uptake was detected, possibly because of a pi-staking interaction between the phenyl group of the dye and the graphene sheets. The p-type dye-sensitized solar cells prepared from these films showed a gradual increase in J(SC) up to 2.6 mA/cm(2) with the higher amount of rGO and a V-OC of 125 mV; in this case, the efficiency was almost 2 times higher with respect to the solar cell with NiO alone. An EIS study showed that the charge recombination resistance was lower in the presence of the rGO, but the hole lifetime was similar. This behavior suggests that the higher J(SC) obtained with the higher amount of rGO can be ascribed to the higher dye uptake, which leads to a higher concentration of holes in the photocathodes.