A new breakthrough for graphene/carbon nanotubes as counter electrodes of dye-sensitized solar cells with up to a 10.69% power conversion efficiency

A three-dimensional graphene/single-walled carbon nanotube counter electrode is developed for dye-sensitized solar cells via a drop-coating method. The resulting graphene/single-walled carbon nanotube counter electrode show satisfactory transmittance (56.6%) compared with that of bare fluorine-doped tin oxide glass (84.71%) at 670 nm. The obtained graphene/single-walled carbon nanotube counter electrode exhibits an excellent power conversion efficiency of 9.24%. When a mirror was set under the cells, the power conversion efficiency increased to 10.56%. Unexpectedly, in the stability test, its power conversion efficiency reached a maximum of 10.69%. This excellent performance was attributed to the three-dimensional structure, which had a large specific surface area. The cyclic voltammetry results indicate that the graphene/single-walled carbon nanotube had a higher catalytic I-3(-) capability than that of Pt counter electrode, whereas the electrochemical impedance spectroscopy and Tafel test results prove that the graphene/single-walled carbon nanotube have a smaller resistance, which is consistent with the I-V characterization. We determine that the three-dimensional graphene/single-walled carbon nanotube counter electrode may have broad application prospects in dye-sensitized solar cells as a Pt-free material.