Novel highly active, optically-transparent electrode catalyst containing Pt, PtOx, graphene oxide and stacked graphene platelet nanofibers is developed for a cathode of Cu(II/I)-mediated dye-sensitized solar cells. The catalyst layer is deposited on a FTO substrate, which thus becomes smoother than the parent FTO, but the button-like Pt/PtOx nanoparticles are still distinguishable. The found electrocatalytic activity for the Cu(tmby)(2)(2+/+) redox couple (tmby is 4,4', 6,6'-tetramethyl-2,2'-bipyridine) is outperforming that of alternative catalysts, such as PEDOT or platinum. Exchange current densities exceeding 20 mA/cm(2) are provided exclusively by our novel catalyst. The synergic boosting of electrocatalytic activity is seen, if we normalize it to the catalytic performance of individual components, i.e. Pt and graphene nanofibers. The outstanding properties of our cathode are reflected by the performance of the corresponding solar cells using the Y123-sensitized titania photoanode. Champion solar-conversion efficiency (11.3% at 0.1 sun) together with a fill factor of 0.783 compare favorably to all other so far reported best values for this kind of solar cells and the given experimental conditions. (C) 2017 Elsevier Ltd. All rights reserved.