Although graphene hydrogel (GH) as counter electrode (CE) has been reported to improve the efficiency of quantum dot sensitized solar cells (QDSCs), the lower current density than that of commonly used brass-based CE is still a limitation. In this work, the investigated CEs containing GH and Cu2S have further improved current density with the resultant high efficiency via controlling the reduction level and property of GH. Results show that the composite CEs in which GH was fabricated via chemical reduction technique exhibit higher current density accompanied by higher voltage and fill factor, leading to the dramatically high efficiency. In comparison with that of hydrothermal reduction, the outperformance of chemically-reduced GH is attributed to the more reduction level, resulting in higher conductivity and better catalytic activity with synergistic effect of Cu2S catalysts. The impressively high efficiency of 11.51% has been obtained for the model CdSeTe QDSC, respectively 23.9% and 7.2% higher than that of brass and thermally-reduced GH based CEs. The exciting results meet our expectations of further improving efficiency through higher current density. The as-prepared GH based composite CE has also been exploited to assemble fully flexible QDSC with high efficiency of 2.84%. (C) 2018 Elsevier Ltd. All rights reserved.