In quantum dots sensitized solar cells (QDSSCs), the back-transport reaction, resulting from the recombination between the photo-generated electrons in transparent conductive oxide (TOC) substrates and the polysulfide species electrolytes, significantly impairs the power conversion efficiency. To solve this, a reduced graphene oxide (rGO) interface layer has been for the first time introduced between the TOC substrate and the porous TiO2 film in a modified CdS QDSSC by a spray-coating method. This rGO interface layer can effectively suppress the back-transport reaction by inhibiting the contact between TOC substrates and electrolytes. Consequently, an improvement in power conversion efficiency of ca. 20% has been achieved on the solar cell. Besides, an interesting threshold effect has also been found on this interface layer, which leads to much variated behavior of this rGO layer on the suppression of the back-transport reaction. Consequently, the optimum rGO interface layer has been obtained with a spray time of 2 min, which leads to the best photovoltaic properties. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.