Polymer solar cells (PSCs) with a structure of transparent electrodes/moderately reduced graphene oxide (GO)/active layer/LiF/Al were fabricated, in which moderately reduced GO serves as a hole transport layer (HTL). The GO based HTLs were prepared using spray process, which is one of the most promising methods to demonstrate cost-efficient high speed mass production of PSCs, on hot substrate fixed at various temperatures of 100, 200, 300, and 400 degrees C, respectively. Power conversion efficiencies (PCEs) of PSCs with sprayed GO HTLs were gradually increased as the temperature of substrate was going up and in particular PSC including sprayed GO at the highest temperature, 400 degrees C, showed best efficiency of 3.71%. Higher performance of PSCs fabricated with GO sprayed at higher temperature was attributed to improved electrical property of GO HTL, resulting from moderate in situ reduction of GO during spray process on hot substrate. (C) 2014 Elsevier B.V. All rights reserved.