Microwave-assisted reduced graphene oxide (MR-GO) layer was applied to hole extraction layer (HEL) of polymer solar cells (PSCs) and was compared with the widely used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in bulk hetero-junction (BHJ) solar cells. The power conversion efficiency (PCE) of 3.57% was achieved with the MR-GO layer, which is 21% higher than that of PSCs with the conventional PEDOT:PSS HEL material. This enhancement of PCE is mainly attributed to the increase of short-circuit current density originated from the hydrophobic surface of the MR-GO layer. The hydrophobic graphene oxide surface is believed to improve wetting property and physical contact of active blends. In addition, the MR-GO interfacial layer is found to show the excellent device stability in atmospheric condition. The PCE of conventional PEDOT:PSS based PSCs showed total degradation when the device was exposed to atmospheric condition for 1000 h without any encapsulation, while that of MR-GO based PSC showed over 85% of PCE. (C) 2015 Elsevier B.V. All rights reserved.