In this report, large area and high-quality graphene films were synthesized under 1000 degrees C by chemical vapor deposition technique. Their structure, optical and electronic properties have been characterized by Raman spectroscopy, Ultraviolet visible spectrophotometry and Hall Effect measurement system, respectively. The films were found to be perfect sp(2) bonding form of graphene with a maximum size of 9 cm(2). By contrasting the current voltage feature of ITO/SiOx/Si, G/ITO/SiOx/Si and ITO/G/SiOx/Si configurations, the different roles of graphene layer to the heterojunction solar cells were discussed. The results showed that an energy conversion efficiency of 7.54% is obtained with the structure of G/ITO/SiOx/Si. The improved solar cell performance was attributed to the roles of graphene in SIS structures, such as improvement of light transmission and current flow, reduction of interfacial states and removal of Fermi pinning effect at the hetero-junctions between graphene films and oxides. The interfacial region between ITO, Graphene and Silicon was analyzed by transmission electron microscope image and X-ray photoelectron spectroscopy. It was found that the a-SiOx, layer does exist between ITO film and Si substrate even without intentional oxidation treatment. The surface passivation and defect-assisted tunneling effect of the ultra-thin oxide layers was reflected by the device performance. (C) 2015 Elsevier Ltd. All rights reserved.