Carbon doped zinc oxide thin films (0-10%) were synthesized by DC sputtering technique on glass substrates. The films were polycrystalline in nature and x-ray diffraction studies confirmed synthesis of phase pure ZnO without any dopant-related peaks. Oxidation states of the elements were determined by x-ray photoelectron spectroscopy. From the high resolution x-ray photoelectron spectroscopic study, it has been confirmed that carbon substituted cationic sites along with some segregation of carbon atoms at the grain boundary region. Carbon substitution at the cationic sites caused a non-linear variation in the band gap energy; band gap initially decreased and then increased for higher doping concentration. Ab-initio calculation based on density functional theory has also been performed to support our experimental observation and it is clearly matching the experimental results. An enhanced non-linearity in the current-voltage characteristic with C doping may increase the varistor application of ZnO. (C) 2016 Elsevier B.V. All rights reserved.