In the present study, the effect of in situ layer-by-layer oxygen plasma treatment (OPT) on optical, nano-mechanical and electrical properties of layer-by-layer diamond-like carbon (DLC) thin films was explored. In situ layer-by-layer OPT on layer-by-layer DLC films led to drastic variation of optical band gap from 1.25 eV to 2.6 eV and hardness from 16.1 GPa to 25.3 GPa. Wide band gap and the band gap feasibility over wide range may lead to its realization as p-type window layer in p-i-n solar cells and variable band gap layers in tandem solar cells. Simulations of a-Si:H based p-i-n solar cells was also carried out by considering OPT DLC films as p-type window layers that yielded maximum efficiency of 8.9%. In addition, due to high hardness and other excellent nano-mechanical properties, these OPT-DLC films can be treated as hard, protective and encapsulate layers on solar cells particularly in n-i-p configuration. It is important to mention that OPT-DLC film as p-layer can minimize the use of additional hard, protective and encapsulate layer. (C) 2011 Elsevier Ltd. All rights reserved.