The effect of organic solar cells (OSCs) by using different power O-2 plasma treatments on indium tin oxide (ITO) substrate was studied. The power of O-2 plasma treatment on ITO substrate was varied from 20 W to 80W, and the power conversion efficiency of device was improved from 1.18% to 1.93% at 20W O-2 plasma treatment. The function of O-2 plasma treatment on ITO substrate was to remove the surface impurity and to improve the work function of ITO, which can reduce the energy offset between the ITO and SubPc layer and depress the leakage current of device, leading to the shunt resistance increased from 897 to 1100 Omega cm(2). The surface roughness of ITO decreased from 3.81 to 3.33 nm and the work function of ITO increased from 4.75 to 5.2 eV after 20W O-2 plasma treatment on ITO substrate. As a result, the open circuit voltage and the fill factor were improved from 0.46 to 0.70 V and from 0.56 to 0.61, respectively. However, the series resistance of device was dramatically increased as the power of O-2 plasma treatment exceeds 40 W, leading to the efficiency reduction. The result is attributed to the variation of oxygen vacancies in ITO film after the 60, 80 W O-2 plasma treatment. As a consequence, the power of O-2 plasma treatment on ITO substrate for the OSCs application should be controlled below 40W to avoid affecting the electricity of ITO film. (C) 2015 Elsevier Inc. All rights reserved.