The effect of mild oxygen plasma etching on poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C(61) butyric acid methyl ester (PCBM) blend film was investigated to improve the efficiency of inverted polymer solar cells. Our study shows that the polymer surface adjacent to the anode was modified into a more hydrophobic P3HT-rich surface, which leads to better interface contact, fine electrode selectivity, and lower series resistance in the device. Although PCBM still aggregates on the top surface, it can be removed with mild oxygen plasma etching. As a result, the power conversion efficiency increased significantly from 3.4% to 4.3% with an enhanced fill factor of 64%. (C) 2010 Elsevier B.V. All rights reserved.