In this study, X-ray photoelectron spectroscopy (XPS) was used to investigate the metal-organic interfaces of a P3HT:PCBM bulk-heterojunction (BHJ) organic solar cell under a high electrical field. This high electrical field was built by applying reverse bias to the solar cell. This electrical field stress enhances the fill factor, short-circuits current and reduces series resistance. As a consequence, the power conversion efficiency of a polymer solar cell enhanced by 8.3% compared to an unstressed solar cell. In addition, the reverse current density was quite low, consumed only a small amount of energy (0.36 mJ/cm(2)) during stressing. The intermixing and atomic concentration gradient of Al and C shown by an XPS depth profile confirmed the penetration of Al into the polymer layer under a large electrical field, which improved the contact properties. (C) 2013 Elsevier B.V. All rights reserved.