We applied the horizontal electric field externally to improve the performances of organic solar cells based on the P3HT:PCBM heterojunction system. The treatment can orientate polymer materials and change the morphology of the active layer. Hence, the fill factor (FF) is enhanced from 63% to 67%. Furthermore, the treatment improves the morphology of the active layer. The root mean square (RMS) roughness of the active layer is increased and the contact areas between the layer and the electrode become larger, which raises current density (J(sc)) from 9.14 mA/cm(2) to 9.55 mA/cm(2). Therefore, the device applied with horizontal electric field has better performance and the power conversion efficiency (PCE) enhances from 3.04% to 3.46%. (C) 2011 Elsevier B.V. All rights reserved.