Bulk heterojunction (BHJ) has emerged over the past years as a promising structure for organic solar cells (OSCs).Different materials for electron and hole contact are usurelly used for most optimized devices,which results in a built-in voltage,Ubi.The electric field in correspondence is called built-in electric field.The built-in voltage is important for OSCs because it sets the upper limit of the open circuit voltage [1],and is hence directly related to the conversion efficiency.