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Planar heterojunction has emerged over the past years as a promising structure for light-emitting devices (OLED)[1-2], organic solar cells (OSC)[3-4] and organic photodetectors[5-6].Different materials for electron and hole contact are usually used for most optimized devices, which results in a built-in voltage.The built-in voltage is important for both OLEDs and OSCs.For OLEDs, Ubi determines direct the turn on voltage.For OSCs, Ubi is the upper limit of the open circuit voltage [7], and hence directly related to the conversion efficiency.Based on the Gaussian distribution of density of states of organic semiconductors, a numerical model for the built-in voltage of planar heterojunction organic devices (PHJ-OD) is developed.The result show that the built-in voltage ofa PHJ-OD is strongly dependent on the workfunction of the electrodes, the energy levels of frontier orbital of the organics as well as the device temperature.