We present results from a device model in which the characteristics of organic light-emitting diodes (OLED), based upon MEH-PPV [poly (2-methoxy,5-(2 ethyl-hexoxy)-1,4-phenylene-vinylene)], are determined by tunneling the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating voltage of the device as well as its efficiency. A model is developed that clearly explains the device characteristics of an indium-tin-oxide / MEH-PPV / Al. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I-V characteristics. In indium-tin-oxide/ MEH-PPV / Al devices, the barrier for hole injection varies between 0.1 eV to 0.6 eV and the barrier for electron injection is about 1.4 eV.