Opto-electronic devices using organic materials are becoming widely desirable for manifold reasons [1-4]. In fact, organic devices have the potential for cost advantages over inorganic devices. In addition, inherent properties of organic materials, such as their flexibility make them well suited for particular applications such as fabrication on a flexible substrate. The materials used in organic light emitting devices involve one of two mechanisms in the electroluminescence produced (fluorescence versus phosphorescence). When properly stacked, these materials result in a device that can achieve the required high efficiency and long lifetime. Such red, green and blue devices can then be combined in matrices to become the core of a display. In general, facile fabrication of large areas is a requirement for the production of low-cost electronics. By increasing the doping level of materials, the barrier to charge carrier injection can be continuously reduced. The use of combinatorial devices allows scientists to quickly screen for the optimum doping level. This concept in organic light emitting diode (OLED) devices with hole or electron limited electroluminescence show that it is possible to significantly reduce the operating voltage while improving the light output and efficiency. Owing to the advantages of solid-state, selfemission, full color capability and flexibility, OLED has been recognized as one of the most promising flat panel display technology and has stepped into commercialization. Keywords: OLED, Electroluminescence, Exciton, Fluorescence, Phosphorescence, flat panel display.