Organic light-emitting devices have attracted much attention owing to their potential application for flat-panel displays. How to improve their efficiency and stability are important problems which people study. Thin film OLEDs usually consist of multiple organic and metallic layers on an ITO-covered glass plate. In this paper, a single-layer device consists of a single organic layer between the anode and cathode. The single organic layer serves as the emitting source for the device, and has good bipolar character which are necessary for the transport of holes and electrons through the organic layer. The light-emitting units were based on 4,4'-bipyridine and oxadiazole derivatives. It was found that the charge carrier mobilities, charge in injection barrier, and the device performance were improved.