| Electroluminescence (EL) from rare earth complex features sharp-band emissions, and there are potential advantages for obtaining high EL efficiency and for realizing laser action in this system. Unfortunately, the EL devices based on rare earth complexes so far reported show very low efficiencies and poor stability. EL properties of several Eu, Tb and Gd complexes are investigated in this paper. Varies emitting color, such as red, green, blue and white, are obtained. Especially, the EL efficiency and degradation property of the device based on an Eu complex Eu(DBM)3bath are investigated in detail. We propose that the injection of the boles into the Eu complex is a very important factor which affect both efficiency and stability of the device. Based on this finding, the device structure is further changed and a mixed layer of the Eu complex with the hole-trasport material is introduced into the device. This change leads to significant improvement on the device performance, i.e. the efficiency, brightness, and the stability of the device is dramatically improved. EL quantum efficiency of higher than 4% is obtained in the device. Compared with the pbotoluminescent efficiency of the Eu complex, this result demonstrates that rare earth complexes are indeed kind of highly efficient emitting materials for EL application. Most double-layered EL devices show obvious photovoltaic properties, and this is rarely revealed. The origin, characteristics, and the potential application of the photovoltaic property in the devices are discussed in this paper as well. Further research on organic electroluminescence needs quantitative understanding the physics of the devices. In this paper, we present a unified model for single layer organic EL device. Based on this model the current-voltage characteristics and the distribution of the electric field, the charge carriers, and the exciton formed etc, are calculated at varies device situation. Some inspiring conclusions are reached.
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