A New Approach to Design Light Emitting Devices Using Electroactive Dyes
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A New Approach to Design Light Emitting Devices Using Electroactive Dyes Michael Pan, Amitava Patra, Christopher S. Friend, Tzu-Chau. Lin, Alexander N. Cartwright, and Paras. N. Prasad Institute for Lasers, Photonics and Biophotonics, Departments of Electrical Engineering and Chemistry, University at Buffalo, The State University of New York, Buffalo, NY 14260 Ryszard Burzynski Laser Photonics Technology, Inc., 1576 Sweet Home Road, Amherst, New York 14228 Abstract Organic electroluminescence (EL) single layer devices using electroactive dyes incorporated in poly-vinylcarbazole (PVK) were fabricated. The molecular structures of the twophoton dyes are the generic D-π−Α, D-π−D, and A-π−Α, structural motif, composed of a diphenylamine donor (D), a distyrylfluorene π-bridge, and an oxadiazole acceptor (A). A single layer type of EL device of ITO/PVK:DYE/Ca/Al was fabricated. The light emission peak and the threshold of the electroluminescence emission depend on the structure and concentration of the dye. The EL intensity increases with the dye concentration and it was found that as the voltage is increased the brightness increases and reaches a value 498 cd/m2 at an applied voltage of 25 V for the D-π−Α dye. We present a physical explanation of this observed behavior and show that this has significant impact on the design of light emitting devices using these organic dyes. Introduction Since the first demonstration of a green organic light emitting diode in 1987 by Tang and VanSlyke [1], numerous organic molecules and polymers have been found that exhibit electroluminescence in the blue, green and red spectral region [2,3]. Organic electroluminescent devices represent a low cost route for display technology. That is, for development of large area light-emitting displays there is considerable interest in the study of electroluminescent polymers because of the structural flexibility, low cost of fabrication and low operation voltage. A variety of organic materials including polymers [1- 6], metal complexes [7] and fluorescent dyes [8,9] have demonstrated electroluminescence since organic materials generally produce emission due to the π−π∗ transition. In fact, the generation of light in the device is due to recombination of electrons and holes injected from the electrodes. The efficiency of a LED depends on the choice of the molecules and on the design of the device structure. Generally, layered devices exhibit better performance than a single layer device because the optimization of injection and recombination of holes and electrons is simplified [6]. Furthermore, the efficiency of the electroluminescent organic light-emitting device can be improved by introduction of a phosphorescent sensitizer to excite fluorescent dyes [10]. The efficiency of electron-hole recombination leading to the creation of singlet excitons is mainly influenced by the overlap of electron and hole densities that originate from carrier injection into the emitter layer [11]. To improve the efficiency of carrier injection organic hole and e
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