Light Emitting Devices with Molecularly Doped Polymer Layers. A New Diamine as a Hole Transporting Molecule
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ABSTRACT In the present contribution a new hole transporting molecule belonging to the triphenyldiamine family (N,N'-diphenyl-N,N'-bis(4-metylphenyl)- 1, 1'-biphenyl-4,4'diamine) in a polycarbonate matrix is studied as the hole transport layer in a two layer device with Alq 3 as emitting layer. The dependence of the electrical and emitting behaviour on the doping level is studied and compared with the vacuum deposited diamine layer also interfaced with Alq 3. INTRODUCTION Molecularly doped polymers (MDP) are widely studied as active materials for organic large area electroluminescent displays because they combine the easiness of the spreading process on large areas and good mechanical properties of a polymer with the big variety of molecules which can be mixed into the polymer matrix to play the active roles in hole transport, electron transport and light emission. Since Tang and Van Slike's report on a double layer electroluminescent structure [1], a great deal of work has been done in improving both efficiency and durability of organic LEDs. The electroluminescence (EL) in organic based devices is a process involving charge carrier injection in the organic layers by the electrodes, their migration through the layers and their recombination to give excited species which loose their excess electronic energy by light emission [2]. The optimization of anyone of these processes is the way to get higher efficiency of the device. Another important parameter to be improved in order to achieve a real commercial interest for organic based devices is their durability. In multilayer devices, the hole transport layer, besides having good hole mobility, plays the important role of confining the charge recombination region far from injecting electrodes thus preventing quenching of the excited states formed by charge recombination. Aromatic diamine derivatives with bulky substituents on the aromatic rings are usually employed as hole transport molecules as they can be layered, by vacuum sublimation, in good quality amorphous films with high hole mobility. In particular a methyl-substituted diamine (TPD) has been largely used as the active molecule in the hole transporting layer of multilayer devices [3-7]. 617
Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
A drawback associated with evaporated layers is a poor durability due to the crystallization of the initially amorphous film [8] with a fall down of the electrical properties and in particular of the electrode/organic-layer interface [9]. Dispersion of the active molecule in a polymer matrix is expected to prevent crystallization of the hole transporting layer and increase the device durability. Such molecularly doped polymers (MDP) allow also high weight or thermally labile molecules, which wouldn't sublime without being destroyed, to be used as the electrically active species in electroluminescent device. MDP have been widely studied as photoconductive layers for electrophotography [10-12] and have also been used in light emitting devices [13-16]. T
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