Electroluminescence of 1,3,4-Oxadiazole and Triphenylamine-Containing Molecules as an Emitter in Organic Multilayer Light Emitting Diodes

We synthesized five new emitter molecules having an oxadiazole group as an electron transport unit and a triphenylamine group as a hole transport unit. We investigated the electroluminescent (EL) properties of these molecules as an emitter layer. The deposited films of all compounds were found to be amorphous and showed strong blue-green fluorescence, ranging from 450 to 490 nm. With the best device using these emitters, the maximum luminance exceeded 19 000 cd/m2. We observed that external EL quantum efficiencies (φEL) were drastically influenced by combinations of hole-transport materials. Formation of exciplexes between the emitters and hole-transport materials was found to greatly influence φEL. We observed that the hole-transport layers (HTLs) having a low ionization potential (Ip) value tended to form exciplexes with the emitter layers, resulting in a low φEL. On the other hand, the HTLs having a large Ip formed no exciplexes with emitter layers and showed a high φEL. In the best device, a φEL∼4% was obtained. Furthermore, we also studied the optimum EL cell structures using the bipolar emitters. We observed that a double hetero (DH) structure was the best device structure and could achieve an external energy conversion efficiency (φenergy) of 3.75 lm/W at a current density of 10 mA/cm2. This is an excellent value among the organic EL devices previously reported. In addition, the durabilities of the EL devices were also measured at constant current density. Performances of the EL device durabilities were quite inferior with these emitter materials. The time for the luminance to decay to half of the initial luminance was below 1 h.