Performance optimization of organic electroluminescent devices

Performance of organic electroluminescent (EL) devices has been found to depend critically on various processing parameters including the purity and deposition rate of organic materials and substrate temperature. The effects of these processing parameters were systematically investigated by using the time-of-flight measurement of carrier mobility, photoluminescence, Raman as well as photoemission spectroscopies. It was observed that carrier mobility in organic EL materials could be improved substantially by either increasing the material purity or decreasing the deposition rate. Concomitantly, the increase in carrier mobility also led to considerable enhancement in the efficiency of EL devices. By depositing organic EL materials at elevated substrate temperatures, significant improvement in EL efficiency was also obtained. The EL devices thus fabricated consisted of a crystalline hole-transporting layer (HTL). Such devices not only showed improved efficiency but also enhanced stability. The improvement in stability is attributed to the fact that the HTL was already crystalline in the fabricated device so that subsequent operation and storage of the device would not lead to further crystallization. Thus, the widely accepted degradation mechanism via operation-induced crystallization and interfacial diffusion appears not important in the devices fabricated with a crystalline HTL.