Effects of insulating layers on the performance of organic electroluminescent devices

In this paper, we demonstrate the enhancement in the performance of organic electroluminescent devices upon the insertion of an insulating layer or layers of LiF in the device structure. Highly efficient and bright organic light-emitting devices were fabricated with this approach. External quantum efficiencies approaching 3 % and light output exceeding 45,000 cd/m² have been achieved for green light-emitting devices with Al cathode. This technique can be extended to fabricate efficient blue and sharp-red light-emitting devices. In this respect, using Al as the electron injecting electrode, blue light-emitting devices with external quantum efficiency of 1.4% and light output more than 4,000 cd/m² have been achieved without the use of dopants. For sharp-red light-emitting devices, record efficiency and light output were obtained when LiF was used. Devices without the LiF layer showed light output levels lower than 5 cd/m², whereas, with the insertion of LiF before the cathode, the external quantum efficiency exceeded 1% and light output was higher than 320 cd/m². All of these devices had lower operational voltage than similar devices without the LiF layer. Preliminary UPS-XPS results revealed a sharp decrease in the work function of aluminum upon the deposition of sub-monolayer of LiF. Although, the use of the LiF layer on the indium-tin-oxide anode showed some enhancement in device performance, the contribution to device performance is lower than the case with the same insulator deposited at the cathode side, indicating that the cathode is more problematic than the hole injecting indium-tin-oxide electrode.