Cyclooctatetrene derivatives as electron transporters and hole blocker in organic LEDs

Abstract

In this paper, we report that novel tetraphenyl- tetraalkynyl-COTs and octaaryl-COTs can be used as electron transport and hole blocker layer in organic LEDs. These materials have the similar reduction potentials compared to traditional electron transporters, such as Alq3. The energy gaps for all of the COT derivatives studied here are very wide (> 3 eV), making it possible to fabricate blue emitting OLEDs with these materials. We have fabricated devices of the structure ITO/NPD400 angstrom/COT-Me 400 angstrom/Mg-Ag. (ITO equals indium-tin-oxide, NPD equals N,N'-diphenyl-N,N'-dinapthylbenzidine). The external quantum yield reaches 0.15% and the emission comes from NPD ((lambda) max equals 436 nm). When 1% perylene (by weight) is doped into the NPD, the (lambda) max of emission shifts to the 454 nm with the external QE reaches 0.60%. It indicates the efficient energy transfer from NPD to perylene. Changing the COT layer thickness from 400 Angstrom to 200 Angstrom, the emission is still from the NPD layer, as seen before. However, the I-V curve and turn-on voltage both shift to lower potentials by ca 2 - 3 V with the QE drops to 0.06%. The device QE can be increased by either using thin Alq layer or replacing Mg:Ag with LiF/Al electrode. Finally, the devices using COTs as hole blocker are fabricated and results are discussed.