Thermally Cross-Linkable Hole-Transporting Materials for Improving Hole Injection in Multilayer Blue-Emitting Phosphorescent Polymer Light-Emitting Diodes

One of the most challenging tasks in fabricating multilayer polymer light-emitting diodes (PLEDs) is to develop robust hole-transporting materials that possess matched energy level with indium tin oxide (ITO) anode and the highest occupied molecular orbital (HOMO) of light-emitting polymers to facilitate efficient hole injection and transport. In this article, a series of thermally cross-linkable 4.4',4 ''-tris(N-carbazolyl)triphenylamine (TCTA) derivatives have been synthesized to explore their function as efficient hole-transporting materials for blue-emitting electrophorphorescent devices. In particular, their excellent solvent resistance enables them to be used in the double hole-transporting layer (HTL) device configuration to facilitate cascade hole injection. The effects of cross-linking temperature and the functional group on the compatibility between two HTLs are investigated through optical and atomic force microscopy. When vinylbenzyl ether is used as the cross-linking group for TCTA, it shows the best compatibility with the bottom HTL. The resulting blue light-emitting device reaches a peak external quantum efficiency of 3.17%, corresponding to a current efficiency of 6.6 cd/A. These values are much higher than those of the PLEDs using conventional PEDOT/PSS as a single HTL. The improvements in the device performance are due to reduced hole-injection barrier and better electron/exciton confinement.