Thermally crosslinked hole-transporting layers for cascade hole-injection and effective electron-blocking/exciton-confinement in phosphorescent polymer light-emitting diodes

A bilayer hole-injection/transport structure was prepared by thermally crosslinking two separate hole-transport layers (HTL). The resulting films possess excellent optical quality and solvent resistance. Cascade hole-injection and effective electron-blocking/exciton confinement can be achieved for light-emitting diodes (LEDs) using blue phosphorescent emitters, such asbis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate. The first HTL was based on tetraphenyldiamine (TPD) has its highest occupied molecular orbit (HOMO) level lies at -5.3 eV, and the second HTL with 4,4',4-tri(N-carbazolyl)triphenylamine has its HOMO level lies at -5.7 eV. The preliminary results from blue LEDs using these cascade HTLs showed much improved device performance than those only use a single layer hole-transporting polymer. (c) 2006 American Institute of Physics.