A bipolar host material containing triphenylamine and diphenylphosphoryl- substituted fluorene units for highly efficient blue electrophosphorescence

Highly efficient blue electrophosphorescent organic light-emitting diodes incorporating a bipolar host, 2,7-bis(diphenylphosphoryl)-9-[4-{N, Ndiphenylamino)phenyl]-9-phenylfluorene {POAPF}, doped with a conventional blue triplet emitter, iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C 2]picolinate {Flrpic) are fabricated. The molecular architecture of POAPF features an electron-donating (p-type) triphenylamine group and an electron-accepting {n-type) 2,7-bis(diphenylphosphoryl)fluorene segment linked through the sp3-hybridized C9 position of the fluorene unit. The lack of conjugation between these p- and n-type groups endows POAPF with a triplet energy gap (ET) of 2.75 eV, which is sufficiently high to confine the triplet excitons on the blue-emitting guest. In addition, the built-in bipolar functionality facilitates both electron and hole injection. As a result, a POAPF-based device doped with 7 wt% Flrpic exhibits a very low turn-on voltage {2.5 V) and high electroluminescence efficiencies (20.6% and 36.7 lm W ....1). Even at the practical brightnesses of 100 and 1000 cd m -2, the efficiencies remain high (20.2%/33.8 lm W-1 and 18.8%/ 24.3 lm W-1, respectively), making POAPF a promising material for use in low-power-consumption devices for next-generation flat-panel displays and light sources.