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,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF), doped with a conventional blue triplet emitter, iridium(III) bis[(4,6-difluoro-phenyl)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 sp 3 -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 (E T ) 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.