Organic electrophosphorescence devices have continued to attract intense interest because their intrinsic efficiencies may be up to four times higher as the fluorescence ones. In the design of highly efficient phosphorescent devices, the selection of a proper material for each layer is of great importance. In particular, the choice of host material for the phosphorescent dopant in the emitting layer fundamentally affects the efficiency of energy transfer from the host to the dopant. In general, to prevent back energy transfer from a dopant to the host, the host matrix should have a larger triplet energy gap (E T ) than the dopant, but a much larger E T of the host would induce charge trapping by the dopant leading to a high driving voltage. High carrier drift mobilities and balance of carrier fluxes are also essential for the host in order to reduce the driving voltage and increase the current and power efficiencies. In addition, a high glass transition temperature (T g ) is desired for a host to increase the thermal and morphological stability to lengthen the device lifetime.
Recently, bipolar materials as hosts for organic electrophosphorescence devices have caught great attention. A great advantage of using a bipolar host is the improvement of carrier mobilities and balance of electron and hole fluxes in the emitting layer. [2] However, a drawback is that the introduction of an electron-donating and -withdrawing moiety to the host molecule would lead to an intramolecular charge transfer resulting in the reduction of energy gap of the molecule. Several bipolar hosts for sky-blue phosphorescent devices were synthesized including a pyridine/ carbazole hybrid, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) by Kido et al., a phosphine oxide/carbazole hybrid, N-(4-diphenylphosphoryl phenyl) carbazole (MPO12) by Sapochak and his coworkers and phosphine oxide/phenylamine bipolar materials, 2,7-bis(diphenylphosphineoxide)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene (PCF) and 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino) phenyl]-9-phenylfluorene (POAPF) by Shu and Chou et al. . Bipolar hosts for green phosphorescent devices were also known. Gao et al. reported a pyridine/carbazole hybrid, 4,7-dicarbazol-9-yl-[1,10]-phenanthroline (BUPH1), for Ir(ppy) 3 -based green device, while Lee et al. used a non conjugated silicon-cored spirobifluorene derivative (SBP-TS-PSB) as a host material for the red phosphorescent Ir(III) complexes. Recently, Ma et al. reported a carbazole/oxadiazole hybrid molecule