Multifunctional triphenylamine/oxadiazole hybrid as host and exciton-blocking material: High efficiency green phosphorescent OLEDs using easily available and common materials

A new triphenylamine/oxadiazole hybrid, namely m-TPA-o-OXD, formed by connecting the meta-position of a phenyl ring in triphenylamine with the ortho-position of 2,5-biphenyl-1,3,4-oxadiazole, is designed and synthesized. The new bipolar compound is applicable in the phosphorescent organic light-emitting diodes (PHOLEDs) as both host and exciton-blocking material. By using the new material and the optimization of the device structures, very high efficiency green and yellow electrophosphorescence are achieved. For example, by introducing 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) to replace 2, 9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP)/tris(8-hydroxyquinoline) aluminium (Alq₃) as hole blocking/electron transporting layer, followed by tuning the thicknesses of hole-transport 1, 4-bis[(1-naphthylphenyl) amino]biphenyl (NPB) layer to manipulate the charge balance, a maximum external quantum efficiency (η_EQE,max) of 23.0% and a maximum power efficiency (η_p,max) of 94.3 lm W^-1 are attained for (ppy)₂Ir(acac) based green electrophosphorescence. Subsequently, by inserting a thin layer of m-TPA-o-OXD as self triplet exciton block layer between hole-transport and emissive layer to confine triplet excitons, a η_EQE,max of 23.7% and η_p,max of 105 lm W ^-1 are achieved. This is the highest efficiency ever reported for (ppy)₂ Ir(acac) based green PHOLEDs. Furthermore, the new host m-TPA-o-OXD is also applicable for other phosphorescent emitters, such as green-emissive Ir(ppy)₃ and yellow-emissive (fbi)₂ Ir(acac). A yellow electrophosphorescent device with η_EQE,max of 20.6%, η_c,max of 62.1 cd A^-1, and η _p,max of 61.7 lm W^-1, is fabricated. To the author's knowledge, this is also the highest effi ciency ever reported for yellow PHOLEDs.