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 (Alq(3)) 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 (eta(EQE,max)) of 23.0% and a maximum power efficiency (eta(p,max)) of 94.3 lm W-1 are attained for (ppy)(2)Ir(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 eta(EQE,max) of 23.7% and eta(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 electro-phosphorescent device with eta(EQE,max) of 20.6%, eta(c,max) of 62.1 cd A(-1), and eta(p,max) of 61.7 lm W-1, is fabricated. To the author's knowledge, this is also the highest efficiency ever reported for yellow PHOLEDs.