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 (ηEQE,max) of 23.0% and a maximum power efficiency (ηp,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 ηEQE,max of 23.7% and ηp,max of 105 lm W−1 are achieved. This is the highest efficiency ever reported for (ppy)2Ir(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)2Ir(acac). A yellow electrophosphorescent device with ηEQE,max of 20.6%, ηc,max of 62.1 cd A−1, and ηp,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. A simple triphenylamine/oxadiazole hybrid can function as efficient bipolar host and exciton blocking material for PHOLEDs. Together with the use of TPBI as electron transporting layer and NPB as hole transporting layer, the charge carrier balance can be manipulated. Accordingly, the best EL performance can be achieved with a maximum EQE of 23.7% and a maximum power efficiency of 105 lm W−1 for a (ppy)2Ir(acac) based green PHOLED and 20.6% and 61.7 lm W−1 for a (fbi)2Ir(acac) based yellow PHOLED.