Abstract
Abstract Three new bipolar molecules composed of carbazole, triarylamine, and bipyridine were synthesized and utilized as host materials in multi-color phosphorescent OLEDs (PhOLEDs). These carbazole-based materials comprise a hole-transport triarylamine at C3 and an electron-transport 2,4′- or 4,4′-bipyridine at N9. The different bipyridine isomers and linking topology of the bipyridine with respect to carbazole N9 not only allows fine-tuning of physical properties but also imparts conformational change which subsequently affects molecular packing and carrier transport properties in the solid state. PhOLEDs were fabricated using green [(ppy)2Ir(acac)], yellow [(bt)2Ir(acac)], and red [(mpq)2Ir(acac)] as doped emitters, which showed low driving voltage, high external quantum efficiency (EQE), and extremely low efficiency roll-off. Among these new bipolar materials, the 2Cz-44Bpy-hosted device doping with 10% (ppy)2Ir(acac) as green emitting layer showed a high EQE of 22% (79.8 cd A−1) and power efficiency (PE) of 102.5 lm W−1 at a practical brightness of 100 cd m−2. In addition, the device showed limited efficiency roll-off (21.6% EQE) and low driving voltage (2.8 V) at a practical brightness of 1000 cd m−2.
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