Abstract
In recent years, phosphorescent organic light-emitting diodes (PhOLEDs) have drawing attention increasingly in solid-state lighting application utilizing excitons to achieve 100% internal quantum efficiency theoretically. For wet process, polymer light-emitting diodes (PLED) are considered to be highly suitable because of good morphology but the polymeric problems are difficult purification, batch-to-batch variations and unclear molecular weight. Therefore, it is a good tendency to develop small molecular bipolar host materials possessing good charge-transporting properties and homogeneous film morphology. In this work, we successfully synthesized two new bipolar host materials FC3O and FC4O utilizing fluorene as a bridge and they possess different ratios (3:1, 4:2) of electron-transporting oxadiazolyl and hole-transporting carbazolyl moieties. The bipolar hosts revealed good thermal stability (Td > 350 oC, Tg > 140 oC) because of intrinsic bulky and non-planar structures. Homogeneous films (RMS roughness < 0.8 nm) were obtained by spin-coating process. Moreover, the HOMO/LUMO levels are closed to that of PEDOT:PSS/BCP so hole-injection/electron-injection are improved as well. In addition, multilayer green PhOLEDs with Ir(ppy)3 as dopant were successfully fabricated by spin-coating process. The maximum luminance and maximum current efficiency of FC3O- and FC4O-based device are (9358 cd/m2, 14.90 cd/A) and (10232 cd/m2, 18.2 cd/A), respectively. The performances of both are better than PVK-based one (1567 cd/m2, 4.0 cd/A).
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