Abstract
Host material with enhanced and balanced charge injection/transport is essential for efficient phosphorescent organic light-emitting diodes (PhOLEDs). We synthesized two new bipolar hosts, TPA-2OXD and TPA-3OXD, composed of hole-transporting triphenylamine core linked with two and three electron-transporting aromatic 1,3,4-oxadiazole derivatives respectively, aiming to enhance and balance charge injection and transport. They revealed good thermal stability (Td>300°C, Tg>100°C) and homogeneous films were readily obtained by spin-coating process (roughness<0.9nm). Multilayer PhOLEDs (ITO/PEDOT:PSS/EML/BCP/LiF/Al) have been successfully fabricated using Ir(ppy)3-doped hosts as emitting layer (EML). The maximum luminance and maximum current efficiency of TPA-2OXD-based device were 8190cd/m2 and 4.50cd/A, much higher than 6299cd/m2 and 1.53cd/A respectively of TPA-3OXD-based one. This has been ascribed to more appropriate ratio (2/1) of electron-/hole-transporting moieties in TPA-2OXD. The TPA-2OXD also excels conventional host PVK in terms of device performance (5774cd/m2, 1.91cd/A). Moreover, the turn-on voltages of TPA-2OXD- and TPA-3OXD-based devices were 4.3V and 5.5V respectively, much lower than 7.4V of PVK-based one. Current results indicate that the bipolar TPA-2OXD and TPA-3OXD are promising host materials capable of being fabricated by wet processes.
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