Highly efficient solution-processed green and red electrophosphorescent devices enabled by small-molecule bipolar host material

A solution-processable host molecule TPO comprised of hole-transporting triphenylamine and electron-transporting oxadiazole has been synthesized. Through meta-linkage between the donor and the acceptor, complete charge localization of the HOMO and LUMO and proper triplet energy are imparted. The new compound shows good thermal stability with a high glass-transition temperature of 131 degrees C. Smooth and homogeneous film can be obtained by spin-coating from a TPO/iridium complex blend as probed by atomic force microscopy. The solution-processed red phosphorescent organic light-emitting device (PhOLED) achieves a maximum current efficiency of 13.3 cd A(-1) with Commission Internationale de l'Eclairage coordinates of (0.64, 0.36); while the green device reaches a maximum current efficiency of 40.8 cd A(-1), and the value is still as high as 39.6 cd A(-1) at a practical luminance of 1000 cd m(-2). The low roll-off can be attributed to the bipolar nature of the host material TPO. An optimized device further elevates the efficiency to 56.8 cd A(-1), which is among the highest ever reported for small-molecule based green PhOLEDs fabricated by a wet process.