Polymers with Carbazole-Oxadiazole Side Chains as Ambipolar Hosts for Phosphorescent Light-Emitting Diodes

Abstract

Polymethacrylates, polystyrenes, and polynorbornenes bearing 2-(3-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(4-(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 2-(3,5-di(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole) groups linked to the polymer backbone through the 3-position of their terminal phenyl groups have been synthesized for use as solution-processable ambipolar hosts for phosphorescent light-emitting diodes. The polymers exhibit good thermal stabilities, with no weight loss below 350 °C, and have glass-transition temperatures in the range 118–209 °C. Spectroscopic, electrochemical, and quantum-chemical data for small-molecule model compounds suggest that the side chain groups are suitable hosts for a range of phosphors, including the green-emitter fac-tris(2-phenylpyridinato-N,C2′)iridium (Ir(ppy)3). Light-emitting diodes were fabricated, each with a solution-processed photo-cross-linked hole-transport layer, a solution-processed emissive layer composed of a carbazole-oxadiazole-functionalized polymethacrylate doped with Ir(ppy)3, and an evaporated electron-transport layer. The polymer with 2-(3,5-(dicarbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole units in the side chain exhibited the lowest turn-on voltage (6.0 V), and the highest efficiency (external quantum efficiency of 10.0%, current efficiency of 34.1 cd/A).