Highly Efficient Electron-Transporting/Injecting and Thermally Stable Naphthyridines for Organic Electrophosphorescent Devices

A series of 1,8-naphthyridine derivatives is synthesized and their electron-transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light-emitting device (OLED) using fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as a green phosphorescent emitter doped into a 4,4-N,N-dicarbazolebiphenyl (CBP) host with 4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (a-NPD) as the hole-transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4-bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole-injecting layer. The turn-on voltage of the device is 2.5 V using 2,7-bis[3-(2-phenyl)-1,8-naphthyridinyl]-9,9-dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole-blocking (HB) and exciton-blocking layer of bathocuproine (BCP), 13.213.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.254.5 lm W1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass-transition temperatures much higher than that of aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq), and decomposition temperatures of 510518 degrees C, comparable to or even higher than those of Alq3.