Bipyridyl-substituted benzo[1,2,3]triazoles as a thermally stable electron transporting material for organic light-emitting devices

Abstract

We developed new electron-transporting materials (ETMs) for organic light-emitting devices (OLEDs) based on benzo[1,2,3]triazole and two bipyridines. Four derivatives based on the same skeleton were synthesized with four different substituents: phenyl (BpyBTAZ-Ph), biphenyl (-BP), m-terphenyl (-mTP), and o-terphenyl (-oTP). These BpyBTAZ compounds have good thermal stabilities, and their decomposition temperatures were greater than 410 °C, which is significantly higher than that of tris(8-quinolinolato)aluminium (Alq), the conventional OLED material. BpyBTAZ compounds show preferable amorphous nature, and moreover, the glass transition temperatures (T_gs) of both BpyBTAZ-TP compounds exceed 100 °C. Furthermore, BpyBTAZ-BP exhibits no melting point and is fully amorphous. The electron affinities of the materials are as large as 3.3 eV and their electron mobility is sufficiently high. These characteristics accounted for a reduction in the operational voltage of OLEDs with BpyBTAZ compounds compared with the reference device with Alq as an ETM. Specifically, the electron mobility of all the BpyBTAZ compounds exceeds 1 × 10⁻⁴ cm² V⁻¹s⁻¹ at an electric field of 1 MV cm⁻¹. In addition, it was revealed that both BpyBTAZ-TP-based devices showed longer luminous lifetimes and smaller voltage increases during continuous operation at 50 mA cm⁻², compared with the Alq reference device.