Novel bipolar host materials based on benzimidazole-hybridized phenanthridine for efficient red PhOLEDs

Abstract

In this work, two innovative bipolar host materials, TPA-BIP and Cz-BIP, were designed and synthesized based on benzimidazole-hybridized phenanthridine (BIP) moiety as the electron transport unit. Compared to the widely used electronic transport unit benzimidazole, the BIP unit possesses a large planar rigid structure that can effectively enhance the thermal stability and the electron transfer capability of materials. Both materials exhibit good thermal stability respectively with Tg of 113 °C and 141 °C, proper HOMO/LUMO energy levels, and balance carrier transport capabilities. The red phosphorescent organic light-emitting diode prepared based on both materials achieve relatively high luminescence efficiency and relatively low efficiency roll-off. In particular, the EQEmax of the R1 device using TPA-BIP as the host material reaches 21.5 %, which is more superior than that of the conventional host material CBP (EQEmax of 9.84 %). Even at 1000 cd/m2, the device still achieves an EQE of 21.3 % with virtually no decrease. In conclusion, the results demonstrate the broad development potential of these BIP-based host materials for red PhOLEDs applications.