New high-Tg bipolar benzimidazole derivatives in improving the stability of high-efficiency OLEDs

Abstract

Four bipolar molecules, named 1-, 2-, 3-, and 4-3cbzBIZ, composed of tricarbazole (3cbz) substituted benzimidazole, were synthesized and characterized. They not only exhibit good thermal stability with remarkably high glass transition temperatures and decomposition temperatures, especially 4-3CbzBIZ (T_g > 193 °C, T_d > 453 °C), but also show superior thermal photoluminescence stability at elevated temperatures. Their morphological stabilities have also been affirmed using atomic force microscopy pictures after 30 min during 80 °C annealing. The FIrpic doped 4-3cbzBIZ organic light-emitting diode (OLED) shows sky-blue electroluminescence from FIrpic with a high external quantum efficiency (η_EQE) of 28.6% with an amazingly small efficiency roll-off (η_EQE = 27.3% at a luminance of 1000 cd m⁻²). Furthermore, the green OLED with Ir(ppy)₃ doped in 4-3cbzBIZ exhibits excellent device thermal stability and a longer operational lifetime than the OLED with CBP as the host. In particular, a significant lifetime improvement by 11.6 times was obtained at an elevated temperature of 80 °C. The high efficiency and high-temperature stability of the OLED were ascribed to the balanced bipolar carrier-transporting properties, high T_g and rigidity of 4-3cbzBIZ. The present work of developing orthogonally linked donor–acceptor hosts provides a strategy of material design to develop high thermal stability OLEDs.