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

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 degrees C, T-d > 453 degrees 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 degrees C annealing. The FIrpic doped 4-3cbzBIZ organic light-emitting diode (OLED) shows sky-blue electroluminescence from FIrpic with a high external quantum efficiency (eta(EQE)) of 28.6% with an amazingly small efficiency roll-off (eta(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 degrees 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.