Triarylboryl-substituted carbazoles as bipolar host materials for efficient green phosphorescent organic light-emitting devices

Two novel bipolar host materials based on carbazole and triarylborane moieties, i.e. B-2CZ and m-B-CZ, are synthesized and characterized. The spatially crowded compound B-2CZ exhibits a high thermal stability (T-d > 340 degrees C) compared to the reported triarylboryl carbazoles, presumably due to the highly compact architecture. As a result, phosphorescent Ir(ppy)(3) combined with both the borylated compounds is selected to construct a host-guest system. Devices A (B-2CZ) and B (m-B-CZ) achieve peak efficiencies of 19.3% (69.1 cd/A and 88.1 lm/W) and 19.1% (66.1 cd/A and 77.2 lm/W), respectively. The respective turn-on voltages of devices A and B are recorded at 2.4 and 2.6 V, which are much lower than that of device with the benchmark host (mCP, 3.3 V). In addition, at a higher practical luminance of 100 cd/m(2) (1000 cd/m(2)), the external quantum efficiencies of devices A and B are still preserved at 19.1% (17.3%) and 18.2% (13.0%) with the mitigated efficiency drops of 1% (10%) and 5% (32%), suggesting the promising advantage for realizing efficient phosphorescent organic light-emitting devices.