Bipolar Phenanthroimidazole Derivatives Containing Bulky Polyaromatic Hydrocarbons for Nondoped Blue Electroluminescence Devices with High Efficiency and Low Efficiency Roll-Off

A series of bipolar phenanthroimidazole derivatives, TTP-TPI, DPT-TPI, and DPF-TPI, were designed and synthesized by incorporating polyaromatic hydrocarbon groups to phenanthroimidazole through a phenyl bridge. The bulky polyaromatic hydrocarbon units endow the molecules with high glass-transition temperatures, nonplanar twisty structures which reduce molecular aggregations. The bipolar transporting natures of these materials are demonstrated by single-carrier devices with supported of theoretical calculations. Nondoped organic light-emitting devices (OLEDs) using these phenanthroimidazole derivatives as emitters show blue-violet to sky-blue emissions with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.05) for TTP-TPI, (0.16, 0.07) for DPT-TPI, and (0.17, 0.24) for DPF-TPI. The TTP-TPI-, DPT-TPI-, and DPF-TPI-based nondoped devices show impressive external quantum efficiencies (EQE) of 5.02, 5.25, 4.85%, respectively (corresponding current efficiencies: 2.10, 3.13, and 8.41 cd/A). These values are the best or among the best comparing to those of the reported nondoped OLEDs with the corresponding color gamuts. These devices also show small efficiency roll-off at high brightness (1000 cd/m2) with EQEs drop by 20.7, 12, and 0%. Moreover, with well-balanced carrier transport, DPF-TPI based device can achieve a higher brightness of 10000 cd/m2 with EQE maintaining at 4.49% (only drop by 7.4%).