Achieving highly efficient fluorescent blue organic light-emitting diodes through optimizing molecular structures and device configuration

Abstract

Based on the results of first-principles calculations of the electronic properties of blue light-emitting materials, the molecular structures of oligofluorenes are optimized by incorporating electron-withdrawing groups into the molecules to balance hole and electron injection and transport for organic light-emitting diodes (OLEDs). The result is a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. Further optimization of the device configurations and processing procedures, e.g., by changing the thickness of the emitting layer and through thermal annealing treatments, leads to a very high maximum EQE of 7.40% for the undoped sky-blue device. Finally, by doping the emitter in a suitable host material, 4,4′-bis(carbazol-9-yl)biphenyl (CBP), at the optimal concentration of 6%, pure blue emission with extremely high maximum EQE of 9.40% and Commission Internationale de l′Eclairage (CIE) coordinates of (0.147, 0.139) is achieved. An external quantum efficiency of 9.4% from pure blue organic light-emitting devices (OLEDs) with color coordinates of (0.15, 0.14) is achieved using highly efficient and stable fluorescent organic emitters designed from first-principles calculations. The efficiency of these low-cost solution-processed pure blue OLEDs maintains >5% even at very high current density of 110 mA cm-2. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.