Highly efficient nondoped blue electroluminescence based on hybridized local and charge-transfer emitter bearing pyrene-imidazole and pyrene

Abstract

• A high-efficiency blue emitter is obtained based on pyrene[4,5- d ]imidazole and pyrene. • The nondoped blue OLED shows the maximum EQE of 9.13% with maximum exciton utilization efficiency of 65%. • Extremely small efficiency roll-offs of 0.9% and 6.9% are realized at high brightness of 10,000 and 50000 cd m −2 . Organic light-emitting diodes (OLEDs) have been explored and utilized in many fields of flat-panel displays nowadays because of their plentiful advantages such as lightweight, high flexibility, high picture quality, etc., and are hopeful to be the next-generation displays. However, improvements to the efficiency of blue OLEDs are still vital to OLEDs as replacements for liquid crystal display technology. A blue light-emitting material is one of the key components in the preparation of OLED displays. Designing molecule based on donor–acceptor (D-A) structure with a hybridized local and charge transfer (HLCT) excited state is a appealing strategy for providing an efficient OLED with high external quantum efficiency through efficacious exciton utilization. Herein, a highly efficient blue emitter, PyI-Py, is constructed combining pyrene[4,5- d ]imidazole (weak donor) and pyrene (weak acceptor). The non-doped blue OLED exhibits excellent performance with a maximum current efficiency of 15.74 cd A −1 , a maximum external quantum efficiency (η ext ) of 9.13% and a maximum brightness of as high as 91097 cd m −2 which is rarely obtained for a non-doped blue OLED. Moreover, the η ext can reach 9.05% at very high brightness of 10000 cd m −2 , displaying extremely low efficiency roll-off of 0.9% which displays great superiorities in contrast with most thermally activated delayed fluorescent materials. The device characteristics lie among the highest values in non-doped blue OLEDs based on HLCT emitter and correspond to the best performance achieved for non-doped pyrene-based blue OLEDs to date. A maximum exciton utilization efficiency of 65% is harvested. The superior performance is attributed to the proper donor–acceptor design strategy which results in a HLCT excited state with the generation of a high proportion of singlet excitons.