Improvement of Quantum Efficiency and Efficiency Roll-off in OLED Devices Using Blue Thermally Activated Delayed Fluorescent Materials

Abstract

The utilization of triplet excitons in the operation of organic light emitting diode play a crucial role for the enhancement of an external quantum efficiency, which will be maximized by the use of thermally activated delayed fluorescent (TADF) materials. In this study, a deep blue TADF materials with a donor-acceptor-donor structure was designed and prepared 9,9'-((2-methylpyrimidine-4,6-diyl) bis(2,1-phenylene)) bis(9H-carbazole) (DCz-mPym), (9,9' -((2- methylpyrimidine-4,6-diyl)bis(2,1-phenylene)) bis(3,6-di-tert-butyl-9H-carbazole) (DtBCz-mPym), and 5,5' -((2-methylpyrimidine-4,6-diyl) bis(2,1-phenylene)) bis (5H-benzofuro[3,2-c]carbazole) (DBFCz-mPym) using a pyrimidine as an acceptor and a carbazole derivatives as a donor. We fabricated an organic light emitting diode (OLED) using the designed three TADF materials as a dopant for light emitting layer of OLED. Correlation between the photophysical data, electrochemical properties, and the efficiency roll-off behavior were investigated for designed materials and device with appropriate structure. As a result, DBFCz-mPym, the material with very small difference between singlet energy and triplet energy, is superior to the DCz-mPym or DtBCz-mPym in terms of efficiency roll-off and harvesting of triplet excitons. Further, as a result of simulation of an efficiency roll-off curves by the increase of the current density, triple-triplet annihilation was confirmed to be the main cause.