In recent years, we have been reported on a series of highly efficient thermally activated delayed fluorescence (TADF) molecules and their OLED performance. We clarified that a large delocalization of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in charge transfer compounds provides a small energy gap between singlet and triplet excited states (EST < 0.2 eV). Simultaneously, even when there is a small overlap between the two wavefunctions, we can successfully keep rather high radiative decay rate (kr) by inducing a large oscillator strength (f). Thus, compatibility of both small EST and large kr is fundamental for high efficiency delayed fluorescence. Based on this design concept, we systematically synthesized a wide variety of TADF molecules and demonstrated high efficiency OLED with a maximum EQE~20%. In this talk, we review materials design, synthesis, photophysics and OLED performance based on TADF. Further, we report advanced device structure of fluorescence-based OLEDs that realize internal quantum efficiencies as high as 100% for blue, green, yellow, and red emission, indicating that the exciton production efficiency reached nearly 100%. The high performance was enabled by utilization of TADF molecules as assistant dopants that permit efficient transfer of all electrically generated singlet and triplet excitons from the assistant dopants to the fluorescent emitters. OLEDs employing this novel exciton harvesting process provide freedom for the selection of emitters from a wide variety of conventional fluorescent molecules.
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