NCTS (South)





SPEAKER Prof. Wen-Yi Hung(洪文誼) - 海洋大學光電所

TITLEEfficient up-conversion from triplet to singlet excited states of exciplex in next-generation organic light-emitting diodes

ABSTRACTExciton (or spin) statistics is a basic physical principle, which determines the singlet and triplet excitons ratio (1:3) formed in an organic semiconductor device. While the first OLEDs displayed low efficiency fluorescent emission which is inherently limited by the underlying physical principles, the second generation OLEDs used phosphorescent materials, enabling to reach the theoretical maximum efficiency of light emission. However, compared to fluorescent OLEDs, phosphorescent OLEDs require expensive metal complexes and their design possibilities are limited. As a third alternative, thermally activated delayed fluorescence (TADF)-based OLEDs can unite the benefits from both the phosphorescence- and the fluorescence-based OLEDs by introducing an upconversion mechanism from triplet to singlet excited states. This kind of materials rejuvenates the spin-statistics theory, and it is expected to develop into a low-cost, high-performance next-generation OLEDs. In the present study, we demonstrate a strategy to realize radiative-exciton production with higher efficiency by using the high reverse ISC efficiency of the intermolecular excited state (that is, exciplex state) between electron-donating and electron-accepting molecules. With this, we prove that a panchromatic range of exciplex emission from blue to red can be attained via systematically tuning HOMO of the HTL material. Further fine-tuning the device structure, a record-high blue OLED was achieved with EQE of 8 %. Standing on this fundamental, we construct a tandem, all- exciplex-based white light organic light-emitting diode (WOLED) configured by the two parallel blend layers, generating blue and yellow exciplex emission, respectively. The resulting device demonstrates for the first the all exciplex-based white light organic light-emitting diode (WOLED) with champion efficiencies with CIE (0.29, 0.35) and CRI 70.6 that are nearly independent of EL intensity. This tandem system, being fully utilizing the exciplex delay fluorescence, provides a paragon for facile tuning of low-cost, abundant organic compounds en route to commercial WOLED.