Rational strategy of exciplex-type thermally activated delayed fluorescent (TADF) emitters: Stacking of donor and acceptor units of the intramolecular TADF molecule

Abstract

• Rational strategy to construct high efficiency exciplex-type chromophores is put forward. • The exciplex PL decay lifetimes can be tuned from 20 μs to 800 μs by the donor structures. • Exciplex-OLEDs show prominent efficiency with EQE of 16.9% and low efficiency roll-off. The lack of the exciplex-type chromophores with high quantum yields hinders their wide applications. Here, a rational strategy to construct exciplexes with thermally activated delayed fluorescence (TADF) were designed by stacking of the donor and acceptor units of an intramolecular charge transfer (CT) chromophore (TXO-PhCz). The intermolecular CT interaction can be anticipated in DFT calculations between the TXO unit and PhCz units. By mixing TXO-P-Si with four donor molecules 1,3-Bis(carbazol-9-yl)benzene (mCP), 4,4′-Bis(carbazol-9-yl)biphenyl (CBP), 3,5-Bis(3-(9H -carbazol-9-yl)phenyl)pyridine (3,5-DCzPPy) and 3-(Diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H -carbazole (PPO21), emission from these exciplexes can be easily observed. And the photophysical properties of these formed exciplexes can be tuned by the structure of the carbazole-based molecules. The exciplexes eliminate the high driving voltage issue, realizing highly efficient green exciplex OLED with a low driving voltage of nearly 3.5, 3.8, and 3.6 V and maximum external quantum efficiency (EQE) up to 16.9% (53.8 cd A −1 , and 48.3 lm W −1 ), 13.7% (39.9 cd A −1 , and 36.2 lm W −1 ), 16.1% (51.0 cd A −1 , and 45.1 lm W −1 ) for mCP:TXO-P-Si, CBP:TXO-P-Si, and 3,5-DCzPPy:TXO-P-Si, respectively. Meanwhile, the efficiency roll off is small, which is comparable with the device with low efficiency roll off. The ideal OLED device based on our strategy with high efficiency and low roll-off has been realized at the same time.