High efficiency and long lifetime fluorescent organic light-emitting diodes based on cascaded energy transfer processes to efficiently utilize triplet excitons via sensitizer

Abstract Fluorescent organic light-emitting diodes (OLEDs) have aroused a lot of attention due to the advantages of lost cost, good operational stability and low efficiency roll-off. However, how to realize high efficiency and long lifetime fluorescent OLEDs simultaneously is still a huge challenge. The use of exciplex and thermally activated delayed fluorescence (TADF) materials as sensitizers for conventional fluorescent emitters is a promising route to improve the efficiency and operational stability of fluorescent OLEDs. Herein, we demonstrate a novel strategy by using a TADF material as sensitizer in exciplex host to realize high efficiency and long lifetime fluorescent OLEDs. The key in our design is the further formation of additional exciplex between TADF sensitizer and electron-acceptor, thus three RISC processes to efficiently convert triplet excitons into singlet excitons by cascaded energy transfer processes. Based on this design strategy, the resulting fluorescent OLEDs achieve the maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 22.6 cd A−1, 29.5 lm W−1 and 13.0%, and the operational lifetime LT50 (decay to 50% of the initial luminance of 1000 cd m−2) reaches 415 h, which are significantly higher than those based on the conventional sensitized methods, fully demonstrating the effectiveness of our design concept to realize higher efficiency and longer lifetime fluorescent OLEDs.