Managing local triplet excited states of boron-based TADF emitters for fast spin-flip process: Toward highly efficient TADF-OLEDs with low efficiency roll-off

Abstract

Improving the rate of a spin-flip process from a triplet to a singlet excited state, which is known as reverse intersystem crossing (RISC), is a challenging issue in realizing efficient and stable organic light-emitting diodes based on thermally activated delayed fluorescence (TADF-OLEDs). Herein, we report two donor-acceptor-type TADF emitters (TMCzBCO and DMACBCO) employing a boron-carbonyl (BCO) hybrid acceptor unit. The emitters possess a (3)n pi* state of the BCO unit as an intermediate local triplet excited state ((LE)-L-3, T-2), leading to the large spin-orbit coupling between the T-2 and excited singlet (S-1) states with significantly low activation barriers <10 meV. Thus, a fast RISC with rate constants (k(RICS)) exceeding 10(6) s(-1), as well as short exciton lifetimes (similar to 1 mu s), are realized. Using the TADF emitters, high-performance TADF-OLEDs with maximum external quantum efficiencies of 24.7% for the green (TMCzBCO) and 28.4% for the yellow (DMACBCO) are presented. Remarkably, the devices sufficiently maintain high efficiency at high luminance, exceeding 20% at 5,000 cd m(-2) and 18% at 10,000 cd m(-2). These results indicate the crucial role of the (3)n pi* state of BCO in the improvement of the RISC rate and shortening of the exciton lifetime, enhancing the optoelectrical performance of TADF-OLEDs.