Achieving over 36% EQE in blue OLEDs using rigid TADF emitters based on spiro-donor and spiro-B-heterotriangulene acceptors

Abstract

The design of robust thermally activated delayed fluorescence (TADF) emitters is crucial in realizing highly efficient and stable blue TADF-organic light-emitting diodes (OLEDs). Here boron-based donor–acceptor-type blue TADF compounds incorporating a triply-bridged, spiro-type boron acceptor; namely, spiro-fluorenyl B-heterotriangulene, and 9,9-diphenyl-9,10-dihydroacridine (DPAC, 1) or 10H-spiro[acridine-9,9′-fluorene] (sAC, 2) donors are presented. Their doped host films show blue emissions with narrow full width at half maximum values of 52 nm (0.29 eV) and unitary photoluminescence quantum yields. The compounds exhibit short-lived delayed fluorescence (τd = 1.8–2.7 μs) and undergo fast reverse intersystem crossing with the rate constants of ∼106 s−1 in the film state. These are attributable to the small singlet–triplet energy splitting of ∼40 meV and the effective spin–orbit coupling between the singlet (1CT, S1) and local triplet (3LE, T2) excited states. From outstanding horizontal transition dipole orientation ratios of 0.89 and 0.90 for the host films of 1 and 2, respectively, high-efficiency blue TADF-OLEDs showing maximum external quantum efficiencies (EQEs) of over 30% are realized. Notably, the device with the fully spiro 2 achieves a state-of-the-art EQEmax of 36.4% and a suppressed efficiency roll-off, with a high EQE of 26.3% even at 1,000 cd/m2, highlighting the synergistic effect between the spiro-structure and the B-heterotriangulene acceptor skeleton.