Highly efficient multiple resonance TADF emitters by hybridizing long-range and short-range charger transfer characteristics to enable narrowband and low roll-off OLEDs

Abstract

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials show great potential for ultrahigh-definition organic light-emitting diodes (OLEDs) owing to their exceptional luminescence efficiencies and narrow emission spectra. Nevertheless, the device performances of MR-TADF emitters typically suffer from significant efficiency loss under high current densities due to the slow reverse intersystem crossing (RISC) rates. Herein, we propose a straightforward yet effective strategy to introduce three typical spiral electron-acceptor fragments to an MR framework featuring hybridized short-range charge-transfer (SRCT) and long-range charge-transfer (LRCT) characteristics. Comprehensive photophysical and computational investigations of these MR-TADF materials demonstrated that the difference in the electron-withdrawing ability among the three acceptor units of the MR framework had a significant influence on the emission color, full-width at half-maximum (FWHM) and RISC rates. Remarkably, the sensitizer-free OLED based on BNAP demonstrated the best device performance, with an electroluminescent peak at 512 nm, an FWHM of 36 nm, CIE coordinates of (0.17, 0.68), and a maximum external quantum efficiency (EQE) of 36.1%. The EQE values at 100 cd m⁻² and 1000 cd m⁻² were 32.8% and 16.1%, respectively, revealing that the introduction of the LRCT feature effectively modulated the energy level and harnessed the high-energy triplet excitons to suppress efficiency roll-off.