Robust luminescent small molecules with aggregation-induced delayed fluorescence for efficient solution-processed OLEDs

Abstract

Purely organic luminescent materials with thermally activated delayed fluorescence have the merits of high exciton utilization and thus excellent electroluminescence (EL) efficiency in OLEDs. However, these devices, particularly solution-processed doped OLEDs, usually encounter the troublesome problem of severe efficiency roll-off at high voltages. Herein, two new organic small molecules consisting of electron-withdrawing benzoyl and electron-donating 9-hexylcarbazole and phenoxazine (or 9,9-dimethyl-9,10-dihydroacridine) moieties are designed and synthesized. The crystal and electronic structures, thermal stabilities, electrochemical behaviors and photophysical properties are thoroughly investigated. Whereas these materials are weak emitters in solution, they can fluoresce strongly in spin-coated neat films with greatly enhanced delayed fluorescence, namely they possess interesting aggregation-induced delayed fluorescence (AIDF). Solution-processed nondoped OLEDs are fabricated by using these materials as light-emitting layers, which provide high EL efficiencies of up to 9.02%. Efficient doped OLEDs with varied doping concentrations (10, 30 and 50 wt%) in a 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host are also obtained by a spin-coating technique, offering higher EL efficiencies of up to 12.1%. More importantly, both solution-processed nondoped and doped OLEDs exhibit an extremely small efficiency roll-off (down to 0.08% at 1000 cd m⁻² luminance), demonstrating the greatly advanced efficiency stability. These results clearly prove that luminescent materials with AIDF properties are promising candidates for the fabrication of high-performance solution-processed OLEDs.