Simultaneous enhancement of efficiency and stability of OLEDs with thermally activated delayed fluorescence materials by modifying carbazoles with peripheral groups

Abstract

Albeit their high efficiencies, the operational stability of the organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters is still far from satisfaction, and few strategies have been proposed to improve their stability. Here, we show that by modifying the carbazole unit, one of the most commonly used donors in TADF emitters, with peripheral groups, both the device efficiency and operational stability can be greatly improved. A well-known TADF molecule—4,5-di(9 H -carbazol-9-yl)phthalonitrile (2CzPN) was chosen as the prototype and modified by introducing peripheral tert -butyl and phenyl groups to the 3,6-positions of the carbazole (named 2 t BuCzPN and 2PhCzPN, respectively). The introduced groups not only improve the compounds’ electrochemical stabilities referred to the cyclic voltammetry multi-sweep results, but also promote their photoluminescence quantum yields. Furthermore, reduced singlet-triplet energy gaps are observed, leading to the shortened exciton lifetimes which are benefit to suppress the exciton annihilations. Besides, the steric hindrance of introduced phenyl groups can partly restrain the concentration quenching of the TADF emitter. Consequently, OLEDs based on 2 t BuCzPN and 2PhCzPN achieved improved maximum external quantum efficiencies (EQEs) of 17.0% and 14.0%, respectively (compared to 8.5% for 2CzPN). Meanwhile, 2PhCzPN based OLED showed reduced roll-off characteristics and a longer lifetime of 7.8 times higher than that of 2CzPN, testifying the effectiveness of subtle modification of the unstable moieties in simultaneous enhancement of efficiency and stability of OLEDs based on TADF emitters.