3-Cyanocarbazole/phosphine oxide hybrid host with increased molecular polarity towards universally enhanced efficiency for TADF OLEDs

Abstract

Very recently, the exploration of new thermally activated delayed fluorescence (TADF) materials has aroused considerable attention. However, the development of suitable organic host materials for TADF organic light-emitting diodes (OLEDs) is lagging far behind. Herein, a new organic host, 3CNCzPO, is designed and synthesized by a simple catalyst-free C–N coupling reaction starting from 3-cyanocarbazole and tris(4-fluorophenyl)phosphine oxide. Compared with the CN-free 3CzPO, the 3CNCzPO exhibits a comparatively high triplet energy of 2.85 eV, lowered HOMO/LUMO energy levels by 0.15–0.2 eV, increased thermal stability, and enhanced electron transport properties. Despite the slightly reduced photoluminescence quantum yield (PLQY) in the 3CNCzPO films doped with TADF emitters, the increased molecular polarity for the CN-substituted 3CNCzPO host could obviously reduce the delayed lifetime for TADF emission, and thus restrict the unbeneficial quenching factors. Therefore, a universally enhanced electroluminescence efficiency is achieved for various TADF OLEDs, with the maximum external quantum efficiency improved from 13.8% to 22.2% and 14.3% to 20.1%, as well as 10.8% to 15.1% for light-blue dCF₃5tCzOXD and 5tCzOXD as well as greenish-yellow 4tCzDOXD-based TADF emitters, respectively.