A review of fused-ring carbazole derivatives as emitter and/or host materials in organic light emitting diode (OLED) applications

Abstract

This review focuses on fused-ring carbazole derivatives, their molecular design, electronic and photophysical properties, and in particular their applications as the emitter and/or the host material in the emitting layer of organic light emitting diodes (OLEDs), with emphasis on recent developments. This review is timely because of the rapidly expanding research into fused-ring carbazoles, predominantly indolocarbazole, indenocarbazole, benzofurocarbazole, benzothienocarbazole and diindolocarbazole derivatives. To our knowledge this class of materials has not been reviewed previously. The appeal of fused-ring carbazoles is their extended π-electron systems with good thermal stability, tunable frontier orbital energies that enable a wide gamut (red, green, blue and white) emission colour, high photoluminescence quantum yields, and versatility for chemical functionalisation at different sites, leading to outstanding OLED efficiencies. This review is divided into sections according to the molecules’ role in OLEDs: namely, as conventional luminescent emitters – especially in the deep-blue region; as state-of-the-art hosts for phosphorescent iridium-based emitters; as thermally activated delayed fluorescence (TADF) emitters with high external quantum efficiency; and as multiresonance (MR) emitters with unprecedented high colour purity. We conclude by highlighting the challenges and the great opportunities for fused-ring carbazole derivatives in OLEDs and other optoelectronic applications.