Modulation of triplet-mediated emission from selenoxanthen-9-one-based D–A–D type emitters through tuning the twist angle to realize electroluminescence efficiency over 25%

Abstract

Room-temperature phosphorescence (RTP) from metal-free purely organic molecules has recently attracted much attention. Unfortunately, the low phosphorescence efficiency and long lifetime of most RTP molecules in the film state restricts their OLED applications. Herein, we design two twisted Donor–Acceptor–Donor (D–A–D) type organic emitters (CzSe and TMCzSe) using selenoxanthen-9-one as the A unit and carbazole and 1,3,6,8-tetramethyl-9H-carbazole as the D units. Through changing the twist angles between the D and A units, the lowest triplet excited states can be modulated from locally excited triplet states (³LE) to triplet charge-transfer states (³CT), which can realize RTP emission and the mixed emission of RTP and thermally activated delayed fluorescence (TADF), respectively. Both CzSe and TMCzSe films exhibit high photoluminescence quantum yields (31.2% and 85.9%) and short lifetimes (360.1 μs and 26.71 μs). In particular, organic light-emitting diodes (OLEDs) fabricated with CzSe achieved a high external quantum efficiency (EQE) for purely organic RTP-based OLEDs up to 8.9%, while TMCzSe-based OLEDs exhibited an EQE of 25.5% for OLEDs with simultaneous TADF and RTP.