Efficient narrowband green OLEDs with TADF sensitizers combining multiple charge-transfer pathways

Abstract

Multiple resonance (MR) thermally activated delayed fluorescence (TADF) emitters have attracted much attention for their narrow emission band and high efficiency. However, organic light-emitting devices based on MR-TADF emitters generally suffer from severe efficiency roll-off at high current density due to their relatively large singlet–triplet splitting energy. In this work, three TADF molecules are designed by an ortho-connected oligo carbazole donor and diphenyltriazine acceptor with the purpose of combining both through-bond charge transfer (TBCT) and through-space charge transfer (TSCT). Through fine tuning the proportion of intramolecular TBCT and TSCT by changing the number of carbazole units, a relatively high reverse intersystem crossing rate (k_RISC) approaching 10⁶ s⁻¹ and radiative transition rate (k_F) of over 10⁶ s⁻¹ are achieved. As a result, the devices using these molecules as emitters give maximum external quantum efficiency (EQE) of over 20% with extremely low efficiency roll-off of 9.9% at the practical luminance of 1000 cd m⁻². Furthermore, by using these materials as assistant hosts and sensitizers and DMAc-BN as a terminal emitter, high efficiencies of 23.9% and 66.0 lm W⁻¹ are achieved with a full width at half maximum of 46 nm, featuring slow efficiency roll-off with EQE of 17.9% at the luminance of 1000 cd m⁻².