Effect of tert-butyl substitution on controlling the orientation of TADF emitters in guest–host systems

Abstract

Organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) materials as emitters have been reported to achieve 100% internal quantum efficiency due to their ability to harvest both singlet and triplet excitons. This is due to the small singlet–triplet energy gap in the emitter material that allows reverse intersystem crossing of excitons formed in the non-emissive triplet state to be endothermally upconverted to the emissive singlet state. Compared to phosphorescent molecules, the less bulky TADF molecules show a stronger tendency for horizontal alignment in vacuum-deposited films, which leads to an enhancement of the light-outcoupling from the OLED. In this work, we compare how changes in the structure of a linear acceptor–donor–acceptor indolocarbazole–triazine TADF emitter with tert-butyl groups affect molecular alignment across a range of different host matrices. We discuss the effects of their molecular electrostatic potentials and inertial moments as well as the glass transition temperatures of the host matrices. We observe that our previously reported molecule ICzTRZ with tert-butyl substitution on the distal phenyl rings of the acceptor yields the strongest horizontal alignment and, accordingly, the highest external quantum efficiency in OLEDs. By contrast, the absence of tert-butyl groups within the emitter leads to significantly lower alignment.