Preserving the Mesophase in the Solution-Processed OLED Emissive Layer Through Liquid-Crystalline Host–Guest Engineering

<jats:p>The fabrication of organic light-emitting diodes (OLEDs) typically proceeds via vacuum deposition (VD); however, solution-processed (SP) devices hold the promise for a much more cost-effective and less energy-intense fabrication. Their performance, however, lags behind 2 vacuum-deposited devices, in part because there is poor control of the orientation of the transition dipole moment (TDM) of the emitter in the SP-OLED, leading to lower external quantum efficiencies. Liquid crystals (LC) have long-range order and thus introduce molecular anisotropy in films. LC multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters have been designed to enhance the preferential alignment of the TDM in the horizontal direction. However, neat films of LC MR-TADF lose the attractive photophysical properties of the isolated molecules due to aggregation caused quenching (ACQ). Herein, we propose a strategy of host:guest films wherein both the host and guest are LC. Firstly, we developed the LC host, mCPLC, which showed a high triplet energy of 2.73 eV and exhibited the smectic A (SmA) mesophase from 121 °C to 180 °C as its neat film. Upon doping the LC MR-TADF emitter DiKTa-Meso into mCPLC at concentrations of 1, 2, and 5 wt%, the SmA mesophase is retained. Spin-coated films containing 5 wt% DiKTa-Meso in mCPLC showed a high photoluminescence quantum yield (ΦPL) of 75% and a modest preferential horizontal orientation of the transition dipole moment (TDM), with an anisotropy factor (a) of 0.30. The SP-OLED fabricated using 5 wt% DiKTa-Meso in mCPLC achieved a maximum external quantum efficiency (EQEmax) of 12.2% and a low efficiency roll-off, with an EQE of 11.8 % at 1000 cd m-2.</jats:p>