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

ABSTRACT

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 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 (LCs) 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 horizontal alignment of the TDM. 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. First, 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 ( Φ _PL ) of 75% and a modest preferential horizontal orientation of the 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 (EQE _max ) of 12.2% and exhibited low efficiency roll‐off, with an EQE of 11.8% at 1000   cd m ⁻² .