Preserving Liquid-Crystalline Order in Organic Light-Emitting Diodes - ChemistryViews

Organic light-emitting diodes have become a dominant display technology in smartphones, televisions, smartwatches, laptops, and virtual-reality headsets. Their individual emissive pixels enable vivid colors as well as true black, high contrast, and thin display architectures, while the use of flexible substrates has opened the way to curved, foldable, and rollable devices. OLEDs are also being explored for transparent displays and augmented-reality applications.

OLEDs are typically fabricated by vacuum deposition but can also be made using solution-processing techniques. The latter are enticing because they offer the potential for lower-cost, scalable, and less energy-intensive manufacturing. However, solution-processed OLEDs still generally perform less efficiently than their vacuum-deposited counterparts. One contributing factor is the limited control over the orientation of the emitter transition dipole moments within the emissive layer, which determines the light outcoupling efficiency of the device. During solution processing, solvent evaporation and molecular relaxation result in the emitters adopting a largely random orientation, restricting the fraction of generated light that can escape from the device.

Liquid crystals offer a promising way to control molecular organisation in solution-processed films. These materials combine fluidity with orientational and positional order and can therefore self-assemble into ordered structures. Using liquid-crystalline emitters in solution-processed OLEDs has the potential to enhance light outcoupling efficiency. However, when liquid-crystalline emitters are used as neat films, strong interactions between their emissive cores frequently result in aggregation-caused quenching, broader emission, and reduced photoluminescence quantum yields. Dispersing the emitters in a conventional small-molecule host can suppress intermolecular interactions and improve their photoluminescence quantum yield. However, dilution can also disrupt the mesophase and its associated long-range supramolecular order, undermining the original purpose of introducing liquid-crystalline functionality. The challenge is therefore to retain an ordered mesophase while simultaneously preserving the favourable emission properties of the emitter.

Researchers at the University of St Andrews, the University of Cologne, and the University of Stuttgart addressed this trade-off by developing an emissive host–guest system in which both components are liquid crystalline. The newly designed host, mCPLC, is derived from the widely used OLED host mCP and contains difluorobiphenyl mesogenic groups. The multi-resonant thermally activated delayed fluorescence emitter DiKTa-Meso bears related mesogenic units, facilitating its incorporation into the ordered host matrix. The neat mCPLC film exhibited a smectic A mesophase between 121 and 180 °C. Importantly, this mesophase was retained when 1, 2, or 5 wt% DiKTa-Meso was incorporated into the film, demonstrating that the emissive dopant could be introduced without destroying the liquid-crystalline organisation of the host at low doping concentrations. The mesomorphic behaviour of the host, emitter, and their mixtures was established using polarised optical microscopy, differential scanning calorimetry, and small- and wide-angle X-ray scattering.

A spin-coated film containing 5 wt% DiKTa-Meso in mCPLC produced bright green emission centred at 534 nm and achieved a photoluminescence quantum yield of 75%. Angle-resolved photoluminescence measurements revealed a modest preference for horizontal alignment of the transition dipole moments of DiKTa-Meso, reflected in an anisotropy factor of 0.30. When this host–guest film was employed as the emissive layer of a solution-processed OLED, the device reached a maximum external

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

Praveen Choudhary, Džiugas Litvinas, Falk Feucht, Franziska Müller, Daniel Rück, Bettina Lutz, Sabine Laschat, Malte C. Gather, Eli Zysman-Colman

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https://doi.org/10.1002/agt2.70465