Seminar: PhD Tom CARDEYNAELS

Diverging from the beaten track - rational design of thermally activated delayed fluorescence emitters inspired by different fields of organic electronics

Organic light-emitting diodes (OLEDs) have emerged as the most successful branch of organic electronics because of their competitiveness with other technologies such as inorganic LEDs, which is related to their ability to achieve high efficiencies (i.e. the light output versus power input), high color purity and color tunability. Especially for display applications, their contrast and color gamut are unparalleled.

In the search for more sustainable OLED devices, active materials based on thermally activated delayed fluorescence (TADF) have been put forward in recent years. OLEDs based on TADF emitters are able to achieve similar device efficiencies to those based on phosphorescent emitters, while removing the need for heavy metal complexes. TADF emitters rely on a thermally enhanced reverse intersystem crossing from the triplet to the singlet state followed by fluorescence emission.

The principle of TADF was first applied to OLED devices in 2009 by Adachi et al. and has been thoroughly investigated ever since. The designs typically make use of donor (D) and acceptor (D) subunits, connected together via cross-coupling chemistry, resulting in charge-transfer type emission from these materials. The reverse intersystem crossing is enhanced by bringing the first singlet and triplet excited states closely together in energy, allowing thermal energy to overcome the energy barrier. The singlet-triplet energy splitting (ΔE_ST) is reduced by separating the HOMO and LUMO topologies of the molecule through space. In practice, this is done by twisting the donor and acceptor parts through steric interactions. Therefore, the active materials usually have dihedral angles between the D and A parts of more than 80°. Quantum-chemical calculations help to predict the required geometrical and excited-state properties of the emitters, but experimental measurements are needed to validate the theoretical findings.

In this thesis, a quantum-chemical method for the determination of the ΔE_ST of typical charge-transfer compounds, including some TADF materials, was optimized. It was then applied to the rational design of novel TADF emitters based on building blocks from different fields of organic electronics (notably organic photovoltaics). Several new donor and acceptor units were introduced successfully, resulting in new D-A-D compounds which were synthesized and characterized using experimental methods. The newly synthesized emitters showed a variety of interesting excited-state properties such as TADF, but some were also found to exhibit phosphorescence at room temperature or triplet-triplet annihilation instead, depending on which D or A units were used.