Study of Thermally Activated Delayed Fluorescent Exciplexes and their Practical Applications in OLEDs

Abstract

Exciplexes are intermolecular charge transfer (CT) complexes in which one electron donating (D) and one electron accepting (A) molecule interact in the excited state. The new bimolecular CT excited state species is the exciplex, a shortening of EXCIted state comPLEX. Until recently exciplexes were avoided in OLEDs structures since they constituted an efficiency loss pathway since they commonly possess low photoluminescence quantum yield (PLQY).
Recently, the rise of thermally activated delayed fluorescence (TADF) applications for triplet harvesting in fluorescent OLEDs has resulted in renewed research interest in these bimolecular excited states. The TADF mechanism in fact, allows to upconvert triplet excited states (which are non-emissive in normal fluorescent emitters) into emissive singlet excited states thus boosting the efficiency of the emitter.
To be efficient, the TADF mechanism needs to have minimal overlap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). Exciplexes intrinsically possess this characteristic since the CT excited state is formed between two different molecules making exciplexes the perfect candidates as TADF emitters. For this reason, TADF exciplexes are attracting more and more attention although always in the shadow of their more successful intramolecular counterpart (covalently linked D-A fragments in a single molecule) since they could be more easily tailored to maximise their efficiency and modify their properties.
The first part of this thesis demonstrates the surprising discovery that exciplex electronic energy and PLQY are not intrinsically fixed by the D/A couple forming the exciplex, and that these characteristics can be tuned and improved through solid state dilution. It is shown that the exciplex electronic energy can be controllably increased by varying average intermolecular distance between the D and A molecule within the exciplex blend by inserting a third inert molecule in the blend forming the film.
The change in the exciplex electronic energy and PLQY is rationalised by a general reduction of the coulombic binding energy with D-A separation. In contrast, the PLQY enhancement is not general and determined to be related to the degree of flexibility of the exciplex forming molecules.
The second part of this thesis showcases work that broadens the range of potential applications of TADF exciplex OLEDs, demonstrating their suitability as emitters for solution processed devices and how they can be used to confine the recombination zone of a standard phosphorescent OLED - leading to performance and stability improvements.