Blue Thermally Activated Delayed Fluorescence OLEDs: towards improved performance

Abstract

Organic molecules are inherently complex entities, their intricacies often veiled from direct observation. Both theoretical and experimental approaches tirelessly endeavour to illuminate their elusive 'dark' side, yet unveiling their mysteries remains a formidable challenge. This work specifically delves into organic light-emitting molecules, focusing on thermally activated delayed fluorescence (TADF) emitters. Light serves as the key tool to indirectly peek into their behaviour. By exciting these TADF molecules with light, time-resolved 'snapshots' of their relaxation from the excited to the ground state are obtained. Through analysis of these 'snapshots,' patterns emerge, holding valuable information such as molecular conformations, quantized energy (sub)levels in the excited and ground state(s), homo- and hetero-molecular interactions. Furthermore, this analysis facilitates the estimation of exciton kinetics within the studied system, whose aim is to efficiently harness its energy and release it in the form of light.
This thesis integrates this experimental approach to explore fundamental aspects of TADF and multiple-resonance (MR)-TADF emitters in guest host systems, alongside organic light-emitting diode (OLED) fabrication and engineering. Its ultimate goal is to address the Achilles’ heel of OLEDs by enhancing blue OLED performance.
Current OLED technologies predominantly rely on fluorescent and/or triplet fusion emitters for commercial blue OLED pixels. While these materials ensure acceptable device lifetimes, they often suffer from lower efficiency compared to blue phosphorescent or TADF emitters. However, all these emitter types share a common challenge: their relatively broad emission spectrum results in unnecessary energy losses to meet stringent emission colour standards. On the contrary, MR-TADF emitters, although encounter issues with relatively poor exciton harvesting efficiency, particularly for triplet excitons, offer a narrow emission spectrum.
Here, through an in-depth investigation of both TADF and MR-TADF emitters, key aspects of each emitter generation can be elucidated, enabling their integration into hyperfluorescence (HF) OLEDs. This innovative approach circumvents the limitations of TADF blue emitters by utilizing them as sensitizers for MR-TADF terminal emitters. The broad emission spectra and low radiative rates of a TADF sensitizer can be offset by the fast and narrowband emission from the terminal emitter while ensuring the utilization of 100% triplet excitons by the former.
Through exploration of the essential requirements for efficient sensitization in HF OLEDs, by pushing the boundaries of sensitization, an unforeseen material combination emerged.
Surprisingly, greenish-emitting sensitizers were found capable of supporting blue HF-OLEDs. This discovery results in a reduction of both singlet and triplet energies for all materials within the emission layer, consequently enhancing device stability.