Dissipative Mechanisms in Organic Light-emitting Diodes: Role of Intramolecular Charge Transfer and Delayed Fluorescence

Abstract

Organic light-emitting diodes (OLEDs) are emerging to replace conventional lighting technology due to their flexible device structures, multicolour emission, and ease of fabrication. However, one of the key challenges in developing efficient emitters for OLEDs is overcoming the dissipative channel of triplet excitons. A common approach to mitigate this challenge is to employ emitter molecules optimized for either thermally activated delayed fluorescence (TADF) or triplet-triplet annihilation (TTA) to enhance the external quantum efficiency (EQE). Another alternative strategy to improve the quantum efficiency is deploying TADF chromophores with fluorescence emitters by recycling dark triplet excitons in a process, namely hyperfluorescence, which also retains narrow emission bandwidth. Therefore, understanding the intricate photophysics of donor-acceptor (D-A) chromophores by exploiting the intramolecular charge transfer (ICT) state to unravel their multifarious applications attracts widespread attention. In my thesis, I have rationally designed a series of aromatic imide-based D-A chromophores that display the TADF and TTA phenomenon, where the suitable choice of donor or acceptor governs the dominant delayed emission pathways by manipulating ICT state. Further, by combining a TADF chromophore with a series of diketopyrrolopyrrole-based fluorescence emitters, efficient energy transfer could be facilitated, thereby enhancing the emission intensity of the fluorescence emitters. Nevertheless, in the quest to design new D-A chromophores, I made a serendipitous observation of stable radical cation formation in carbazole-based diketopyrrolopyrrole derivatives, which offers a plethora of promising applications. Our in-depth photophysical studies provide insights into the importance of developing new chromophores, which offer myriad applications in optoelectronics.