Efficient Organic Light-emitting Diodes with Singlet, Triplet and Doublet Excitons

Abstract

Organic light-emitting diodes have shown a large potential and significant research progress recently. Tightly-bound excitons play an important role in the function of energy conversion and light emission for OLEDs. Many properties of excitons are determined by the spin state including singlets, triplets and doublets. Triplet excitons are regarded as dark states due to their spin-forbidden decay process, whereas effective triplet harvesting or triplet circumvention can help to improve the OLED performance. In this thesis, methods and mechanisms to avoid loss pathways were explored from the spin management perspective, which is not only limited to the spin-flip in singlet-triplet photophysics but also extend to the energy transfer in the singlet-triplet-doublet playground. We first demonstrate a molecular design concept on carbene-metal-amide (CMA) complexes as an effective way to optimise the triplet upconversion process in the singlet-triplet manifold. Through a partially twist and tilt between the donor and acceptor, it is able to realise highly efficient CMA materials with low exchange energy while maintaining the high radiative decay rate. These superior properties can be translated to OLEDs with great electroluminescent behaviour and improved device stability. We then explore the triplet and singlet exciton harvesting for energy transfer to doublets. In the first CMA−CF3:TTM-3PCz system, ultrafast intersystem crossing process in CMA− CF3 leads to the generation of triplet excitons in high yield, enabling efficient spin-allowed Dexter type energy transfer to the doublet state in TTM-3PCz in ps−ns timescales. We also discovered other sulfone-based thermally activated delayed fluorescent (TADF) hosts which can induce efficient singlet-to-doublet energy transfer to TTM-3NCz. These mechanisms can be successfully translated to devices with maintained OLED performance and enhanced device lifetimes. Finnaly, we present the tuneability of the two novel classes of materials, CMA and doublet radicals discussed above, in solution-process near-infrared (NIR) luminescent OLEDs. The novel properties of doublet spins are maintained in both non-conjugated and conjugated radical polymers, with fast radiative decay rate and NIR emission around 700 nm. Similarly, the strong TADF and spin-orbit coupling characters are reserved in the NIR CMA complexes which exhibit high external quantum efficiency and tuneable emission colours.