Exciton dynamics in organic semiconductors for efficient electroluminescent devices

Abstract

In this thesis, new approaches to realise efficient and stable visible and near-infrared (NIR) electroluminescence (EL) devices are explored by controlling exciton energy transfer and charge transport in organic semiconductors. In the first part of the thesis, ‘hyperfluorescence’, which is the concept that thermally activated delayed fluorescence (TADF) materials are exploited as an exciton sensitiser to a fluorescent emitter, is investigated. Firstly, the possibility of excluding the host matrix is investigated based on the exploration of exciton energy transfer between donors and acceptors. Since a host matrix is removed, charge transport in TADF donors is studied in addition to energy transfer. Space-charge-limited current (SCLC) measurement based on single-carrier devices is employed. Secondly, encapsulated fluorophores are studied. To prevent aggregation and energy loss by Dexter energy transfer (DET), encapsulation of the molecules is used to increase intermolecular spacing so that the short-range energy transfer process can be effectively suppressed. Furthermore, newly designed deep blue multiple resonance emitters are explored. Using this concept, highly efficient deep blue hyperfluorescent organic light-emitting diodes (OLEDs) with ultra-narrow emission are demonstrated with the study of exciton dynamics on the basis of transient analysis. Additionally, they are also applied in conventional fluorescent OLEDs. With effective Förster resonance energy transfer (FRET), efficient deep blue OLEDs showing ultra-narrow emission with 450 nm peak emission and CIEy ≤ 0.05 are demonstrated. In the second part of the thesis, organic radicals are explored for efficient NIR EL devices. Recently, doublet fluorescent emission from organic radicals has emerged as a new route to more efficient light-emitting devices than those using established non-radical organic emitters. Charge recombination in radical-based devices results in doublet excitons with nanosecond emission and avoids the efficiency limit usually associated with singlets and triplets. Firstly, a novel NIR organic radical emitter with an 820 nm emission peak is used for efficient NIR EL devices. To improve charge transport and balance in an emitting layer (EML), mixed-host systems are utilised to improve the efficiency roll-off and operational lifetime. Secondly, a novel energy transfer mechanism between exciton donors and radical acceptors is devised to optimise device performance in devices. One of the anthracene derivatives is selected as an exciton donor to activate effective intersystem dual energy transfer, which means that singlet excitons are transferred to doublet excited states via FRET, and triplet excitons are transferred to doublet excited states via DET. Utilising this approach, highly efficient NIR EL devices are demonstrated based on the study of transient EL and magneto EL, as well as transient photophysical measurements.