Towards organic materials exhibiting thermally activated delayed fluorescence and circularly polarised luminescence

Abstract

Organic light emitting diodes (OLEDs) have come to the fore as the stateof-the-art technology for displays and lighting. Indeed, they are now standard for most recent smartphone designs. However, despite their success, advancements are still required to improve energy efficiency and sustainability, especially with the onset of 5G which has increased power consumption. This thesis studied two mechanisms to improve OLED efficiency. The first seeks to improve the internal quantum efficiency (IQE) of the emissive materials used within the OLED device by making non-emissive triplet states emit light. Statistically, only 25% of excitons formed by electrical excitation within the emissive material are singlets and can radiatively relax via fluorescence. The remaining 75% are triplet excitons which do not emit in conventional fluorescent materials and therefore, their energy is lost to heat. Recently, thermally activated delayed fluorescence (TADF) has emerged as a powerful approach to harvest these non-emissive triplet states. Importantly, previous work has demonstrated the importance of specific vibrational degrees of freedom to optimise the TADF process. This work examines how rotaxanes can be used to control vibrational modes of TADF molecules and therefore, fine-tune their properties and thus more efficiently re-engage the ’lost’ triplet states. In many of cases in organic electronics, due to the size of the molecules of interest, the excited state properties are described using time-dependent density functional theory (TDDFT) which provides the required balance between computational efficiency and accuracy. However, TADF materials display charge transfer (CT) characteristics, a property which is poorly described by standard approximations of the exchange correlation functional in TDDFT. Herein, we apply two tuning approaches to improve the description of excited states within TDDFT, with a particular focus upon the accuracy of excited state energies and chiroptical response. The latter being connected to the second way of improving the OLED efficiency. Indeed, OLED display technologies require anti-glare filters to improve viewing contrast when routinely used under conditions of high ambient light. Anti-glare filters use a double layer of linear polarisers and quarter wave plates to ensure that any ambient light which enters the device is not reflected off the cathode, back towards the viewer. However, this also absorbs ∼50% of the light generated by the OLED, meaning higher driving voltages are required to achieve the set brightness levels. This increases power consumption and decreases device operation lifetimes. Circularly polarised luminescence (CPL) is not absorbed by these filters, thus the integration of CPL could increase the energy efficiency of an OLED display. This work explores two mechanisms for generating CPL, firstly using small molecules and secondly with large polymer systems, which have been shown to exhibit large dissymmetry