rISC - the game of strategic molecular design for high efficiency OLEDs

Lighting and displays form essential parts of our daily lives and consume approximately 20% of the electricity used worldwide. It has been shown that lights visible from space at night time are an effective measure of a country's economic development, while energy capture through the development of materials that absorb more sunlight is at the heart of attempts to reduce our reliance upon unsustainable fuel sources.

In the context of generating light, Organic Light Emitting Diodes (OLEDs) are a rapidly expanding technology which are highly appealing owing to their potential high energy efficiency and ability to be printed roll-to-roll on a plastic substrate. However, in terms of efficiency, initial attempts to implement OLEDs based upon purely organic materials were restricted by the type of excited state which emits the light. Indeed, upon electrical excitation 25% of the emitting molecules are in emissive singlet excited state, whilst 75% are in non-emissive triplet excited states. However, conventional organic materials cannot emit from the triplet excited states.

The proposal consists of developing a new class of OLEDs that exploits thermally activated delayed fluorescence (TADF). Here a triplet state is thermally activated to become iso-energetic with a singlet excited state, enabling efficient rISC into a radiative singlet manifold. One of the major challenges that must be overcome in TADF materials is that the use of pure-organic materials means that the often weak spin-orbit coupling between the singlet and triplet states can lead to rISC rates that extend into the millisecond range. This causes poor roll-off in device efficiency at higher current densities. However, through a new conceptual design of TADF molecules, we have shown that it is possible to achieve both a reverse intersystem crossing (rISC) rate > 1E7 s-1 and a unity photoluminescence quantum yield (PLQY), a combination previously considered untenable.

In this proposal we will combine detailed synthetic, computational and photophysical studies to develop approaches for exploiting steric hindrance and non-covalent interactions to exert finer conformational control of the excited state dynamics to enhance functional properties. This is performed in conjunction with detailed studies to establish a deeper understanding of the excited states and their geometries formed by charge recombination. This proposal will deliver new understanding about the emission processes in TADF OLEDs and how to enhance the rISC rate to beyond 1E8 s-1 whilst retaining PLQY ~ 1. Achieving these outcomes will have a major disruptive impact on the OLED industry and ensure that the UK remains in the lucrative OLED materials supply chain.

This research proposal seeks to address a number of important questions aimed at achieving a deeper understanding and control of molecules that exploit thermally activated delayed fluorescence (TADF) for applications within 3rd generation organic light emitting diodes (OLEDs). The core research objectives contain strong elements of knowledge generation that are both closely related to fundamental academic research and, importantly, which are of direct relevance to applied practical questions for realising significantly improved devices. Consequently, it is anticipated that the successful execution of this proposal will have implications over a broad field of research with, on the longer term, the potential for improved and cheaper devices. Our new materials offer a step-change for OLEDs in that they yield highly efficient devices, >30% EQE, greatly enhanced stability to roll-off at high brightness levels and thus far better resistance to photochemical degradation. In this grant, by establishing new routes to controlling the triplet harvesting pathways in these materials, we will further increase the efficiency of these devices. This will then impact very strongly on the OLED industry as stable, high efficiency materials are still demanded. Moreover, it will allow full colour displays to be fabricated in a far simpler way, greatly increasing manufacturing yield and reducing cost.

Our new device results and materials will be very attractive to the high-growth and high-volume markets for OLED displays, as well as OLED lighting. According to IDTechEx, the plastic and flexible active-matrix OLED (AMOLED) display market will grow to $16bn by 2020 and the OLED lighting market will reach $1.9bn by 2025. The demand is fuelled by the key advantages that OLED technology offers over traditional display and lighting technologies: OLEDs consume less power; OLED technology enables thin, flexible, and transparent panels; OLED displays have wider viewing angles. These advantages will be further strengthened by our new materials. This is not only relevant to televisions and large-size screens. According to Global Industry Analysts, the global market for microdisplays is projected to reach $2.9bn by 2020, driven by the technology's use in a wide range of applications, including virtual reality. In particular, OLED microdisplays are expected to experience strong demand in the years ahead, as they are lighter, brighter, more efficient, and less power-consuming than other microdisplay technologies. By demonstrating long-lived high-efficiency blue OLEDs, enabling a step-change in display architecture and simplified manufacturing, these markets can grow more rapidly, and OLED TV can displace LCD at the mid-price bracket, as well as dominating the high end. This major impact will strongly reflect the UK's leading role in materials development. In addition, achieving this has the potential for a broad impact to the general public, leading to a significant economical and societal impact.

The impact on people will, in the short term, arise from the training and career development of the researchers involved in this multidisciplinary proposal. This funding will provide the research team the opportunities to develop exciting collaborations with academic and industrial partners. The postdoctoral researchers funded through this proposal will work in close collaboration with existing groups at Glasgow, Durham and Newcastle. They will have the opportunity to supervise projects run by Masters students and also contribute to the supervision of the postgraduate researchers, providing opportunities to develop academic management skills. The postdoctoral researchers will be encouraged to attend training courses to enhance their multidisciplinary skills. Specific training and career development requirements will be identified at the start of employment in a meeting with the PI.