Enhanced Fluorescent OLEDs, through Triplet Fusion

This project is clearly focused on understanding one of the most fundamental aspects of organic light emitting devices, which as yet is little recognised as being so important. With CDT we have shown that up to 37% of light coming from an OLED device arises from triplet fusion, in itself an absolutely fundamental excitonic process which if not at all well understood. By elucidating the triplet fusion mechanism and demonstrating that 0.50 singlet yield is possible with correct triplet energy levels we aim to build devices that could attain internal quantum efficiencies of 0.62 from fluorescent emitters. This would provide a real step change in OLED performances both for displays and lighting. Together with CDT we shall work to develop a new device architecture to fully exploit this possibility. This will be a highly adventurous piece of applied science. We will also collaborate with Kyushu University on small molecule systems in evaporated devices.

A range of state of the art spectroscopic techniques will be used, most developed in and unique to Durham, to fully elucidate the photophysical process underpinning triplet triplet annihilation and triplet fusion, some of the most fundamental excitonic processes in organic materials but still poorly understood. We will determine how singlet yield from triplet fusion is controlled by energy levels of the molecule. Further, we shall also determine the efficiency of upper excited state triplet inter system crossing to the singlet manifold, a further process which we believe also contributes to the overall singlet yield both in an optical experiment and in an OLED device.

Together with CDT will will develop a novel device based on a polymer dyad where triplet fusion is maximised and aim to produce fluorescent OLEDs with efficiencies approaching those of phosphorescent devices. In this way we can produce blue fluorescent devices of high efficiency without the short lifetime associated with phosphorescent systems. This would be a major step change for OLED device.

This project has the full support of Cambridge Display Technology who will provide £45,000 to off set the EPSRC cost of the project and another £30,000 in new materials developed specifically for this project and further resource for modelling and project management.

The current global lighting market (commercial and domestic) is $80 Billion and based on historical evidence is expected to continue growing at approximately 3% per annum. With rising energy costs and political drivers to reduce CO2 production, organisations and consumers are becoming increasingly interested in more energy efficient lighting solutions. For example, lighting in the NHS accounts for ~37% of the total electricity bill and globally 20% of all electrical energy produced is consumed for artifical lighting. The introduction of high effiency organic solid-state lighting products would have a dramatic impact on energy consumption. The current road map is to demonstrate the manufacturability of low voltage PLED devices suitable for the eventual replacement (2013) of fluorescent lighting systems, and when linked to central energy storage systems and renewable power sources, offers potential for non-metered lighting systems, i.e. your lighitng is free after installation. This will ensure the UK can produce the lighting technology for tomorrow and will allow significant progress towards the Kyoto targets set for 2020.

The US DoE has reported that the potential benefits from developing solid-state lighting (SSL) are: (i) by 2025 US national electricity consumption for lighting could be reduced by more than 300 TWh, which corresponds to 8% of electricity production in 2002; (ii) the cumulative savings on US consumer electricity bills could be >$125B between 2005 and 2025; (iii) the building of more than forty 1000 MW power stations could be deferred, contributing to a cleaner environment and more reliable grid operation. If an efficiency of 120 lm/W is achieved (predicted for ca. 2015) then about 30% of the electrical energy used for general lighting purposes in Europe could be saved. That translates into savings of 40 GW electrical peak power supply or an equivalent of 50 Million tons of CO2 per year. By 2025, SSL could reduce the global amount of electricity used for lighting by 50%! SSL light sources are also free of poisonous heavy metals, e.g. mercury, and as a consequence are much easier to dispose of, i.e. the PLED lamp can simply be thrown back into the glass furnace and recycled. These overall benefits accrued from the transfer to SSL would have massive social impact in all major Western countries. This project will help to maintain the UK at the forefront of this positive social and environmental revolution. There will be significant social impact on employment in the construction, electrical installation and lighting manufacturing industries as building and interior designs adapt to the new lighting technology and existing businesses are replaced by new ones exploiting the advantages of OLED lighting. Further benefit to UK PLC will come from future interaction with the vibrant UK design sector. The myriad possibilities opened up by a combination of high value design concept lighting and flexible lighting manufacture can only be of benefit to the economy. Thus in the mid- to long-term this project, if successful, will be seen to make a major impact on the achievement of these goals.

A succesful outcome from this project will provide the fluorescent polymers and device architectures that will effectively use triplet excitons to produce light at high efficiency without recourse to phosphorescent emitters. This will yield devices which can attain both an efficacy of >40 lm/W (without outcoupling) and a colour temperature of 4000K with simple device structures which cannot be made using phosphorescent blue emiters, moreover at present there are no suitable, stable blue phosphorescent emitters or high triplet energy host for them. This project will provide a viable alternative route for efficent blue light production, open new avenues for the production of high efficiency all fluorecence OLEDs and facilitate new possibilities to use triplet fusion in many spin out application areas.