High triplet energy polymers for blue phosphorescent, solution-processable multilayer PLEDs to develop solid-state lighting

Leading on from the highly acclaimed TSB-funded Project TOPLESS, we will work towards developing a materials set of polymers, all of which posses high triplet energies, which will allow all-phosphorescent, multi-layer polymer light-emitting devices (PLEDs) to be made specifically for use in solid-state lighting applications. Project TOPLESS, lead by Thorn Lighting with CDT and Durham has demonstrated that all-solution-processed PLED panels giving high quality white light at 25 lm/W without out-coupling can be readily made. The key here was the use of a fluorescent blue emitter along with phosphorescent green and red emitters to generate a tri-white spectrum. Calculations show that to make a further step change in efficiency, towards 40 lm/W or more, devices must be made utilising all-phosphorescent emitters. This dictates radical changes to the materials used in the multi-layer device structure, such that all the layers (polymers) must have sufficiently high triplet energies to prevent quenching of the blue phosphor. Such polymers do not yet exist. In this research project we take the first steps towards the design and synthesis of high triplet hole-transport, electron-transport and ambipolar (emitter host) materials. Materials will be fully characterised using a range of spectroscopic techniques. Devices will be fabricated and tested, leading on to new generations of materials. Key building blocks have been identified as starting points for this work and several promising motifs have been patented with Thorn Lighting (via Project TOPLESS and through other work at Durham). During the project we shall not make the emitters themselves; a new family of blue phosphors has been developed in Durham, as part of the TOPLESS project and new phosphors will be explored in collaboration with Dr J. A. G. Williams in Durham. Here we focus on the materials for hosts and transport layers for blue phosphors. Along with the design, synthesis and characterisation of new materials, we will explore device architectures best suited to high efficiency blue and white emission, exploiting multi-layer fabrication techniques developed in Durham (also recently patented). Further, detailed analysis of triplet exciton migration within multilayer structures will be made using both novel spectroscopy (developed in Durham) and modelling developed in a collaboration with Kodak and continued in collaboration with Prof Chris Winscom at Brunel University. This will enable us to investigate ideas of triplet exciton confinement within an emitter layer such that transport layers do not act as quenching sites. This would then make the design and synthesis of transport layers much more simple. Critical decision points following the progress of this work will be made by the management team at the end of the first year of the project.We aim to fast-track new industrialisation of materials made during this project. This will be achieved by input from Thorn on the project management team and the fact that this project will run in parallel with the successor to Project TOPLESS, namely Project TOPDRAWER. Promising new materials sets can be rapidly feed into Project TOPDRAWER for printing trials and development of an all-phosphorescent white PLED panel. The development of host materials for blue phosphors is so vital to Thorn Lighting's plans to commercialise organic solid-state lighting that they will accelerate research in this area by giving the strongest possible support to this project. They have committed to support this project by contributing 150,000 to the cost of the project, reducing the EPSRC's funding by this amount.

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 polymers required to utilise a fully phosphorescent white PLED architecture. This will yield devices which can attain both an efficacy of >40 lm/W (without outcoupling) and a colour temperature of 4000K. This will enable polymer solid state lighting to better current fluorescent luminaires, enable PLED solid state lighting to move to commercialisation and start realise the major benefits highlited above. In so doing this project will have a major impact on the UK economy and environment.