The Energy Agenda: Exciplex blend small-molecule OLEDs; high performance fluorescent devices from E-type triplet harvesting

The energy agenda demands more efficient display and lighting technologies to meet the UK government's targets. Professors Monkman and Bryce propose a new paradigm for highly efficient organic LEDs (OLEDs) using only organic fluorescent emitters, rather than organometallic phosphorescent emitters that are currently very fashionable. Phosphorescent emitters have major limitations. Blue-emitting phosphorescent complexes especially suffer from poor stability and short lifetimes, and they have not yet produced the deep-blue emission required for both display and solid-state lighting applications. Our new devices will have as their emitters fully blended organic molecules which form exciplexes. An exciplex is a special type of electronically excited state that can occur at the interface of electron donating and electron accepting molecules. Exciplexes can emit light with high efficiency if the donor part also has high photoluminescence quantum yield. This idea of intentionally generating exciplexes is very different from accidental 'mixed' exciplex emission layers or exciplex contributions from interfacial states between transport and emission layers. The reason for doing this research is two-fold: firstly, exciplexes can have vanishingly small electron exchange energies resulting in nearly equivalent singlet and triplet energies. This means that it is efficient for triplet exciplexes to (thermally) reverse intersystem crossing back to the singlet manifold, thereby giving a method of 'harvesting' up to 100% of triplet exciplex states formed from charge recombination, i.e. they can be as efficient as a phosphorescent emitter. Secondly, using a very simple device structure comprising a hole transport layer (the donor), a blend donor-acceptor emission layer, and an electron transport layer (the acceptor) one achieves direct injection of charges into the exciplex which gives very low turn-on voltages (ca. 2.5 V) which also gives very high power efficiencies (lm/W). These features are very important for both mobile, battery-operated display devices and for lighting applications. Thus, exciplex based devices offer a step change in OLED design.

It is particularly important to replace phosphorescent blue emitters, as they chemically degrade during the vacuum deposition process used in device fabrication; they have short working lifetimes and do not emit deep-blue light which is essential for both high quality displays and lighting. We have demonstrated an initial deep blue exciplex system which is very promising: the device emits at 450 nm and has an efficiency of 2.7% at 2.8 V, using 'off-the-shelf' materials. There is considerable scope for improved design of materials. If the photoluminescence yield of the donor could be increased from 30% to 90% we could expect the exciplex emission to increase 4-fold in efficiency: this would yield a 10% efficient deep-blue device with a power efficiency approaching 30 lm/W. Improving the charge mobility of the donor should increase the efficiency still further. Colour tuning is also relatively simple as chemically modifying the donor or acceptor components will alter the molecular energy levels which dictate the exciplex energy.

The very simple exciplex device structure is perfect for industrial application as reducing the numbers of layers in a device greatly improves manufacturing yield and lowers the cost. During the project we will concentrate on monochrome devices with the goal of producing alternatives to red, green and blue phosphorescent systems, and then move to fabricate initial white-emitting devices to trial further new ideas to produce very simple white device structures.

New exciplex devices could provide a real alternative to phosphorescent systems and bring with them higher power efficiencies, longer lifetimes and far simpler device architectures. Doing this research in the UK will maintain our world capability and strength in OLED and OLED-lighting research and development.

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 artificial lighting. The introduction of high effiency organic solid-state lighting (OSSL) 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 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 lighting 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 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 buildings 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 new deep blue emitters for both display and lighting applications. This will yield devices which can attain both an efficacy of >60 lm/W and colour temperatures >4000K using stable emitters with long lifetimes, replacing the current poorly performing blue/green phosphorescent emitters. This will enable OLED solid-state lighting to surpass current fluorescent luminaires, to move to commercialisation and to start to realise the major benefits highlighted above. In so doing this project will have a major impact on the UK economy and environment.