OLEDs without Iridium. 100% efficient triplet harvesting by Thermally Activated Delayed Fluorescence.

The energy agenda demands more efficient display and lighting technologies to meet the UK government's targets. Professors Monkman, Dias and Bryce propose a new paradigm for highly efficient organic LEDs (OLEDs) using only organic fluorescent emitters, rather than Ir based 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. Further, they mostly contain Ir as the core heavy metal, this is the fourth most scarce element on the planet, and so basing high volume mass produced lighting technology on such a scarce resource is highly risky. Our new devices will have as their emitters organic internal charge transfer (ICT) molecules. The excited states of ICT molecules have strong charge transfer character which can have vanishingly small electron exchange energies resulting in nearly equivalent singlet and triplet energies. This means that it is efficient for triplet CT states to (thermally) reverse intersystem crossing back to the singlet manifold, thereby giving a method of 'harvesting' up to 100% of triplet states formed by charge recombination in an OLED device, i.e. they can be as efficient as the best phosphorescent emitter. Thus ICT emitters can combine the most desirable properties of phosphorescent emitters, namely 100% triplet harvesting, with the added benefit of the long term stability of a fluorescent emitter. This is what both display and lighting manufactures demand but as yet do not have in the blue spectral region.

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. Our Initial studies have so far demonstrated that TADF molecules can harvest triplets with up to 100% efficiency, and also in the solid state, isolation of TADF molecules in a host affords ideal conditions for very efficient TADF emission. Thus these materials are perfectly suited for OLED applications. Furthermore, our collaborators in Japan have demonstrated simple monochrome TADF OLEDs having up to 85% internal quantum efficiency. TADF emitters are looking well set to provide a unique challenge to Ir phosphors.

During this project we shall undertake detail photophysical investigations of ICT molecules that show very efficient TADF in the solid state. We will develop models to understand fully this triplet harvesting method and structure-property relationships to optimise the design and synthesis of new families of efficient TADF emitters covering the whole visible spectrum. OLEDs will be fabricated from these new materials and fully characterized. Finally, device architectures that combine two or more TADF emitters will be evaluated to determine the best routes to producing white emission from TADF emitters in simple device structures.

The UK printed electronics community has identified OLEDs and organic solid state lighting as a key technology in which the UK holds a world leading position. Through the activities of CDT, Thorn Lighting and Durham University, via projects TOPLESS and TOPDRAWER, polymer solid state lighting has become a reality. Accordingly, £20.5M has been invested in new facilities at the CPI Printed Electronics Centre, £5M of which funded LACE, a pre-production manufacturing line for polymer OLEDs, OPV and solid state lighting on 6" tiles. This project will have great impact on this facility and OLED lighting research in the UK and Europe, including a new project, ENAB-SPOLED, an OLAE+ BRITE EURAM (TSB) project that brings together CDT, Durham, Tridonic, Zumtobal, Novaled and Fraunhofer IAP.

However, none of these devices are all phosphorescent; as yet there is no deep blue phosphorescent emitter available which gives the correct colour or adequate lifetime. The impact of moving from a fluorescent blue and green, phosphorescent red architecture (18-20 lm/W) to an all-phosphorescent architecture (40-50 lm/W) are obvious. LG Chem 80 lm/W lighting panels require a tandem OLED architecture with complex charge regeneration layers. Ir based blue phosphors degrade during deposition and Ir is the fourth rarest element on earth: current estimates of the global reserve base of Ir (estimated from zinc ore stocks) stands at 6000 tonnes or 13 years supply at current usage (infomine.com) and there is a major resistance to base a new global lighting technology on such a scarce commodity. Fluorescent emitters give far longer lifetimes, and TADF gives the near 100% efficiency of a phosphor. Thus it is vital that the proposed work is undertaken, to retain our world leading position in the UK. The uses of E-type TADF emission are not confined to organic solid state lighting: it will give exactly the same up-lift to the blue in RGB colour OLED displays, In all OLED technology Ir phosphors will need to be superseded.

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, solid-state lighting (SSL) could reduce the global amount of electricity used for lighting by 50%! SSL sources are also free of poisonous heavy metals, e.g. mercury, and as a consequence are much easier to dispose of - the OLED lamp can simply be placed in a glass furnace and recycled. These overall benefits which will arise from the transfer to solid-state lighting should have massive social impact around the globe. This project can help the UK achieve the Kyoto targets set for 2020. 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.

A successful outcome from this project will provide replacement high efficiency emitters for current Ir materials suitable for lighting applications. This will enable OLED solid-state lighting 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.