The main research goal of the multi-disciplinary TADFORCE project is to explore CT and exciplex emitters and their application in OLED devices by training Experienced Researcher through joint research in chemical, physical and material science in both academia and industry.
The high demand for flexible OLEDs will increase the need for very expensive and rare iridium. The TADFORCE project aims to explore exciplex emitters and thermally activated delayed fluorescence (TADF) in OLEDs i) to replace currently used Ir complexes, and ii) to show how to easily tune emitter, resulting in reduced production cost, especially blue, where Ir based emitters fails.
The OLED worldwide market is growing rapidly and Europe needs experts possessing a comprehensive knowledge and practical experience in this technology. OLED technology is used in small devices such as smart phones and tablets but also in high-end TVs and lighting, as OLEDs are still relatively expensive compared to LCD. But with research progressing towards lower cost and longer lifetime, together with a growing trend to use flexible displays in smartphones etc., the OLED market is growing fast. Europe is a huge supplier of the materials for OLED displays, taking in to account that electronic market is changing very fast and companies are search for new cheaper materials the input of research in this area is needed. The flexible OLED display market is predicted to quadruple next year, with predicted global market revenue for flexible OLEDs to increase from $21.9 million in 2013 to $12 billion by 2020 and this project will help to maintain Europe place as a major supplier of OLED materials.
The engagement of industrial groups in TADFORCE consolidates both academic and industrial sectors in order to provide a high quality training programme. A goal of the work is to transfer new TADF materials and knowledge to the industry partners to help maintain Europe as a strong OLED materials leader.
Currently the OLED displays industry and R&D is based mainly in Asia (Samsung, LG etc.), in Europe there are many organic electronic materials companies (Novaled, Merck, Cynora etc). Samsung and LG are starting to invest a lot of money into TADF materials development but in Europe there is only Cynora and Merck at present and few Universities, lead by the University of Durham, trying to develop TADF technology. Thanks to this project, together we will have a chance to maintain a competitiveness with Asia.
The main research goal of the multi-disciplinary TADFORCE project is to explore charge transfer (CT) and exciplex emitters and their application in OLED devices by training Dr Przemyslaw Data through joint research in chemical, physical and material science in both academia and industry. Our research will focus on:
- Developing new knowledge on CT and exciplex TADF emitters and their application in OLED devices.
- Defining the role and influence of the host and dopant molecules for CT emitters or donor and acceptor molecules of the exciplex emitters in overall OLED device efficiency.
- Validating novel small molecules yielding efficient TADF.
- Applying TADF emissive layers in ultra-efficient OLED devices.
Dr Przemyslaw Data will gain experience in conducting research in a multidisciplinary environment to produce important data in this new OLED field to enable the development, modeling and tailoring of TADF OLED devices, which are at the forefront of new OLED research and development.
The scientific excellence of the TADFORCE project is based on three pillars:
- Development of TADF emitters based on molecular CT and exciplex states, definition of the role, and influence of the host-dopant (H-D) molecules for CT emitters or donor-acceptor (D-A) molecules of the exciplex, using state of the art spectroscopy and innovative chemistry.
- Development of novel H-D and D-A systems from knowledge gained in the project.
- Validation of novel small molecules and their emitters forms as well as the application of the novel emissive layers to ultra-efficient OLED devices.
In OLEDs, injection of both electrons and holes into an active material leads to recombine to form various excited states such as singlet excitons and triplet excitons. By this process singlet and triplet excitons are formed theoretically in the ratio one to three. With a fluorescent emitter, this electron–hole recombination creates 25% singlet excitons that decay radiatively producing light directly, and 75% non-radiative triplet excitons that are wasted. Thus, to make an efficient OLED, these triplets must in some way be converted into singlets. To gain light generation from a triplet exciton without using a phosphorescent dopant, the phenomena of delayed fluorescence can be used, either from the process of triplet fusion, yielding a maximum 62.5% device efficiency, or better by TADF attaining 100% efficiency. For TADF though, the excited singlet and triplet state must have very similar energies. In this case the triplet excited state can back transfer to the singlet excited state by thermal activation..The maximum efficiency can theoretically be 100% and dependents very strongly on the energy gap between the singlet and triplet states, i.e. the electron exchange energy. It is this ‘E-type’ delayed fluorescence process which nowadays is usually called ‘thermally activated delayed fluorescence’ (TADF), that shall be explored in this project
Current research into TADF, TADF materials and devices is only in its infancy. The state-of-the-art in terms of materials is led by the research group of Professor C Adachi, and photophysical characterisation and physical understanding of the TADF mechanism by Professor Monkman at UDUR. Exciplexes, comprising an electron-donating (D) and an electron-accepting (A) molecule, are characterised by a very small gap between the singlet and triplet state. This property of this excited state is what is required for efficient TADF. The exciplexes provide simple possibilities to make a wide range of emitters because the wavelength of the emission in such systems is not dependent on the band-gap value of a single compound, but the HOMO-LUMO offset between donor and acceptor molecules. A small range of exciplex TADF materials and devices has been reported, yielding very high external quantum yields1, reaching a leading value of 19% EQE2, clearly indicating that very efficient triplet harvesting is occurring and that 100% internal quantum efficiency is possible. These devices importantly have very simply stack structure, typically 3 to 5 organic layers, and without the need of a p-i-n structure3 the devices have ultra-low turn on voltages, of the order of 2.7 V. This is because charges are directly injected into the exciplex (LUMO of the acceptor, HOMO of the donor). Initial studies of the photophysics of these systems4 have shown the importance of the nπ* orbitals of heteroatoms, the critical role of the triplet levels in the system5 and that true energy gaps of the charge transfer states are very low < 50 meV6. Further, UDUR has shown through spectroscopic measurements that 100% triplet harvesting does occur within the charge transfer states. An unexpected result arising from exciplex devices is the observation that the emitting dipoles in these systems are partially aligned and that such anisotropic emission gives rise to better intrinsic light out-coupling, enabling more than the accepted intrinsic 21.5% out-coupling. Devices with external quantum yields > 29%, reaching up to 35% have been demonstrated7. Several other groups have verified Adachi’s initial results8.
So far, the TADF mechanism has been shown to be a promising way to harvest triplets, at near 100% efficiency without the use of Ir based complexes. However, many questions still remain, including proper design rules to yield emitters at specific wavelengths, how to control the emission bandwidth, the long term stability of TADF emitters and devices and most important, a highly efficient deep blue TADF system which as yet no materials fill. Further, recent work from the Durham group has shown that dopant host interactions and DD interactions readily occur in devices leading to complex emission and introduce inefficiencies. These need to be fully understood and new materials sets developed to yield fully controlled emission layers.
1 K. Goushi et al., Nat. Photonics, 2012, 6, 253; 2 K Gouchi et al, APL, 2012, 101, 023306; 3 GF He et al, J.Appl. Phys. 2004, 95, 5773; 4 F. B. Dias, et al., Adv. Mater. 2013, 25, 3707; 5 V. Jankus, et al Adv. Mat., vol. 25, pp. 1455-9, 2013; 6 D. Greaves et al, Adv. Func. Mat., DOI: 10.1002/adfm.201303389; 7 Jang-Joo Kim et al Adv. Func. Mat. 30, 0547, 2013; 8 Ping Chen et al, APL, 21013, 102, 063301