New Generation of Organic Light Emitting Diodes (OLED's)

Organic light emitting diodes, OLEDs, are now the must-have displays in mobile phones, televisions and medical displays. OLED has introduced such innovations as formable, ‘wrap round’ phone displays, fully flexible displays, ultrahigh resolution 4K, expanded colour gamut and high dynamic range displays. So compelling are they as the premium display, that Apple have finally succumbed and the iPhone X is now their first OLED phone. Indeed, every new generation of OLED TV has been acclaimed as the best TV screen ever by many independent reviewers. OLED displays are beginning to appear in laptop computers, and automotive lighting is slowly taking off.

According to IDTechEx, the plastic and flexible AMOLED display market will grow to $16bn by 2020 and the OLED lighting market will reach $1.9bn by 2025. The demand is fuelled by the key advantages that OLED technology offers over traditional display and lighting technologies: low power consumption; thin, flexible, and transparent panels; wider viewing angles. According to Global Industry Analysts, the global market for microdisplays is projected to reach US$2.9 billion by 2020, driven by the technology’s use in a wide range of applications including virtual reality and AR devices.
But, all these displays are still based on fluorescence, not phosphorescence blue emitters, which are 75% less efficient. The simple fact is, that after more than 10 years of development, no long lifetime, stable blue phosphorescent emitter has been demonstrated to meet commercial performance targets. The cost of using metals such as Ir and Pt in emitters is also still a major concern, severely limiting the impact of OLED lighting.

However, a new generation of OLED emitter has emerged, based on thermally activated delayed fluorescence (TADF). Like phosphorescence, TADF is a 100% efficient mechanism for converting triplet states into emissive singlet excited state, thereby enabling 100% internal quantum efficiency. Recent rapid advances have shown that deep blue emission with external quantum efficiency (EQE) above 22% is possible, and coupled with enhanced out-coupling through self-orientation of emitter molecules, combined with low refractive index transport layers can boost EQE above 40%. This creates a step change for OLEDs. Displays could be manufactured with just blue pixels, with red and green generated by down converting phosphors. This would greatly simplify panel fabrication, increasing panel yield and driving down cost. TADF emitters do not consume scarce precious metal resources. Thus, if Europe can dominate the blue TADF materials sector, it can retain an extremely strong position in the displays, lighting and all OLED applications industries regardless of where panels are manufactured.

At present, the lifetime of TADF devices do not reach commercial targets. We strongly believe that we can produce new device models that address this issue through a dedicated and well-planned program that studies both the materials aspects of TADF (focusing on emitter-host interactions, molecular orientation and its effects on TADF as well as out-coupling, charge transport and trapping mechanisms in TADF emitter host systems) in combination with detailed device measurement (seeking materials and device architecture correlations on device efficiency and lifetime). From this, we can predict the factors affecting device lifetime and degradation so that new high purity materials set can be synthesised to overcome them. For this, we propose a comprehensive approach, embracing synthesis with spectroscopy guided by quantum chemistry, out-coupling analysis, electrical and trap characterisation, detailed device physics and simulation. From this holistic approach, and introducing the concept of the ‘smart matrix’ to OLED devices, we will maximise TADF device lifetime without compromising device performance. We firmly believe that only through this all-embracing approach can we succeed in our goals. Because this requires a multitude of skill sets and disciplines to achieve, it is the ideal project for a MCSA-ITN, where the cohort of ESRs working closely together can solve these problems whilst at the same time learning critical skills, techniques and training, based around this common, industrially relevant scientific question. They will earn their PhD’s whilst being immersed in a large scale scientific multidisciplinary project. In this way, we ensure that the ESRs are exposed to cutting edge research and learn to place their own projects and skills in the context of a large multifaceted project. Ultimately, this is preparing them very well for scientific life after their PhD studies in industry or academia.

Objectives: We aim to maximize TADF OLED efficiency and, critically, lifetime simultaneously. This will be achieved using the broad methodologies outline below. Addressing such a multifaceted and complex issue, that is of critical importance to the development of the OLED industry, requires a cross-sectorial, multidisciplinary approach that is beyond the capability of a single institution. The objectives of project TADFlife are therefore

  1. to combine the leading European academic and industrial expertise in synthesis, quantum chemistry, spectroscopy, photophysics, device physics, simulation and technology to elucidate the simple, yet profound question of how to make efficient and long-lived TADF OLEDs, especially with deep blue emission. We will collectively develop models to identify the causes of poor lifetimes and from this eradicate these causes.
  2. to meet the demand for a new generation of highly mobile cross-disciplinary chemists, physicists and materials scientists for growing industries, qualified in the area of OLED innovation, research and development. They will have a distinctive training experience combining both academic and industrial work experience in Europe with unique experiences gained by secondments in leading East Asian and American laboratories. Their accumulated training can be transferred to other classes of functional materials and electronic or photonic devices, thus contributing to creating a much needed broad and versatile European work force.