Using the smart matrix approach to enhance TADF-OLED efficiency and lifetime

Project Information

TADFlife

Grant agreement ID: 812872

DOI 10.3030/812872

Project closed

EC signature date 21 August 2018

Start date 1 October 2018

End date 31 December 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost € 3 942 036,72

EU contribution € 3 942 036,72

Coordinated by

UNIVERSITAT BAYREUTH
Germany

Periodic Reporting for period 2 - TADFlife (Using the smart matrix approach to enhance TADF-OLED efficiency and lifetime)

Reporting period: 2020-10-01 to 2022-12-31

Summary of the context and overall objectives of the project

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. According to IDTechEx4, the plastic and flexible AMOLED display market will grow to $16bn by 2020 and the OLED lighting market will reach $1.9bn by 2025. 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. To adress this, we used 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 aimed to maximise TADF device lifetime without compromising device performance. 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. Being provided with unique scientific and professional skills, the highly trained TADFlife ESRs will make a direct contribution to the European players. This is an important economical factor as Europe is in direct competition with East Asia in the highly competitive field of organic electronics.

The objectives of project TADFlife are therefore
-to combine the leading European academic and industrial expertise in synthesis, quantum chemistry, spectrosco-py, 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.
-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 con-tributing to creating a much needed broad and versatile European work force.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Systematic series of emitter and host materials have been synthesized and were investigated regarding their optoelectronic properties and their performance in device architectures. Time-resolved, temperature dependent spectroscopic and device measurements were carried out on these to determine photoluminescence and electroluminescence quantum yields as a function of emitter concentration in the host, to determine singlet-triplet gaps, the rates for forward and reverse intersystem crossing, the emitter orientations and to obtain information of charge trapping in neat and blend films. The information is exchanged between all partners using a central file depository. Further, the computer-based methodology was imporoved. We carried out calculations on dipolar and quadrupolar dyes with a view to establish the most suited TD-DFT functional and to investigate environmental effects. Monte-Carlo codes were written to study the role of host-guest transfer on triplet-triplet annihilation (TTA) and delayed fluorescence (DF), and its dependence of on chromophore orientation
We aimed at establishing how the choice of emitter and host, their interaction and orientation dictate TADF efficiencies. The current progress pertains mainly to the selection and characterization of suitable focus systems for the consortium. This will allow future work to focus on addressing how current flow across interfaces impact on device performance.

Overall, we were able to produce new host materials that were improving the OLED lifetime by a factor of two over the current benchmark hosts, and the use of hyperfluorescent emitters provides a viable avenue in the blue spectral range. Surface orientation polarization was identified as a majore contributor to device degradation and means to circumvene this are shown. Thermally stimulated luminescence measurements annd CELIV measurements were identified as most helpful in assessing degradation processes. Modelling methods, both Drift-Diffusion and kinetic Monte Carlo, were advanced such as to assist the identification of the most dominant factors causing device degratdation.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We were able to improve device lifetime by a factor of two over the current state of the art and to identify major processes leading to degradation. This technological advance will contribute significantly to the further development and expansion of the OLED market, allowing for more demanding consumer-oriented applications.
The training of our ESRs, with secondments for each of them despite the COVID pandemic, will render them a highly attractive workforce with excellent career perspectives in an area of great demand.
In the mid- to long-term, the research goals of the TADFlife project will have a major impact on the OLED industry enabling Europe to maintain a dominant position in the OLED materials supply chain and retain critical research strength and infrastructure to support those Industries.

TADFlife start-up meeting

OLED