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

Periodic Reporting for period 1 - TADFlife (Using the smart matrix approach to enhance TADF-OLED efficiency and lifetime)

Reporting period: 2018-10-01 to 2020-09-30

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. We strongly believe that we can produce new device models that address this issue. 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. 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.
Systematic series of emitter and host materials have been synthesized, and further compounds from partners were received. Focus systems, e.g. of one particular emitter in 4 hosts, or of an emitter series in on host, have been identified and were distributed to the beneficaries. 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 developed. 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.
Why TADF OLED you may ask. After all, the efficiency-limiting spin-statistics issue is already solved using phosphorescent Ir complexes! However, in the blue, Ir complexes start to absorb light in a direct d-d* Ir transition which rapidly leads to degradation of the complex giving poor blue device lifetime. The only way around this is to find an alternative, 100% efficient blue emitting system. This will also circumvent the requirement of using expensive and scarce metals such as Ir or Pt. Today’s current blue fluorescent technology is limited to an EQE of 11.5%. Using a TADF emitter, devices surpassing 35% EQE have already been demonstrated. What is missing, however, is TADF device operational lifetime. Currently it seems you can have either high device efficiency or long device lifetime, but not both! Why? Nobody knows!
In TADFlife we will develop comprehensive device models to understand this. Moreover, we will use this knowledge to develop new optimised device architectures and material sets to yield both high EQE >30% and lifetime >10,000 hrs. We firmly believe that only by tackling all aspects of this problem together, in the holistic approach set out above, can this problem be solved.
TADFlife therefore consists of a team of internationally leading scientists with complementary skills who will address this question following two proposed parallel routes. These are complimentary yet at the same time highly interlinked, and tackle very different aspects of OLED properties which we believe are critical to understanding lifetime.

The research project underpinning TADFlife offers a truly new research idea which brings together academic and private partners with joint interests, to solve a demanding set of problems for TADF materials and devices. It gives a unique opportunity to educate talented researchers from across Europe in an area in which European Industry can be play a dominant role.

Our ESRs will meet the future needs of academia and industry in the rapidly emerging area of OLEDs. Working periods in different world leading research groups will add a broader perspective. We aim for them to become the OLED research leaders of the future in both academia and industry. This will render them a highly attractive workforce with excellent career perspectives in an area of great demand. In the short term, our ESRs will deliver tangible research results and have acquired substantial transferable skills. 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 fabrication
organic semiconductor device