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H2020

PHEBE Report Summary

Project ID: 641725
Funded under: H2020-EU.2.1.1.6.

Periodic Reporting for period 1 - PHEBE (New paradigms for high efficiency blue emitters for white OLEDS)

Reporting period: 2015-02-01 to 2016-01-31

Summary of the context and overall objectives of the project

The overall objective of the PHEBE project is to develop innovative, high-efficiency, blue emitters for white OLEDS, which will create a major breakthrough in the cost performance of OLED lighting. To produce the innovative blue emitters, two new types of molecular systems – without rare earth complexes - will be investigated:

• intramolecular charge transfer systems that enable thermally activated delayed fluorescence (ICT-TADF)
• intermolecular exciplex charge transfer systems that enable thermally activated delayed fluorescence (Exciplex-TADF)

In order to develop the ICT-TADF and Exciplex-TADF based emitters, the following scientific and technical objectives will be targeted:

• Objective 1: Screen potential ICT-TADF and Exciplex-TADF compounds with theoretical models
• Objective 2: Synthesise the most promising ICT-TADF and Exciplex-TADF model compounds
• Objective 3: Characterise and select the best ICT-TADF and Exciplex-TADF synthesised compounds
• Objective 4: Design white stack units employing the selected TADF based emitter and block materials
• Objective 5: Design close-to-production OLED lighting panel demonstrators

To show the project’s overall objective has been achieved, white stack tandem units (2 x 2 cm2 with 90 nm ITO) and OLED lighting panel demonstrators (e.g. 25 cm2 circular panels) - based on the new blue emitters – will be produced and tested that meet the performance targets indicated in the H2020 call ICT 29 – 2014. The PHEBE project will be undertaken by a strong consortium of partners that span the complete value chain for the development and commercialisation of the new, high-efficiency, blue emitters for white OLEDS: OLED lighting research organisations (UDUR, TUD and KTU), OLED component producer (Novaled), and OLED lighting device manufacturer (Astron-FIAMM). Overall, the PHEBE consortium is well-balanced in terms of the number of industrial and academic partners as well as their geographic spread.

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

The work performed and main results achieved by the PHEBE project from the start until the end of Period 1 have been as follows:

WP1 Modelling
• TUD has implemented a workflow model to screen possible TADF emitters.
• Novaled has designed and provided over 80 molecules for modelling.
• TUD has developed a tool for modelling molecular pairs and a scheme to evaluate CT processes.
• Deliverable D1.1 “Table with state-of-the-art TADF emitters and their properties collated” was submitted on schedule.
• Deliverable D1.2 “Report on initial model to predict TADF PLQY” was submitted on schedule.

WP2 Synthesis
• KTU has synthesised and characterised a number of exciplex and ICT compounds.

WP3 Characterisation
• UDUR has photophysically characterised a series of donor‐π bridge‐acceptor (D‐π‐A) molecules. Although the molecules showed strong ICT character, the delayed fluorescence was found to occur by a triplet‐triplet annihilation mechanism.
• TUD and UDUR have photophysically characterized the phenyl-carbazole, the trimer family, and a series of benzoate molecules. While the phenyl-carbazoles and the benzoates show significant TADF contribution, TADF is only weakly observed in the trimers.
• UDUR have identified three regimes for TADF based upon the spin-orbit charge transfer (SOCT) theory for reverse intersystem crossing (RISC).
• Deliverable D3.1 “Report on the effect of electric fields on TADF” was submitted on schedule.

WP4 Emitter Layer Design and White Stack Integration
• Novaled has tested a number of potential reference TADF emitters in OLEDs.
• Novaled optimised OLED architectures (PIN) on commercially available materials using the first batch of ICT-emitters synthesized by TUD (phenyl-carbazole family).

WP5 OLED Lighting Panel Demonstrators
• Deliverable D5.1 “Report on market study and customer specifications for white OLED stacks” was submitted on schedule.

WP6 Dissemination and Exploitation
• TUD has filed two patents and a third patent is expected to be filed early in Period 2.
• Deliverable D6.1 “Data management plan – initial version” was submitted on schedule.
• Deliverable D6.2 “Project website” was submitted on schedule.
• Deliverable D6.3 “Promotional material (Powerpoint presentation, short film, leaflet and poster)” has been submitted.
• Deliverable D6.5 “Exploitation and use plan – midterm version” was submitted on schedule.

WP7 Project Management
• Kick-off meeting successfully held at Novaled in Dresden during 3-4/2/15.
• Second consortium meeting successfully held at UDUR in Durham on 9/7/15.
• Deliverable 7.1 Management and Quality Plan was submitted on schedule.

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)

Progress beyond the state of the art
Two issues currently hold back organic light emitting diodes, ‘OLEDs’, from gaining a major fraction of the world lighting markets. The first is the necessary use of iridium containing phosphorescent emitters. Iridium (Ir) is the 4th rarest naturally-occurring element on the planet, so basing a large-scale high volume industry, such as lighting, on this resource is risky and highly detrimental to the environment. Ir is a highly unsustainable resource. Secondly, Ir based blue phosphor devices have short working lifetimes, far below industry expectation. Therefore, new types of emitters, particularly for blue, are required to replace the current iridium complexes and this is the ambitious goal for this project.

The EU desires OLED solid-state lighting to achieve T97 lifetimes (time taken to fall to 97% of original light output) of several hundred hours and so T90 lifetimes reaching several 1000 hours in the future. The limited operating lifetimes of current devices are dictated by the blue Ir complex emitters. For the most common blue (aqua) phosphorescent complex, FIrpic, vacuum deposition causes partial loss of fluorine substituents from the ligands, and partial de-complexation of the picolinate (pic) ancillary ligand. More critically, as the metal-to-ligand charge-transfer (MLCT) transition is pushed deeper into the blue, thermal excitation into the higher lying 3dd* metal orbitals can occur, causing quenching and degradation of the complex, casting doubt on the achievability of stable deep blue Ir phosphors. Currently the best blue phosphor, from BASF, has peak emission at 454 nm, and efficiency of 36 lm W-1, EQE 18.6%, but devices suffer from serious efficiency roll-off at high brightness and have poor lifetimes. Given these problems with blue phosphorescent emitters, and the scarcity of Ir, there is a real need for new efficient deep blue emitters for high efficiency lighting, with long lifetime.

This is not a trivial matter because of the way an OLED works. Electron–hole recombination in the organic emitter layer creates 25% singlet excitons that decay radiatively producing light directly, and 75% non-radiative triplet excitons. Thus, to make an efficient OLED, the triplets must in some way be converted into singlets. Heavy metal complexes have efficiently intermixed singlet and triplet states, via effective heavy atom spin orbit coupling, yielding near 100% light emission by phosphorescence. Without this ‘triplet harvesting’, a fluorescent OLED would be limited to a maximum 25% internal quantum efficiency. However, this is not the case in reality. The high triplet density generated within the thin emitter region gives rise to efficient triplet annihilation; one decay channel of which is triplet fusion (TF), producing singlet excitons from a pair of annihilating triplet excitons. This singlet state can then emit delayed fluorescence, which contributes to the total electroluminescence emitted by the OLED. From detailed studies of the TF process it has been shown that typically TF contributes around an extra 20% to the light output giving, in total, internal quantum efficiencies (IQE) of 40%, i.e. 25% direct singlet + 0.75 x 20% DF. Theoretically, with the correct energetics (within the triplet manifold) TF could have a maximum efficiency of 50% yielding device IQE of 62.5%. This is a big improvement over 25% but still well short of the near 100% IQE that Ir phosphors can give.

Recently, however, E-type delayed fluorescence, a neglected process in OLEDs, has been shown to work very efficiently in converting triplets into singlets in organic intramolecular charge transfer (ICT) molecules. Here, the triplet exciton is thermally activated and becomes iso-energetic with a singlet state allowing it to cross to the singlet manifold. The efficiency of the process is dictated by the magnitude of the energy barrier, which in most materials is ∆EST ≫ kT, and so such thermally activated delayed emission (TADF) is rare and usually weak. However, in charge-transfer molecules the singlet-triplet energy gap can be rather small, and in this case TADF can become very efficient: UDUR have measured 100% interconversion and it has been demonstrated that an ICT-TADF emitter can yield efficient OLEDs, >19% EQE. Thus, the TADF mechanism offers excellent prospects for new materials to replace Ir phosphors. As yet, little is known about the photophysics of TADF, especially in the solid state, and only monochromatic devices have been made. The challenges which drive this project are therefore: (i) to fully understand the TADF mechanism particularly in the solid state, so that new, highly efficient emitters can be designed from the red to the blue, with a major focus on the blue; (ii) synthesis of highly efficient TADF emitters to replace Ir complexes and iii) investigate multilayer TADF devices and new device architectures for white light emission.

Beens and Weller predicted that there should be a negligible gap between the singlet and triplet states in charge transfer systems, i.e. zero exchange energy, and this was first shown experimentally by Staerk. However, initial solution based studies at UDUR on a family of D-A-D molecules which have strong ICT clearly show that the thermally activated energy gap crossed in reality is as large as 0.4 eV in solution, whilst still achieving 100% efficient crossing. The above UDUR results indicate an internal energy ladder from a 3* state to a 3n* state to the charge transfer triplet state (3CT), giving intermediate energy steps to facilitate thermally activated population of the 3CT which is nearly isoenergetic with 1CT, giving efficient reverse ISC (RISC) to the 1CT singlet state. The results also show that in materials where the S-T gap is much larger than 0.4 eV, TADF and TF coexist, both producing DF. TADF is observed above a threshold temperature, even in materials where the singlet-triplet energy gap is as large as 0.8 eV. This strongly suggests that the first TF event may be triggering the thermally assisted step, but we need to further clarify this. In the solid state, initial UDUR results show very different behaviour; in high triplet hosts TF can be effectively switched off and only TADF occurs on the ICT dopant. Here the emission yield increases from 20% to 50%, and the emission intensity is constant for over the first 400 ns. Such behaviour has never been seen before and is ascribed to be a consequence of energy exchange back and forth between the isoenergetic singlet and triplet ICT states (energy difference < 25 meV). UDUR also have evidence that the CT states lie below the 3ππ* at least at low temperature. Also for the first time both CT emission and phosphorescence are observed at different delay times in the same sample at low temperature. In this case it becomes clear how the PLQY increases so dramatically as DF accounts for 2/3 of emission at room temperature and how TADF OLEDs can be so efficient. However, we have, so far, merely scratched the surface on the photophysics, there is a vast amount still to clarify and learn.

In recent work addressing TADF emitters suitable for highly efficient OLEDs, time-dependent density functional theory (TD-DFT) calculations of transition energies have been performed routinely with standard hybrid functionals like B3LYP, ignoring possible problems when applying this scheme to CT transitions. On the other hand, more suitable functionals for quantifying CT transitions have emerged from fundamental studies comparing TD-DFT with Hartree-Fock-based computational approaches. However, till now the most suitable range-separated hybrid functionals like CAM-B3LYP have only been applied occasionally to CT emitters. TUD shall identify and apply the most suitable theoretical tools to technologically relevant classes of blue emitters, going beyond previous applications of standard DFT methods to CT emitters. Advanced methods like RI-CC2 have not been applied to the technologically most interesting CT emitter systems, but benchmark calculations applying RI-CC2 to pairs of perylene chromphores demonstrate that this computationally demanding scheme can be applied to systems in the size range of interest.

In terms of devices utilising TADF emitters, several groups have tentatively reported external quantum efficiencies reaching >20%, indicative of 100% triplet harvesting in line with UDUR’s photophysical results. UDUR’s own materials have reached a verified 15% EQE. Moreover, charge injection occurs directly into the charge transfer states in the emission layers, leading to device turn on voltages as low as 2.5 V, meaning extremely high power efficiencies can be attained, a prerequisite for reaching > 100lm/W devices. TADF based emitters can be created with three simple layers, meaning their overall device construction is much easier, giving far higher fabrication yields, cheaper devices (allowing us to reach the target of 1€/100 lm), and longer lifetimes. The TADF emission mechanism is ideal for OLED lighting applications, and when the PHEBE consortium become the first to integrate it into an OLED, Europe will gain a major technological lead.

Finally, TADF molecules are intrinsically shape anisotropic. This gives rise to orientation of the emitter molecules parallel to the electrode plane in a device so that more light is propagated away from wave-guiding modes. This allows more light to escape from the device without recourse to external out-coupling measures, up to a theoretical 40% external quantum efficiency. Combined with state of the art out-coupling techniques, will open the possibility to better than 60% EQE in total, a step change in efficient light extraction.

Expected potential impacts
Expected impacts set out in the H2020 ICT 2014-2015 work programme:
The PHEBE project will directly help to create the expected impacts of call ICT 29 – 2014 Development of Novel Materials and Systems for OLED lighting. Below, we systematically step through each of the expected impacts from the work programme (indicated in italics), explaining how they will be achieved by the PHEBE project.

“Cost performance breakthroughs - lighting systems with production costs of 1€/100 lm.”
The PHEBE project’s focus on TADF emitter development and outcoupling improvements will help to achieve this cost performance impact on OLED lighting device production.

This belief is based on the recent tentative findings of several research groups working with early stage TADF emitter devices. They have reported external quantum efficiencies reaching >20%, indicative of 100% triplet harvesting, and in line with UDUR’s own photophysical results. UDUR’s own materials have reached a verified 15% EQE. Moreover, charge injection occurs directly into the charge transfer states in the emission layers, leading to TADF devices with turn on voltages as low as 2.5 V, meaning extremely high power efficiencies can be attained, a prerequisite for reaching > 100 lm/W devices.

TADF based emitters can be created with three simple layers, meaning their overall device construction is much easier, giving far higher fabrication yields, cheaper devices, and longer lifetimes.

Furthermore, TADF molecules are intrinsically shape anisotropic. This gives rise to orientation of the emitter molecules parallel to the electrode plane in a device so that more light is propagated away from wave-guiding modes. This allows more light to escape from the device without recourse to external out-coupling measures, up to a theoretical 40% external quantum efficiency. Combined with state of the art out-coupling techniques, this opens the possibility to better than 60% EQE in total, a step change in efficient light extraction.

Concerning state of the art out-coupling techniques, the PHEBE project will investigate methods compatible with low-cost, high-volume manufacturing. Methods considered will be direct light extraction, macro-extractors, or a combination of both. The former method will improve power efficacy by freeing up trapped internal wave-guided modes (organic, ITO, plasmon modes). Then macroextractors could be used to extract these modes from a substrate into air.

Outcoupling is also heavily dependent on device architecture. In this respect, Novaled will examine use of their advanced PIN technology to enhance outcoupling. The PIN’s very thick electron transport layers can be used to shift the emission zone as needed, without a trade-off in high operating voltage.

“Secured and reinforced industrial technology leadership and substantially increased market presence in lighting.”
The PHEBE project will help to create this impact, because it directly addresses the two key issues currently holding back the OLED lighting industry from gaining a major share of the world lighting markets.

Firstly, the TADF based emitters that will be developed in the project do not use iridium (Ir), as is the case for currently available phosphorescent OLED emitters. This metal is amongst the rarest naturally-occurring elements on the planet, so basing a large-scale high volume industry, such as lighting, on this resource is risky and highly detrimental to the environment.

Secondly, current blue phosphorescent OLED devices have short working lifetimes due to their blue Ir complexes. The EU industry desires OLED solid-state lighting to achieve T97 lifetimes (time taken to fall to 97% of original light output) of several hundred hours and so T90 lifetimes reaching several 1000 hours in the future. For the most common blue (aqua) phosphorescent complex, FIrpic, vacuum deposition causes partial loss of fluorine substituents from the ligands, and partial de-complexation of the picolinate (pic) ancillary ligand. More critically, as the metal-to-ligand charge-transfer (MLCT) transition is pushed deeper into the blue, thermal excitation into the higher lying 3dd* metal orbitals can occur, causing quenching and degradation of the complex, casting doubt on the achievability of stable deep blue Ir phosphors. Currently the best blue phosphor, from BASF, has peak emission at 454 nm, and efficiency of 36 lm W-1, EQE 18.6%, but devices suffer from serious efficiency roll-off at high brightness and have poor lifetimes. Given these problems with blue phosphorescent emitters, and the scarcity of Ir, there is a real need for new efficient deep blue emitters for high efficiency lighting, with long lifetime.

Again, with its focus on non-iridium TADF based emitters, the PHEBE project tackles this issue head on.

“Improved business opportunities and value creation in Europe in lighting by reinforced cooperation along the value chain.”
The PHEBE project will help to create this impact thanks to its strong consortium of partners that span the complete value chain for the development and commercialisation of the new, high-efficiency, blue emitters for white OLEDS: OLED lighting research organisations (UDUR, TUD and KTU), OLED component producer (Novaled), and OLED lighting device manufacturer (Astron-FIAMM). Overall, the PHEBE consortium is well-balanced in terms of the number of industrial and academic partners as well as their geographic spread.

Other environmental and socially important impacts:
The PHEBE project will help contribute to the development of energy efficient lighting and so create a number of social benefits. These include improved visual comfort and light quality, improved safety, reduced light pollution and improved health. There are clear and significant environmental benefits from adopting energy efficient lighting technologies. According to Photonics21, the European technology platform for photonics, it is estimated that 40 - 70% energy saving can be achieved using energy efficient lighting rather than current technologies. This would lead to a saving of over 640 million tonnes of CO2 per annum.

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Related information

Record Number: 186606 / Last updated on: 2016-07-14
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