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PHEBE Report Summary

Project ID: 641725
Funded under: H2020-EU.

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 act

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