OLEDs without Iridium. 100% efficient triplet harvesting by Thermally Activated Delayed Fluorescence.
Lead Research Organisation:
Durham University
Department Name: Physics
Abstract
The energy agenda demands more efficient display and lighting technologies to meet the UK government's targets. Professors Monkman, Dias and Bryce propose a new paradigm for highly efficient organic LEDs (OLEDs) using only organic fluorescent emitters, rather than Ir based organometallic phosphorescent emitters that are currently very fashionable. Phosphorescent emitters have major limitations. Blue-emitting phosphorescent complexes especially suffer from poor stability and short lifetimes, and they have not yet produced the deep-blue emission required for both display and solid-state lighting applications. Further, they mostly contain Ir as the core heavy metal, this is the fourth most scarce element on the planet, and so basing high volume mass produced lighting technology on such a scarce resource is highly risky. Our new devices will have as their emitters organic internal charge transfer (ICT) molecules. The excited states of ICT molecules have strong charge transfer character which can have vanishingly small electron exchange energies resulting in nearly equivalent singlet and triplet energies. This means that it is efficient for triplet CT states to (thermally) reverse intersystem crossing back to the singlet manifold, thereby giving a method of 'harvesting' up to 100% of triplet states formed by charge recombination in an OLED device, i.e. they can be as efficient as the best phosphorescent emitter. Thus ICT emitters can combine the most desirable properties of phosphorescent emitters, namely 100% triplet harvesting, with the added benefit of the long term stability of a fluorescent emitter. This is what both display and lighting manufactures demand but as yet do not have in the blue spectral region.
It is particularly important to replace phosphorescent blue emitters, as they chemically degrade during the vacuum deposition process used in device fabrication; they have short working lifetimes and do not emit deep-blue light which is essential for both high quality displays and lighting. Our Initial studies have so far demonstrated that TADF molecules can harvest triplets with up to 100% efficiency, and also in the solid state, isolation of TADF molecules in a host affords ideal conditions for very efficient TADF emission. Thus these materials are perfectly suited for OLED applications. Furthermore, our collaborators in Japan have demonstrated simple monochrome TADF OLEDs having up to 85% internal quantum efficiency. TADF emitters are looking well set to provide a unique challenge to Ir phosphors.
During this project we shall undertake detail photophysical investigations of ICT molecules that show very efficient TADF in the solid state. We will develop models to understand fully this triplet harvesting method and structure-property relationships to optimise the design and synthesis of new families of efficient TADF emitters covering the whole visible spectrum. OLEDs will be fabricated from these new materials and fully characterized. Finally, device architectures that combine two or more TADF emitters will be evaluated to determine the best routes to producing white emission from TADF emitters in simple device structures.
It is particularly important to replace phosphorescent blue emitters, as they chemically degrade during the vacuum deposition process used in device fabrication; they have short working lifetimes and do not emit deep-blue light which is essential for both high quality displays and lighting. Our Initial studies have so far demonstrated that TADF molecules can harvest triplets with up to 100% efficiency, and also in the solid state, isolation of TADF molecules in a host affords ideal conditions for very efficient TADF emission. Thus these materials are perfectly suited for OLED applications. Furthermore, our collaborators in Japan have demonstrated simple monochrome TADF OLEDs having up to 85% internal quantum efficiency. TADF emitters are looking well set to provide a unique challenge to Ir phosphors.
During this project we shall undertake detail photophysical investigations of ICT molecules that show very efficient TADF in the solid state. We will develop models to understand fully this triplet harvesting method and structure-property relationships to optimise the design and synthesis of new families of efficient TADF emitters covering the whole visible spectrum. OLEDs will be fabricated from these new materials and fully characterized. Finally, device architectures that combine two or more TADF emitters will be evaluated to determine the best routes to producing white emission from TADF emitters in simple device structures.
Planned Impact
The UK printed electronics community has identified OLEDs and organic solid state lighting as a key technology in which the UK holds a world leading position. Through the activities of CDT, Thorn Lighting and Durham University, via projects TOPLESS and TOPDRAWER, polymer solid state lighting has become a reality. Accordingly, £20.5M has been invested in new facilities at the CPI Printed Electronics Centre, £5M of which funded LACE, a pre-production manufacturing line for polymer OLEDs, OPV and solid state lighting on 6" tiles. This project will have great impact on this facility and OLED lighting research in the UK and Europe, including a new project, ENAB-SPOLED, an OLAE+ BRITE EURAM (TSB) project that brings together CDT, Durham, Tridonic, Zumtobal, Novaled and Fraunhofer IAP.
However, none of these devices are all phosphorescent; as yet there is no deep blue phosphorescent emitter available which gives the correct colour or adequate lifetime. The impact of moving from a fluorescent blue and green, phosphorescent red architecture (18-20 lm/W) to an all-phosphorescent architecture (40-50 lm/W) are obvious. LG Chem 80 lm/W lighting panels require a tandem OLED architecture with complex charge regeneration layers. Ir based blue phosphors degrade during deposition and Ir is the fourth rarest element on earth: current estimates of the global reserve base of Ir (estimated from zinc ore stocks) stands at 6000 tonnes or 13 years supply at current usage (infomine.com) and there is a major resistance to base a new global lighting technology on such a scarce commodity. Fluorescent emitters give far longer lifetimes, and TADF gives the near 100% efficiency of a phosphor. Thus it is vital that the proposed work is undertaken, to retain our world leading position in the UK. The uses of E-type TADF emission are not confined to organic solid state lighting: it will give exactly the same up-lift to the blue in RGB colour OLED displays, In all OLED technology Ir phosphors will need to be superseded.
The US DoE has reported that the potential benefits from developing solid-state lighting are: (i) by 2025 US national electricity consumption for lighting could be reduced by more than 300 TWh, which corresponds to 8% of electricity production in 2002; (ii) the cumulative savings on US consumer electricity bills could be >$125B between 2005 and 2025; (iii) the building of more than forty 1000 MW power stations could be deferred, contributing to a cleaner environment and more reliable grid operation. If an efficiency of 120 lm/W is achieved (predicted for ca. 2015) then about 30% of the electrical energy used for general lighting purposes in Europe could be saved. That translates into savings of 40 GW electrical peak power supply or an equivalent of 50 Million tons of CO2 per year. By 2025, solid-state lighting (SSL) could reduce the global amount of electricity used for lighting by 50%! SSL sources are also free of poisonous heavy metals, e.g. mercury, and as a consequence are much easier to dispose of - the OLED lamp can simply be placed in a glass furnace and recycled. These overall benefits which will arise from the transfer to solid-state lighting should have massive social impact around the globe. This project can help the UK achieve the Kyoto targets set for 2020. There will be significant social impact on employment in the construction, electrical installation and lighting manufacturing industries as buildings and interior designs adapt to the new lighting technology, and existing businesses are replaced by new ones exploiting the advantages of OLED lighting.
A successful outcome from this project will provide replacement high efficiency emitters for current Ir materials suitable for lighting applications. This will enable OLED solid-state lighting to move to commercialisation and to start to realise the major benefits highlighted above. In so doing this project will have a major impact on the UK economy and environment.
However, none of these devices are all phosphorescent; as yet there is no deep blue phosphorescent emitter available which gives the correct colour or adequate lifetime. The impact of moving from a fluorescent blue and green, phosphorescent red architecture (18-20 lm/W) to an all-phosphorescent architecture (40-50 lm/W) are obvious. LG Chem 80 lm/W lighting panels require a tandem OLED architecture with complex charge regeneration layers. Ir based blue phosphors degrade during deposition and Ir is the fourth rarest element on earth: current estimates of the global reserve base of Ir (estimated from zinc ore stocks) stands at 6000 tonnes or 13 years supply at current usage (infomine.com) and there is a major resistance to base a new global lighting technology on such a scarce commodity. Fluorescent emitters give far longer lifetimes, and TADF gives the near 100% efficiency of a phosphor. Thus it is vital that the proposed work is undertaken, to retain our world leading position in the UK. The uses of E-type TADF emission are not confined to organic solid state lighting: it will give exactly the same up-lift to the blue in RGB colour OLED displays, In all OLED technology Ir phosphors will need to be superseded.
The US DoE has reported that the potential benefits from developing solid-state lighting are: (i) by 2025 US national electricity consumption for lighting could be reduced by more than 300 TWh, which corresponds to 8% of electricity production in 2002; (ii) the cumulative savings on US consumer electricity bills could be >$125B between 2005 and 2025; (iii) the building of more than forty 1000 MW power stations could be deferred, contributing to a cleaner environment and more reliable grid operation. If an efficiency of 120 lm/W is achieved (predicted for ca. 2015) then about 30% of the electrical energy used for general lighting purposes in Europe could be saved. That translates into savings of 40 GW electrical peak power supply or an equivalent of 50 Million tons of CO2 per year. By 2025, solid-state lighting (SSL) could reduce the global amount of electricity used for lighting by 50%! SSL sources are also free of poisonous heavy metals, e.g. mercury, and as a consequence are much easier to dispose of - the OLED lamp can simply be placed in a glass furnace and recycled. These overall benefits which will arise from the transfer to solid-state lighting should have massive social impact around the globe. This project can help the UK achieve the Kyoto targets set for 2020. There will be significant social impact on employment in the construction, electrical installation and lighting manufacturing industries as buildings and interior designs adapt to the new lighting technology, and existing businesses are replaced by new ones exploiting the advantages of OLED lighting.
A successful outcome from this project will provide replacement high efficiency emitters for current Ir materials suitable for lighting applications. This will enable OLED solid-state lighting to move to commercialisation and to start to realise the major benefits highlighted above. In so doing this project will have a major impact on the UK economy and environment.
Organisations
Publications
Aydemir M
(2017)
Photophysics of an Asymmetric Donor-Acceptor-Donor' TADF Molecule and Reinterpretation of Aggregation-Induced TADF Emission in These Materials
in The Journal of Physical Chemistry C
Aydemir M
(2014)
The key role of geminate electron-hole pair recombination in the delayed fluorescence in rhodamine 6G and ATTO-532.
in Physical chemistry chemical physics : PCCP
Aydemir M
(2015)
Inter/Intrachain Interactions Behind the Formation of Charge Transfer States in Polyspirobifluorene: A Case Study for Complex Excited-State Dynamics in Different Polarity Index Solvents
in The Journal of Physical Chemistry C
Aydemir M
(2016)
High efficiency OLEDs based on anthracene derivatives: The impact of electron donating and withdrawing group on the performance of OLED
in Organic Electronics
Aydemir M
(2015)
Synthesis and investigation of intra-molecular charge transfer state properties of novel donor-acceptor-donor pyridine derivatives: the effects of temperature and environment on molecular configurations and the origin of delayed fluorescence.
in Physical chemistry chemical physics : PCCP
Che W
(2018)
Selective sensing of 2,4,6-trinitrophenol (TNP) in aqueous media with "aggregation-induced emission enhancement" (AIEE)-active iridium(iii) complexes.
in Chemical communications (Cambridge, England)
Chen C
(2018)
Intramolecular Charge Transfer Controls Switching Between Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence
in Angewandte Chemie
Chen C
(2018)
Intramolecular Charge Transfer Controls Switching Between Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence.
in Angewandte Chemie (International ed. in English)
Chulkin P
(2017)
Determination of standard redox rate constants of OLED active compounds by electrochemical impedance spectroscopy
in Electrochimica Acta
Congrave D
(2018)
Intramolecular p-p Interactions with a Chiral Auxiliary Ligand Control Diastereoselectivity in a Cyclometalated Ir(III) Complex
in Inorganic Chemistry
Congrave DG
(2018)
Sky-blue emitting bridged diiridium complexes: beneficial effects of intramolecular p-p stacking.
in Dalton transactions (Cambridge, England : 2003)
Costa BB
(2017)
Indirect consequences of exciplex states on the phosphorescence lifetime of phenazine-based 1,2,3-triazole luminescent probes.
in Physical chemistry chemical physics : PCCP
Data P
(2015)
Efficient p-phenylene based OLEDs with mixed interfacial exciplex emission
in Electrochimica Acta
Data P
(2016)
Dibenzo[a,j]phenazine-Cored Donor-Acceptor-Donor Compounds as Green-to-Red/NIR Thermally Activated Delayed Fluorescence Organic Light Emitters.
in Angewandte Chemie (International ed. in English)
Data P
(2016)
Exciplex Enhancement as a Tool to Increase OLED Device Efficiency
in The Journal of Physical Chemistry C
De Sa Pereira D
(2019)
Electroabsorption Spectroscopy as a Tool for Probing Charge Transfer and State Mixing in Thermally Activated Delayed Fluorescence Emitters.
in The journal of physical chemistry letters
Dias FB
(2017)
Photophysics of thermally activated delayed fluorescence molecules.
in Methods and applications in fluorescence
Dias FB
(2016)
The Role of Local Triplet Excited States and D-A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices.
in Advanced science (Weinheim, Baden-Wurttemberg, Germany)
Dos Santos P
(2018)
Chemical and conformational control of the energy gaps involved in the thermally activated delayed fluorescence mechanism
in Journal of Materials Chemistry C
Dos Santos P
(2017)
Optical and Polarity Control of Donor-Acceptor Conformation and Their Charge-Transfer States in Thermally Activated Delayed-Fluorescence Molecules
in The Journal of Physical Chemistry C
Dos Santos P
(2016)
Investigation of the Mechanisms Giving Rise to TADF in Exciplex States
in The Journal of Physical Chemistry C
Dos Santos PL
(2018)
Triazatruxene: A Rigid Central Donor Unit for a D-A3 Thermally Activated Delayed Fluorescence Material Exhibiting Sub-Microsecond Reverse Intersystem Crossing and Unity Quantum Yield via Multiple Singlet-Triplet State Pairs.
in Advanced science (Weinheim, Baden-Wurttemberg, Germany)
Dos Santos PL
(2016)
Using Guest-Host Interactions To Optimize the Efficiency of TADF OLEDs.
in The journal of physical chemistry letters
Etherington MK
(2019)
Persistent Dimer Emission in Thermally Activated Delayed Fluorescence Materials.
in The journal of physical chemistry. C, Nanomaterials and interfaces
Graves D
(2013)
Photophysical Investigation of the Thermally Activated Delayed Emission from Films of m-MTDATA:PBD Exciplex
in Advanced Functional Materials
Haase N
(2018)
Kinetic Modeling of Transient Photoluminescence from Thermally Activated Delayed Fluorescence
in The Journal of Physical Chemistry C
Hempe M
(2021)
Cyclophane Molecules Exhibiting Thermally Activated Delayed Fluorescence: Linking Donor Units to Influence Molecular Conformation.
in The Journal of organic chemistry
Hempe M
(2022)
Intramolecular Hydrogen Bonding in Thermally Activated Delayed Fluorescence Emitters: Is There Evidence Beyond Reasonable Doubt?
in The journal of physical chemistry letters
Higginbotham H
(2018)
Effects of Ortho-Phenyl Substitution on the rISC Rate of D-A Type TADF Molecules
in The Journal of Physical Chemistry C
Higginbotham H
(2018)
Triphenylamine disubstituted naphthalene diimide: elucidation of excited states involved in TADF and application in near-infrared organic light emitting diodes
in Journal of Materials Chemistry C
Higginbotham HF
(2017)
Fluorescence and Phosphorescence Anisotropy from Oriented Films of Thermally Activated Delayed Fluorescence Emitters.
in The journal of physical chemistry letters
Huang R
(2019)
Balancing charge-transfer strength and triplet states for deep-blue thermally activated delayed fluorescence with an unconventional electron rich dibenzothiophene acceptor
in Journal of Materials Chemistry C
Huang R
(2017)
The contributions of molecular vibrations and higher triplet levels to the intersystem crossing mechanism in metal-free organic emitters
in Journal of Materials Chemistry C
Huang R
(2018)
The influence of molecular conformation on the photophysics of organic room temperature phosphorescent luminophores
in Journal of Materials Chemistry C
Jankus V
(2015)
The role of exciplex states in phosphorescent OLEDs with poly(vinylcarbazole) (PVK) host
in Organic Electronics
Jankus V
(2016)
Generating Light from Upper Excited Triplet States: A Contribution to the Indirect Singlet Yield of a Polymer OLED, Helping to Exceed the 25% Singlet Exciton Limit.
in Advanced science (Weinheim, Baden-Wurttemberg, Germany)
Jiang N
(2023)
Multicolor Luminescence of a Polyurethane Derivative Driven by Heat/Light-Induced Aggregation
in Macromolecules
Kukhta N
(2019)
Revealing resonance effects and intramolecular dipole interactions in the positional isomers of benzonitrile-core thermally activated delayed fluorescence materials
in Journal of Materials Chemistry C
Kukhta N
(2018)
Importance of Chromophore Rigidity on the Efficiency of Blue Thermally Activated Delayed Fluorescence Emitters
in The Journal of Physical Chemistry C
Kukhta N
(2019)
Achieving Conformational Control in Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence Emitters by Functionalization of the Central Core
in The Journal of Physical Chemistry C
Kukhta NA
(2017)
Deep-Blue High-Efficiency TTA OLED Using Para- and Meta-Conjugated Cyanotriphenylbenzene and Carbazole Derivatives as Emitter and Host.
in The journal of physical chemistry letters
Laba K
(2015)
Diquinoline Derivatives as Materials for Potential Optoelectronic Applications
in The Journal of Physical Chemistry C
Li C
(2022)
TADF dendronized polymer with vibrationally enhanced direct spin-flip between charge-transfer states for efficient non-doped solution-processed OLEDs
in Chemical Engineering Journal
Liu S
(2022)
AIE-active iridium( iii ) complex integrated with upconversion nanoparticles for NIR-irradiated photodynamic therapy
in Chemical Communications
Liu S
(2022)
AIE-active Ir(III) complexes functionalised with a cationic Schiff base ligand: synthesis, photophysical properties and applications in photodynamic therapy.
in Dalton transactions (Cambridge, England : 2003)
Monkman A
(2016)
Explaining rISC and 100% efficient TADF (Conference Presentation)
Monkman A
(2015)
Organic semiconductor spintronics: utilizing triplet excitons in organic electronics.
in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences
Description | The true mechanism for TADF has been elucidated enabling far better design of TADF OLEDs to be made. First new generation material gives deep blue TADF OLEDs with EQE 24% |
Exploitation Route | Better design of effiicient TADF emitters |
Sectors | Chemicals,Electronics |
Description | EU materials companies taking up TADF emitters New TADF emitter deign patented and in negotiations with companies to take forward |
Sector | Chemicals |
Impact Types | Economic |
Description | (TADFlife) - Using the smart matrix approach to enhance TADF-OLED efficiency and lifetime |
Amount | € 3,942,036 (EUR) |
Funding ID | 812872 |
Organisation | European Commission |
Sector | Public |
Country | European Union (EU) |
Start | 10/2018 |
End | 09/2022 |
Description | EPSRC Understanding and Design Beyond Born-Oppenheimer using Time-Domain Vibrational Spectroscopy |
Amount | £680,000 (GBP) |
Funding ID | EP/P012167/1 |
Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
Sector | Public |
Country | United Kingdom |
Start | 03/2017 |
End | 02/2020 |
Description | EU 2020 Project HyperOLED |
Amount | € 840,000 (EUR) |
Funding ID | 732013 |
Organisation | European Commission |
Department | Horizon 2020 |
Sector | Public |
Country | European Union (EU) |
Start | 02/2017 |
End | 01/2020 |
Description | EU Horizon 2020 Project Phebe |
Amount | € 720,000 (EUR) |
Funding ID | 641725 |
Organisation | European Commission |
Sector | Public |
Country | European Union (EU) |
Start | 02/2015 |
End | 01/2018 |
Description | Excilight Marie Currie ITN ESR project |
Amount | € 2,600,000 (EUR) |
Organisation | European Commission |
Department | Horizon 2020 |
Sector | Public |
Country | European Union (EU) |
Start | 09/2015 |
End | 08/2019 |
Description | H2020-TWINN-2015 project ORZEL |
Amount | € 460,000 (EUR) |
Funding ID | 691684 |
Organisation | European Commission |
Department | Horizon 2020 |
Sector | Public |
Country | European Union (EU) |
Start | 02/2016 |
End | 01/2019 |
Description | Horizon 2020 Marie Currie Fellowship for Dr Prezmek Data |
Amount | € 200,000 (EUR) |
Organisation | European Commission |
Department | Horizon 2020 |
Sector | Public |
Country | European Union (EU) |
Start | 05/2015 |
End | 04/2017 |
Description | Merck PhD Studentship |
Amount | € 105,000 (EUR) |
Organisation | Merck |
Sector | Private |
Country | Germany |
Start | 10/2014 |
End | 09/2018 |
Description | Merck funded PhD studentship |
Amount | € 125,000 (EUR) |
Funding ID | Merck LCM 240620 |
Organisation | Merck |
Sector | Private |
Country | Germany |
Start | 02/2018 |
End | 07/2022 |