Organic materials that exhibit thermally activated delayed fluorescence (TADF) are an attractive class of functional materials that have witnessed a booming development in recent years. Since Adachi et al. reported high-performance TADF-OLED devices in 2012, there have been many reports regarding the design and synthesis of new TADF luminogens, which have various molecular structures and are used for different applications. In this review, we summarize and discuss the latest progress concerning this rapidly developing research field, in which the majority of the reported TADF systems are discussed, along with their derived structure–property relationships, TADF mechanisms and applications. We hope that such a review provides a clear outlook of these novel functional materials for a broad range of scientists within different disciplinary areas and attracts more researchers to devote themselves to this interesting research field.

Recent advances in organic thermally activated delayed fluorescence materials.

We thank the University of St Andrews for support. EZ-C thanks the Leverhulme Trust for financial support (RPG-2016-047). and the EPSRC (EP/P010482/1) for financial support.

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Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes.

Thermally activated delayed fluorescence (TADF) emitters, which produce light by harvesting both singlet and triplet excitons without noble metals, are emerging as next-generation organic electroluminescent materials. In the past few years, there have been rapid advances in molecular design criteria, our understanding of the photophysics underlying TADF and the applications of TADF materials as emitters in organic light-emitting diodes (OLEDs). This topic is set to remain at the forefront of research in optoelectronic organic materials for the foreseeable future. In this Review, we focus on state-of-the-art materials design and understanding of the photophysical processes, which are being leveraged to optimize the performance of OLED devices. Notably, we also appraise dendritic and polymeric TADF emitters — macromolecular materials that offer the potential advantages of low cost, solution processable and large-area OLED fabrication. Thermally activated delayed fluorescence (TADF) emitters are promising electroluminescent materials for next-generation organic light-emitting diodes (OLEDs). In this Review, the molecular design, photophysical characteristics and OLEDs composed of small-molecule, dendritic and polymeric TADF emitters are discussed.

All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes

Recently, organic thermally activated delayed fluorescence (TADF) emitters have attracted a great deal of attention because they can theoretically realize 100% internal quantum efficiency. Many TADF emitters have been developed since the first demonstration of close to 20% external quantum efficiency in the devices. Recently developed TADF emitters demonstrated close to 37% external quantum efficiency in blue, above 30% external quantum efficiency in green, and close to 18% external quantum efficiency in red devices. Therefore, TADF organic light-emitting diodes could potentially be substituted for high-efficiency phosphorescent organic light-emitting diodes. In this work, we reviewed molecular design strategies of organic-based TADF emitters by classifying them into several categories depending on the material parameters required for the TADF emitters. In addition, we proposed a future development direction of TADF emitters to make them competitive with phosphorescent emitters.

Molecular Design Strategy of Organic Thermally Activated Delayed Fluorescence Emitters

Materials that exhibit thermally activated delayed fluorescence are promising for the realization of efficient organic light-emitting diodes. However, finding suitable deep-blue thermally activated delayed fluorescence materials is still challenging. Here, we report two highly efficient deep-blue thermally activated delayed fluorescence emitters, TDBA–Ac and TDBA–DI, containing oxygen-bridged, symmetric and rigid boron acceptor moieties. Both emitters have been designed to have high photoluminescence quantum yield and narrow-band blue emission. TDBA–Ac and TDBA–DI exhibited deep-blue emission and a small singlet–triplet energy gap of 0.06 eV and 0.11 eV, respectively, in toluene. The 20wt%-doped films of TDBA–Ac and TDBA–DI in DBFPO host exhibited high photoluminescence quantum yields of 93% and 99%, respectively. The fabricated TDBA–DI device showed an extremely high external quantum efficiency of 38.15 ± 0.42% in the blue region with low roll-off characteristics of 25.2% at high luminance of up to 5,000 cd m–2. The TDBA–Ac-doped device exhibited a high external quantum efficiency of 21.50 ± 0.22% with deep-blue colour coordinates of (0.15, 0.06). The discovery of two deep-blue organic emitters of light could aid the development of next-generation organic light-emitting devices.

Highly efficient blue thermally activated delayed fluorescence emitters based on symmetrical and rigid oxygen-bridged boron acceptors

Efficient sky-blue organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) display a three orders of magnitude increase in lifetime, which is superior to those of controlled phosphorescent OLEDs used in this study. The combination of electro-oxidation and photo-oxidation of the TADF emitters in their triplet excited-states is suppressed through molecule design and device engineering.

Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices.

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m2, the time to reach 90% of initial luminance (LT90) increased eight times, resulting in LT90 = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT95 was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.

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Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers

Scientific Reports 6: Article number: 2246310.1038/srep22463; published online: March012016; updated: May312016

Toshinori Matsushima was omitted from the author list in the original version of this Article. This has been corrected in the PDF and HTML versions of the Article.

The Acknowledgements section now reads:

This work was supported by the Exploratory Research for Advanced Technology (ERATO) and the Kumamoto Collaborations on Organic Electronics under the Regional Innovation Strategy Support Program sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We would like to acknowledge Mr. Ko Inada for his assistance with calculation of molecular energy levels. We also thank Dr. William J. Potscavage Jr. for his assistance with the preparation of this manuscript.

The Author Contributions section now reads:

D.T. performed the OLED and lifetime experiments. T.M. performed the TSC measurements and analysis. D.T. and C.A. wrote the paper with input from T.M. C.A. supervised the project.

/pdf/corrigendum-operational-stability-enhancement-in-organic-1b2ao03kgo.pdf

Corrigendum: Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers

Blue OLEDs: Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices (Adv. Mater. 35/2016)

Daniel Ping Kuen Tsang

Papers

Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices.

Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers

Corrigendum: Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers

Blue OLEDs: Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices (Adv. Mater. 35/2016)