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

Extremely efficient sky-blue organic electroluminescence with external quantum efficiency of ≈37% is achieved in a conventional planar device structure, using a highly efficient thermally activated delayed fluorescence emitter based on the spiroacridine-triazine hybrid and simultaneously possessing nearly unitary (100%) photoluminescence quantum yield, excellent thermal stability, and strongly horizontally oriented emitting dipoles (with a horizontal dipole ratio of 83%).

Sky-Blue Organic Light Emitting Diode with 37% External Quantum Efficiency Using Thermally Activated Delayed Fluorescence from Spiroacridine-Triazine Hybrid.

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

A versatile thermally activated delayed fluorescence emitter for both highly efficient doped and non-doped organic light emitting devices

A series of twisted D–π–A type emitters based on the acridine donor unit and CN-substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge-transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN-substitution pattern but also by the subtle control of in-acceptor conformation (twist angles). Small triplet-to-singlet energy gaps (ΔEST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in-acceptor conformation, yielding a series of efficient blue-green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue-green to yellow TADF organic light-emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.

Efficient and Tunable Thermally Activated Delayed Fluorescence Emitters Having Orientation‐Adjustable CN‐Substituted Pyridine and Pyrimidine Acceptor Units

Delayed Fluorescence Emitters: Efficient and Tunable Thermally Activated Delayed Fluorescence Emitters Having Orientation‐Adjustable CN‐Substituted Pyridine and Pyrimidine Acceptor Units (Adv. Funct. Mater. 42/2016)

An organic fluorescent luminescent material and an organic electro-luminescence device are provided. The organic fluorescent luminescent material may represented by the formula, (Ar)-(Ph)-(X), wherein Ar is (DMAC), Ph include at least one benzene ring, and X is a six-membered aromatic ring, and at least one nitrogen atom is located on the six-membered aromatic ring. The organic fluorescent luminescent material can be used in organic electro-luminescence device.

Organic fluorescent luminescent material and organic electroluminescent device

A pixel array including a plurality of pixel units is provided. Each of the pixel unit includes at least a first color subpixel, a second color subpixel and a Nth color subpixel, wherein N is an integer and N≥3. The first color subpixel includes a first stacked light-emitting layer, the second color subpixel includes a second stacked light-emitting layer, and the Nth color subpixel includes a Nth stacked light-emitting layer. The first stacked light-emitting layer, the second stacked light-emitting layer, and the third stacked light-emitting layer each include a main light-emitting layer and an auxiliary light-emitting layer. The main light-emitting layer of the first color subpixel and the auxiliary light-emitting layer of the second color subpixel are the same material layer. The main light-emitting layer of the Nth color subpixel and the auxiliary light-emitting layer of the first color subpixel are the same material layer.

Hu Hung-Chieh

Papers

A versatile thermally activated delayed fluorescence emitter for both highly efficient doped and non-doped organic light emitting devices

Efficient and Tunable Thermally Activated Delayed Fluorescence Emitters Having Orientation‐Adjustable CN‐Substituted Pyridine and Pyrimidine Acceptor Units

Delayed Fluorescence Emitters: Efficient and Tunable Thermally Activated Delayed Fluorescence Emitters Having Orientation‐Adjustable CN‐Substituted Pyridine and Pyrimidine Acceptor Units (Adv. Funct. Mater. 42/2016)

Organic fluorescent luminescent material and organic electroluminescent device

Pixel array with sub-pixels comprising vertically stacked light emitting layers