TADF：Frontiers of OLEDs Revolution

Being able to control the evolution, energy and spin of excitons in advanced materials underpins technologies ranging from organic light-emitting diodes (OLEDs), to solar cells, to optical sensing and imaging, to photocatalysis and wider technological applications. Many of these applications rely on efficient radiative decay of the generated exciton, that is, the generation of light. Light is not only generated as a result of photo-excitation (photoluminescence) but can be produced following electrical excitation (electroluminescence), chemical reaction (chemiluminescence), biochemical reaction (bioluminescence), application of mechanical force (mechanoluminescence), changes in crystallographic structure (crystalloluminescence), external sound (sonoluminescence), or high-energy ionized particle bombardment (cathodoluminescence, radioluminescence).

In particular, the use of OLEDs (applied electroluminescent devices) has exploded over the last decade due to their superior performance in displays and promise for solid-state lighting (SSL) over preceding technologies such as liquid crystalline displays (LCDs), plasma display panels(PDPs), and inorganic light-emitting diodes (LEDs).

Unlike now-ubiquitous LCDs, OLED display pixels are self-illuminating and individually addressable, and so do not require a uniform backlight pane. This allows pure black to be produced, resulting in a simpler and more energy-efficient display architecture with deeper achievable visual contrast.

Unlike LCD or inorganic LED displays, OLED displays can also be fabricated on a wide range of substrates, offering ultrathin, foldable, flexible and even transparent displays supporting innovative technological applications. Primarily because of their superior picture quality and color gamut (supported by the endless tunability of photophysical properties of the organic materials) OLED displays are now used in the majority of high-end smartphone and smartwatch screens, and are being increasingly adopted in the large-area television,monitor, and automotive markets.

OLED mechanism

OLEDs consist of a multilayer stack of organic semiconductor materials that are sandwiched between the cathode and anode. These devices produce light upon the application of a voltage, which leads to the injection of charges (holes from the anode and electrons from the cathode) that migrate through the layers of the device, ultimately recombining within the emissive layer (EML) to form excitons (bound electron- hole pairs, Figure 1a). As both holes and electrons − which correspond directly to molecular radical cations and anions possess spin 1/2, random recombination and Fermionic spin statistics dictate that the excitons formed will exist in a 1:3 ratio of singlet:triplet excited states (Figure 1b).Subsequent radiative decay from the excited states to the ground state produces light emission.

History of OLED

The first generation of OLEDs used simple fluorescent emitters, which could only produce light through the radiative decay of singlet excitons, as radiative decay of triplet excitons is a spin-forbidden process, leading to non-radiative decay in these devices. Consequently, the internal quantum efficiency (IQE) of these devices was capped at 25%. In 1987, Tang and VanSlyke first reported a functional fluorescent OLED using Alq3 as the emitter, with an external quantum efficiency (EQE) of about 1%. Despite exploring a variety of fluorescent emitters, the IQE cap and typical outcoupling efficiency limited the EQE of the first-generation OLEDs to around 5%.

In 1998, Baldo and colleagues broke through the 5% EQE limit with devices using phosphorescent emitter materials, leading to the development of so-called PhOLEDs. Organometallic phosphorescent emitters can utilize both singlet and triplet excitons to produce light due to strong spin-orbit coupling (SOC) mediated by a central heavy transition metal ion (such as Pt(II), Ir(III)) within the material. This large SOC facilitates the intersystem crossing (ISC) of singlet excitons to become triplets and radiative decay from the triplet excited state in the form of phosphorescence. PhOLEDs can achieve up to 100% IQE.

However, blue phosphorescent emitters have not yet met the stability required by the industry and may be fundamentally incapable of doing so. Therefore, blue subpixels typically contain a fluorescent TTA material. These TTA materials, although stable, require two triplet excitons to generate one singlet, thus having an IQE cap of about 63%. Researchers are currently seeking new emitter materials to address the color and stability issues of blue phosphorescent complexes and to reduce costs.

In addition to phosphorescence, there are other exciton harvesting mechanisms that can convert both singlet and triplet excitons into light, including TTA, excited state dynamics with hybridized local and intramolecular charge transfer (HLCT) characteristics, materials with inverted singlet-triplet gap (INVEST), doublet organic radical emitters, and thermally activated delayed fluorescence (TADF). The INVEST mechanism violates Hund's rule, where the S1 state is lower in energy than the T1 state, making the reverse intersystem crossing (RISC) process exothermic and thus accelerated. The core challenge for INVEST research is to fully understand and apply design rules that can deliver materials with this 'impossible' ordering of excited states.

Beyond the singlet-triplet picture of excited states, recent work has highlighted that organic radicals can be used as emitters in OLEDs. As open shell systems, the excited states have spin multiplicity, and thus there are no non-radiative triplets, yet IQEmax can still reach 100%. Despite this promise, the chemical space is narrowly explored, based only on donor-decorated tris(trichlorophenyl) radicals, and emission is limited to the red region.

Application of TADF

Now an established research theme globally, TADF involves the endothermic upconversion of triplet excitons into singlets followed by radiative decay, ensuring 100% IQEmax is possible (Figure 2).The research and development of TADF-based materials has progressed rapidly since the first report of a TADF material used in an OLED in 2009.As well as driving progress in state-of-the-art device efficiency, the use of TADF materials has also branched out to include other uses in OLEDs such as host materials,exciton harvesting materials in hyperfluorescent OLEDs, in other electroluminescent devices such as light-emitting electrochemical cells (LECs), as photocatalysts, bioimaging reagents,optical components in sensors,and as materials in photovoltaics and lasing.

Reference：

John MS, David H, Biju B, Megan B, Dongyang C, Eli ZC, et  al. The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation. CHEM.REV.–2024 ASAP

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