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Design of Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes
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Design of Thermally Activated Delayed Fluorescence Materials
for Organic Light-Emitting Diodes



Organic light-emitting diodes (OLEDs) are ultrathin two-dimensional light sources based on soft amorphous organic semiconductors. Owing to their high color rendering, high flexibility, and low power consumption, OLEDs have gained tremendous interest as next-generation displays and lighting sources. Here, we discuss a concept of thermally activated delayed fluorescence (TADF) that has recently been introduced to realize 100% internal quantum efficiencies of electron-to-photon conversion in OLEDs. We aim to provide a rational guide for designing intramolecular donor−acceptor systems that exhibit highly efficient TADF.


Organic light-emitting diodes (OLEDs), which consist of very thin, stacked layers of organic semiconductors, can transform electrical energy into light (Fig. 1(a)). Owing to their form factor, OLEDs can serve as two-dimensional light sources, which enables practical commercial applications in general solid-lighting and display devices, including mobile phones and flat-panel televisions. Moreover, OLEDs have many excellent characteristics, such as self-luminescence, high color rendering, high electroluminescence (EL) efficiency, high flexibility, and low power consumption. It is expected that the number of products and applications utilizing OLED technologies will increase to a large extent in the future.

Fig. 1: (a) Photograph of an OLED that transforms electricity into visible light. (b) Device architecture of typical OLEDs. (c) Schematic energy-level diagram of typical OLEDs.

The prospect of using organic semiconductor materials for OLEDs has flourished over the last three decades since the seminal work of Tang and Van Slyke in 1987 [1]. Their prototype OLED using tris(8-quinolinolato)aluminum(III) (Alq3) as a fluorescence emitter exhibited an external EL quantum efficiency (ηext) of 1% and a brightness of 1000 cd m-2. In 1990, Burroughes et al. demonstrated the first OLED based on a fluorescent polymer emitter, poly(p-phenylene vinylene) [2].

Driven by an external electrical field, the holes and electrons in OLEDs are injected from the anode and cathode, respectively, and transported to an emission layer, located at the center of the device (Fig. 1(b), 1(c)). Upon recombination of holes and electrons in the emission layer, the emitter molecules are electronically excited. In general, the theoretical maximum of ηext, which corresponds to the number of photons emitted from the OLED device per charge carriers injected into the device, is expressed by the following equation.

Here, ηint is the internal EL quantum efficiency, ηout is the light out-coupling efficiency, γ is the charge balance factor, ηST is the fraction of radiative excitons, and ΦPL is the photoluminescence (PL) quantum efficiency of the emitters. Ideally, γ and ΦPL can be close to unity if holes and electrons are fully balanced and recombined to generate excitons in the emission layer, and the emitter material possesses an appropriately high luminescence property. ηout is generally assumed to be approximately 20% in a randomly oriented emitter for a device without out-coupling enhancement. Meanwhile, the electrical excitation typically leads to the formation of singlet and triplet excitons with a probability of 25% and 75%, respectively, according to spin statistics [3]. Therefore, for conventional fluorescent OLEDs, the theoretical maximum of ηint is limited to 25% (Fig. 2(a)), because only the singlet excitons enable spin-allowed emissive transitions, and the remaining triplet excitons are lost as heat.

Fig. 2: Schematic representation of three different types of electroluminescence mechanisms in OLEDs: (a) fluorescence (up to 25% internal quantum efficiency), (b) phosphorescence, and (c) thermally activated delayed fluorescence (TADF) (up to 100% internal quantum efficiency).

An effective way to circumvent this problem is to utilize phosphorescence [4], i.e., the radiative transition from the lowest excited triplet (T1) states to the singlet ground (S0) state (Fig. 2(b)), from heavy transition metal-containing organometallic complexes, such as tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) [5]. Phosphorescence is a spin-forbidden transition, yet it becomes allowable due to strong spin−orbit coupling induced by the heavy transition metal center. As a result, all electro-generated excitons can be harvested by the T1 state via intersystem crossing (ISC), leading to ηint of nearly 100% in phosphorescent OLEDs [6]. However, phosphorescent materials rely on precious metals such as iridium and platinum. The expense and rarity of such precious-metal complexes limit the cost effectiveness and long-term mass production of phosphorescent OLEDs. Moreover, it is still difficult to produce efficient pure-blue phosphorescent materials exhibiting high quantum efficiency, color purity, and long-term stability, which would hamper the widespread application of phosphorescent OLEDs in the future.

Recent studies by Adachi and coworkers and other research groups have paved an alternative way for the realization of high-efficiency, precious-metal-free OLEDs by utilizing the mechanism of thermally activated delayed fluorescence (TADF) (Fig. 2(c)) [7,8]. In the TADF mechanism, purely organic luminescent compounds with very small singlet−triplet energy splitting (ΔEST) enable efficient up-conversion of the excited T1 states to the emissive lowest excited singlet (S1) state via reverse intersystem crossing (RISC). In the last couple years, considerable research efforts have been invested for exploring efficient TADF materials and understanding their unique photophysical properties, in order to realize ηint of 100% without using precious metals in OLEDs [8]. In this Feature Article, we focus on recent progress on purely organic TADF materials, and describe the molecular design and photophysical properties of TADF materials as well as their OLED performances.


TADF is typically observed in luminescent organic materials having a very small ΔEST, since the thermal energy (kBT ≈ 25.6 meV) at room temperature can assist RISC from the T1 state to the S1 state. The origin of RISC involves the mixing of the excited T1 and S1 states, which is caused by spin−orbit coupling (HSO). According to first-order perturbation theory, the first-order mixing coefficient (λ) is expressed by the following equation.

Since there is finite HSO even in purely organic materials, minimizing the ΔEST can lead to the enhancement of λ, and hence, an efficient RISC from the T1 to S1 states. The key molecular design for achieving the desired minimal ΔEST is to reduce the spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Assuming S0 energy is zero, ΔEST is given by the following equation [9].

Here, ES and ET are the energies of the S1 and the T1 states, respectively, and J is the electron exchange integral associated with the Pauli principle, and is expressed by the following equation.

Here, ΦH and ΦL are the wave functions of the HOMO and LUMO, respectively, e is the charge on an electron, and r12 is the distance separating the electrons. From these equations, it is crucial to separate the HOMO and LUMO wave functions in rational TADF molecular design, in order to minimize ΔEST and to accelerate the RISC process.

Donor−acceptor (D−A) molecular systems, which are structurally defined by an electron-rich moiety covalently linked to an electron-deficient moiety, are commonly used for the design of TADF materials. Distorting the π-conjugations of these D−A molecules leads to well separated HOMO and LUMO distributions [10]. These D−A systems can also provide the additional advantage of precise control over the emission wavelength (i.e., the energy level of the lowest excited charge-transfer singlet (1CT) state) by varying the strength of the composed donor and acceptor moieties. Recent research activities have been focused on the design and synthesis of new D−A TADF molecules containing aryl amines (e.g., carbazole, diphenylamine, phenoxazine, and acridan) as a donor moiety. This is likely due to the fact that these aryl amines not only have an electron-donating ability but also offer effective intramolecular steric repulsion to the adjacent acceptor moiety, which induces a large dihedral angle between the donor and acceptor moieties. This propensity leads to an effective spatial separation of the HOMO and LUMO, and thus, a small ΔEST of the emitter molecules. Dias et al. pointed out that the origin of TADF involves population of triplet excitons to the 3nπ* states of these aryl amines, followed by the RISC from the 3CT to 1CT states [11]. The selection of the acceptor moiety also significantly affects the photophysical properties of the TADF molecules. Representative TADF materials are discussed below, by classifying them based on the structural features of the acceptor moieties.


Benzonitriles are one of the most widely used acceptor units in the design of TADF materials [8,12-22]. Compound 1 (Spiro-CN) was developed as a yellow TADF material by Nakagawa et al. in 2012 [12]. In Spiro-CN, a 2,7-bis(di-p-tolylamino)fluorene donor unit and 2,7-dicyanofluorene acceptor unit were orthogonally connected through a spiro bridge. As can be seen from Fig. 3, the connection via spiro bridge effectively breaks the π-conjugation between the donor and acceptor moieties, resulting in well separated HOMO and LUMO distributions. Spiro-CN exhibited a yellow TADF emission with a peak at around 540 nm and a TADF emission lifetime of 14 μs. Indeed, the ΔEST of Spiro-CN was determined to be 57 meV, which is much smaller than that of C70 (0.26 eV) [23] and tin(IV) fluorideporphyrin (0.24 eV) [7], suggesting that the spiro-based D−A structure is promising in obtaining small ΔEST. Because of its low ΦPL of 27%, the ηext of a Spiro-CN-based OLED was limited to 4.4%. A further structural modification by replacing the donor moiety of Spiro-CN with N-phenylacridan afforded compound 2 (ACRFLCN), resulting in a higher ΦPL of 67% and ηext of 10.1% [13,14].

Fig. 3: (a) Optimized molecular structure and (b,c) the distributions of the HOMO and LUMO of Spiro-CN calculated at the B3LYP/6-31G(d) level of theory.

As described above, the key design rule for obtaining a small ΔEST is to reduce the overlap between the HOMO and LUMO. However, this molecular design in turn results in the decrease of the oscillator strength between S1 and S0, and thus, lowers the ΦPL. In 2012, Uoyama et al. attained both small ΔEST and high ΦPL values in phthalonitrile-based TADF molecules (compounds 3−8) by carefully tuning the spatial overlap between the HOMO and LUMO [8]. Compound 3 (4CzIPN) bearing an isophthalonitrile acceptor core and four carbazole donor arms was found to show a small ΔEST of 83 meV and a high ΦPL of 94% with an emission peak at 507 nm in toluene solution. Besides having a high ΦPL, TADF emission lifetime of 4CzIPN was very short (5.1 μs). For a 4CzIPN-based OLED, the ηext reached 19.3%, which was considerably higher than those of conventional fluorescent OLEDs (ηext < 5%) and even comparable with those of phosphorescent OLEDs. Time-dependent density functional theory (TD-DFT) calculations predicted that the highest occupied and lowest unoccupied natural transition orbitals (NTOs) of 4CzIPN were located on the donor and acceptor moieties, respectively, while they were moderately overlapped on the central benzene ring. Additionally, the reorganization energies in the excited state geometries were calculated to be relatively small for these intramolecular D−A molecules. These structural and electronic features can contribute to the high ΦPL in 4CzIPN.

Further device optimization improved the ηext and operational lifetime of 4CzIPN-based OLEDs [24-27]. Sun et al. reported that the transition dipole moments of the 4CzIPN molecules tend to orient horizontally in a host matrix, which results in an enhancement of ηout [28]. As a result, a high ηext of 29.6% was obtained, which was consistent with the theoretical maximum ηext value predicted by the optical simulations of the device. Nakanotani et al. reported that the operational lifetime of 4CzIPN-based OLEDs was improved to 2800 hours for aging to 50% of an initial luminance (LT50) of 1000 cd m-2 when the concentration of 4CzIPN in the emission layer was increased [29]. This improvement of the device stability can be attributed to the shift of the recombination zone to the center of the emission layer at the high concentration of 4CzIPN, which acts as an electron-trapping site in a carbazole-based host matrix.

The promising TADF properties of the phthalonitrile-based TADF materials encouraged further exploration of their analogues and derivatives. Nishimoto et al. developed compound 9 (CzTPN) incorporating an electron-accepting terephthalonitrile unit and two electron-donating carbazole units [15]. CzTPN exhibited a blue-green emission with a peak at 494 nm and a high ηext of 15.0% when embedded in a cyclotriphosphazene-based host material with a high T1 energy. Park et al. demonstrated that the emission colors of TADF can be tuned systematically by varying the donor and acceptor strengths in a simple D−A system (Fig. 4) [16]. Depending on the combination of the donor and acceptor units, compounds 10−14 displayed a wide variety of emission colors originating from TADF, covering the entire visible region from 450 nm for Cz-VPN to 610 nm for Px-CNP. As an example, compound 11 (AcVPN) incorporating a phthalonitrile acceptor unit and two 9,9-dimethylacridan donor units was found to show both a small ΔEST of 0.2 eV and a high ΦPL of 86%. An OLED based on AcVPN exhibited a high ηext of 18.9% for a sky-blue TADF emission with a peak at 504 nm [16].

Fig. 4: (a) Photographs showing full-color TADF emissions and (b) PL spectra of the thin films of compound 10−14 embedded in a host matrix under UV irradiation. (c)ηext−current density characteristics of the TADF OLEDs based on compounds 10−14.


A highly electron-withdrawing 1,3,5-triazine is another useful acceptor building block for designing a wide variety of efficient TADF materials [10,30-38]. 1,3,5-Triazine was first utilized in TADF molecules by Endo et al. in 2011 [10]. Compound 15 (PIC-TRZ), where a biphenyltriazine unit functions as an acceptor and two indolocarbazole units perform as a donor, exhibited green TADF emission with a peak at 495 nm and a TADF emission lifetime of 230 μs [10]. The ΔEST of PIC-TRZ was determined to be 0.11 eV. Owing to its relatively large ΔEST, a large portion of the T1 excitons underwent non-radiative decay rather than RISC to the S1 state, resulting in a relatively low ηext of 5.3%. With a suitable structural modification, compound 16 (PIC-TRZ2) was found to exhibit a smaller ΔEST of 20 meV and a higher ηext of 14.0% [30]. Lee et al. reported that compound 17 (CC2TA) incorporating a phenyltriazine acceptor unit and two bicarbazole donor units exhibited a small ΔEST of 60 meV and a high ηext of 11% [31]. DFT calculations for CC2TA revealed that the outermost carbazoles were more electron rich, compared to the inner carbazoles in the bicarbazole moieties. This electron-distribution gradient in the bicarbazole moieties can give rise to an effective spatial separation of the HOMO and LUMO and a small ΔEST.

Hirata et al. reported that for triazine-based TADF materials, extending the π-conjugation of the donor moieties increases the oscillator strength without change in ΔEST values [32], which originates from an increased transition dipole moment induced by delocalization of the HOMO on the extended donor moieties. As a representative example, compound 18 achieved both small ΔEST of 90 meV and high ΦPL of 100%. A TADF OLED based on compound 18 exhibited a high ηext of 20.6% with an emission peak at 487 nm. This result suggested that ηint of nearly 100% can be attained, assuming a ηout of 20% (assuming the refractive index of 1.8 for the organic layers). Compound 19 (DACT-II) possessing a stronger electron-donating 3,6-diphenylaminocarbazole unit exhibited a red-shifted TADF emission at 520 nm in an OLED [33]. The ηext of DACT-II-based TADF OLED reached 29% because DACT-II molecules tend to orient parallel to the substrate in a host matrix, giving rise to an improved ηout to 29.6%.


An electron-accepting aryl ketone unit was introduced as an acceptor moiety for the design of a spiro-type TADF molecule by Nasu et al. in 2013 [39]. Compound 20 (ACRSA), consisting of an anthrone acceptor unit and an N-phenylacridan donor unit through a spiro bridge, exhibited a greenish blue TADF emission with a peak at around 480 nm with a high ΦPL of 81%, when doped in a host matrix. The fluorescence and phosphorescence spectra of ACRSA were nearly identical with a very small ΔEST of 30 meV. It is known that aryl ketones intrinsically possess a small ΔEST; however, their ΦPL is typically less than 1% at room temperature because of the noticeably small overlap integral for the 1nπ* transitions. The highly emissive feature of ACRSA suggests that the S1 state should correspond to a 1CT excited state. Thus, ACRSA exhibited a high ηext of 16.5% in a TADF OLED, which was four times higher than the theoretical maximum for a conventional fluorescent material with the same ΦPL of 81% [39].

In 2014, Lee et al. reported a series of TADF molecules based on a benzophenone as an acceptor moiety [40]. Depending on the strengths of the donor and acceptor moieties, compounds 21−25 (Cz2BP, CC2BP, Px2BP, m-Px2BBP, and p-Px2BBP) displayed tunable TADF emission colors with the emission maxima at 438, 462, 509, 566, and 600 nm, respectively, covering the entire visible region (Fig. 5). Moreover, the combination of light-blue-emitting CC2BP and yellow-emitting m-Px2BBP gave a white TADF emission in OLEDs with a ηext of 6.7%. One particular feature of these benzophenone-based TADF molecules is the enhanced ΦPL in solid states. In fact, the ΦPL of CCz2BP was 73% in a host matrix, while it was 38% in a toluene solution. Similar emission behavior was also observed in compound 26 (AcPmBPX) and 27 (PxPmBPX) incorporating a benzoylbenzophenone acceptor unit [41]. The decreased ΦPL in a solution can be explained by the presence of a non-radiative decay process associated with intramolecular motions, which can be suppressed in a solid state.

Fig. 5: EL emission color coordinates in the CIE chromaticity diagram and photographs of full-color TADF OLEDs based on compounds 21−25.

Zhang et al. developed anthraquinone-based TADF materials [42], and reported that the introduction of a phenylene (Ph) spacer suppressed the twisting and stretching motions of the N−C bond, resulting in enhanced ΦPL. D−Ph−A−Ph−D structured compound 28 exhibited a ΦPL of 80% in a host matrix, which was 1.6 times higher than that observed for the corresponding D−A−D structured compound 29.


Five-membered heteroaromatic rings, such as 1,3,4-oxadiazole and 1,2,4-triazole can function as acceptor moieties, and are used in the design of TADF materials [43-45]. Lee et al. developed compound 30 (PXZ-OXD), where a 2,5-diphenyl-1,3,4-oxadiazole unit served as an acceptor moiety and two phenoxazine units served as a donor moiety [43]. The PXZ-OXD-based OLED showed a green TADF with a peak at 508 nm and a ηext of 14.9%. Replacement of the acceptor moiety of PXZ-OXD with 3,4,5-triphenyl-1,2,4-triazole gave compound 31 (PXZ-TAZ), leading to a blue-shifted TADF emission with a peak at 456 nm. The ΔEST of PXZ-TAZ determined from the onsets of the fluorescent and phosphorescent spectra was 0.23 eV. Because of its large ΔEST, PXZ-TAZ exhibited a long TADF emission lifetime of 2.09 ms. Consequently, the ηext of a PXZ-TAZ-based OLED was limited to 5.3%. Similar millisecond-order decay times were observed for compound 32 (PPZ-3TPT) and compound 33 (PPZ-4TPT) as reported by Zhang et al [45]. TD-DFT calculations revealed that the locally excited triplet (3LE) states on the triazole acceptor moieties were lower in energy than the 3CT states in PPZ-3TPT and PPZ-4TPT. Although the 1CT and 3CT states were energetically almost equivalent owing to the minimal overlap between the HOMO and LUMO distributions, these lower-lying 3LE states led to a large ΔEST, and underwent non-radiative decay for the triplet excitons.


Given the role of the interplay between 3CT and 3LE states in governing ΔEST, one of the viable candidates for use as an acceptor moiety of blue TADF molecules is diphenylsulfone, which possess a 3LE of 3.02 eV [45-50]. In 2012, Zhang et al. reported a series of blue TADF molecules based on diphenylsulfone as an acceptor moiety [46]. Compound 34 (DTC-DPS) with 3,6-di-t-butylcarbazole donor moieties exhibited a ηext of 10% for pure blue TADF emission with a peak at 423 nm and Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.07) in a TADF OLED. The low-lying 3LE (3ππ*) states of the donor moieties resulted in a deleterious large ΔEST of 0.32 eV and a millisecond-order lifetime of the triplet excitons in DTC-DPS. These conditions aided bimolecular singlet−triplet and triplet−triplet annihilations at a high luminance in OLEDs, causing a significant decrease in ηext value (known as efficiency roll-off). A chemical modification of DTC-DPS by replacing the t-butyl substituents on the carbazole donor moieties with stronger electron-donating methoxy substituents, gave compound 35 (DMOC-DPS) with a smaller ΔEST of 0.21 eV by stabilizing the 1CT states while maintaining the 3LE state [47]. This smaller ΔEST of DMOC-DPS resulted in an improved ηext of 14.5% and reduced efficiency roll-off for deep blue emission with a peak at 460 nm in a TADF OLED. Compound 36 (DMAC-DPS) was designed to possess a small ΔEST of 80 meV by introducing 9,9-dimethylacridan as a donor moiety [45]. A DMAC-DPS-based TADF OLED exhibited a blue emission with a peak at 470 nm and a high ηext of 19.5%, which was comparable to those observed in the state-of-art blue phosphorescent OLEDs.


Aryl borons are another successful acceptor scaffold for designing blue TADF molecules [51-55]. Numata et al. reported a series of blue TADF molecules featuring a phenoxaborin unit [51]. Compound 37, incorporating a phenoxaborin acceptor unit and a 9,9-dimethylacridan donor unit, exhibited a small ΔEST of 0.10 eV and a high ΦPL of 100% for a blue TADF emission with a peak at 475 nm. A blue TADF OLED using compound 37 exhibited a notably high ηext of 21.7% (Fig. 6). Moreover, the emission peak was shifted to a shorter wavelength by about 20 nm when replacing the donor moiety with spiroacridan units, such as 10H-spiro(acridine-9,9'-fluorene) and 10H-spiro(acridine-9,9'-xanthene). The resulting compounds 38 and 39 exhibited ηext of 19.0% and 20.1% for pure blue emissions with peaks at 456 and 451 nm, respectively [51].

Currently, deep-blue phosphorescent emitters with sufficient performance, especially in terms of device stability, are not available and blue OLEDs in commercial devices are still built with fluorescent emitter having intrinsically low ηext. Therefore, the exploration for deep-blue TADF materials should be very attractive.

Fig. 6: (a) ηext−current density characteristics, (b) EL spectra at 1 mA cm-2, and (c) photographs of EL emissions of blue TADF OLEDs based on compounds 37−39.


This Feature Article has reviewed recent progress on OLEDs based on purely organic D−A molecules that exhibit efficient TADF. These D−A electronic systems are advantageous for achieving a small ΔEST and high ΦPL by precisely tuning the HOMO and LUMO distributions, which ultimately contribute to realizing a high ηint approaching 100% without having to use precious metals. The high EL efficiency as well as the lower cost of purely organic TADF materials make them a promising alternative to conventional phosphorescent organometallic complexes, as an emitter in a variety of OLED applications. Although there are further challenges to be addressed regarding the long-term device stability, given the current speed of the development of TADF technology, their mass production for general solid-lighting and display devices is expected to commence in the near future.


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Naoya Aizawa received his PhD from Yamagata University in 2015 under the supervision of Prof. Junji Kido and was a visiting research scholar in the group of Prof. Bernard Kippelen at the Georgia Institute of Technology. He is currently an assistant professor in the research group of Prof. Takuma Yasuda at Kyushu University. His research focuses on the synthesis and utilization of π-conjugated organic materials for printed optoelectronic devices.

In Seob Park received his M.S. degree from Pukyong National University in 2012. Since 2014, he has been conducting his PhD studies in the Department of Applied Chemistry at Kyushu University under the supervision of Prof. Takuma Yasuda and Prof. Chihaya Adachi. His research focuses on the development of novel luminescent organic materials for applications in organic light-emitting diodes.

Takuma Yasuda received his PhD in 2005 from Tokyo Institute of Technology under the supervision of Prof. Takakazu Yamamoto. After completing postdoctoral research under Prof. Takashi Kato at the University of Tokyo, he was appointed as an assistant professor at the University of Tokyo in 2008. In 2010, he joined the Department of Applied Chemistry at Kyushu University as an associate professor in the research group of Prof. Chihaya Adachi. He was also a concurrent researcher of PRESTO, Japan Science and Technology Agency from 2011 to 2014. In 2014, he was appointed as a full professor at the INAMORI Frontier Research Center (IFRC) of Kyushu University. His research is directed toward the design, synthesis, and functionalization of electro- and photoactive π-conjugated molecular materials. His current topics of research include applications of these materials to organic photonics and electronics devices.