Molecular Design Strategy of Organic Thermally Activated Delayed

Review pubs.acs.org/cm

Molecular Design Strategy of Organic Thermally Activated Delayed Fluorescence Emitters Yirang Im,†,§ Mounggon Kim,‡,§ Yong Joo Cho,‡ Jeong-A Seo,† Kyoung Soo Yook,† and Jun Yeob Lee*,† †

School of Chemical and Engineering, Sunkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 446-740, Korea Department of Polymer Science and Engineering, Dankook University, 152, Jukjeon-ro, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Korea

‡

ABSTRACT: 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.

1. INTRODUCTION Organic light-emitting diodes (OLEDs) have been widely developed for the last 30 years since the first discovery of electroluminescence (EL) in organic materials inserted between two electrodes and are being used in many applications such as in mobile displays, in large OLED televisions, in flexible displays, and as lighting.1 The rapid penetration of OLEDs in the display and lighting markets happened because of the unique combination of advantages that they offer such as design flexibility, thinness, lightweightness, fast response time, and superior device performance. In particular, the design flexibility of OLEDs that came about due to OLED fabrication on plastic films has appeal in many application areas because it means that display or lighting products can be produced in various forms. Although the OLEDs have these several advantages over liquid crystal displays, there are several weaknesses such as high power consumption and short lifetimes that need to be improved upon before the range of OLED applications can be expanded. High power consumption is one of the critical issues of OLEDs, and two main parameters for power consumption are quantum efficiency (QE) and driving voltage. Mostly, the high power consumption of the OLEDs is due to the low QE of OLEDs rather than to the high driving voltage. In order to improve the QE of OLEDs, there have been extensive studies about highly efficient organic emitting materials for OLEDs. At first, fluorescent emitting materials that can convert only 25% of injected electrons into photons were used as the emitting materials.2−4 In general, holes and electrons injected © 2017 American Chemical Society

from electrodes to emitters generate excitons, and the excitons are classified into singlet and triplet excitons that are formed at a ratio of 25:75.3,5−7 In the case of fluorescent emitting materials, only singlet excitons can be transformed into photons, and so only 25% internal QE is theoretically possible. However, the low internal QE of the fluorescent materials prompted researchers to develop high-efficiency phosphorescent emitting materials that have a high internal QE.8 Fluorescent materials can convert only singlet excitons into photons, but the phosphorescent emitting materials can convert both singlet and triplet excitons generated by hole and electron injection into photons by a radiative transition process. Therefore, theoretically 100% internal QE can be reached if there is no loss process during the radiative transition. In fact, nearly 100% internal QE has already been achieved in phosphorescent OLEDs (PHOLEDs) through material and device engineering9−15 since the first publication of phosphorescent emitting materials in 1998.8 The progress of the QE of PHOLEDs was made by developing Ir metal-based emitters16−23 and that enabled the real application of red and green phosphorescent emitting materials in commercial products instead of traditional fluorescent emitting materials. The other critical issue of OLEDs is lifetime, which is related to the long-term stability of the OLEDs. OLEDs are self-emissive Received: December 15, 2016 Revised: February 6, 2017 Published: February 7, 2017 1946

DOI: 10.1021/acs.chemmater.6b05324 Chem. Mater. 2017, 29, 1946−1963

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Chemistry of Materials

development of high-efficiency and stable TADF emitters for practical applications will be suggested. Mostly, current issues and problems of the TADF emitters will be discussed, and a molecular design approach to overcome those issues will be described.

devices that have different light-emitting materials for the red, green, and blue emissions and have the intrinsic problem of poor long-term stability because organic materials cannot be as stable as inorganic materials. The poor stability of organic materials is one of the most important factors limiting the lifetime of OLEDs.24 Therefore, the use of stable emitting materials can enhance the lifetime of OLEDs. Comparing the fluorescent and phosphorescent emitting materials, pure organic-based fluorescent emitting materials have potential as stable emitters because the coordinated chemical bond of organic ligands to Ir metal of phosphorescent emitters is not as stable as the π-conjugated chemical bonds of fluorescent emitters.25 As described above, fluorescent materials are stable emitters but have low efficiency. Therefore, it is essential to develop emitting materials satisfying the requirements of high efficiency and a stable lifetime at the same time using the fluorescent emitters. It would be desirable to develop emitters with the stability of fluorescent organic emitters and the high efficiency of the phosphorescent emitters. Recently, it was found that thermally activated delayed fluorescence (TADF) emitters can reach the same QE level as PHOLEDs using pure organic compounds which can harvest both singlet and triplet excitons for radiative transition.26 As the TADF emitters have organicbased molecular structure for stability and exciton harvesting capability for high efficiency, they are suitable as high-efficiency and stable emitters for OLEDs that can replace fluorescent and phosphorescent emitters. In fact, there has been great progress in the improvement of the external QE (EQE) of the TADF OLEDs over the last couple of years, as is presented in Figure 1,

2. BASICS OF TADF EMITTERS 2.1. Basic Mechanism of TADFs. Generally, fluorescent materials emit light by a fast radiative transition with a time scale of approximately nanoseconds from a singlet excited state to a singlet ground state. However, some fluorescent materials give additional light emission at a time scale of approximately microseconds, which is called delayed fluorescence because the emission is delayed by the triplet to singlet conversion process. The delayed fluorescence process can be classified into E-type and P-type delayed fluorescence processes by the type of triplet to singlet conversion process.27−31 In the E-type delayed fluorescence originated from eosin, triplet excitons can be converted into singlet excitons by reverse intersystem crossing, which is activated by thermal energy and across a small singlet− triplet energy gap (ΔEST).32 This process is also called the TADF process. In the P-type delayed fluorescence originated from pyrene, triplet−triplet annihilation (TTA) or triplet− triplet fusion generates singlet excitons and contributes to fluorescent emission.33 The TADF emission and TTA emission are the same in that both emission processes utilize delayed fluorescence, but the two emission processes are different in terms of the delayed fluorescence component dependence on the temperature. The delayed fluorescence component gradually increases in the TADF and TTA processes, but there is an onset temperature only for the TADF emission as the delayed emission is activated by thermal activation in the TADF process. Theoretically, the internal QE of the TADF process is 100% because all triplet excitons can be transformed into singlet excitons, while that of the TTA process is 62.5% because the collision of two triplet excitons can make only one singlet exciton.34 Therefore, TADF emission is more efficient than TTA emission and can be comparable to phosphorescent emission. In the ideal TADF process by electroluminescence (EL), 25% of light emission is from prompt fluorescence, and 75% of light emission is from delayed fluorescence by reverse intersystem crossing. The prompt fluorescence is caused by fast S1 → S0 transition with an excited state lifetime (τ) of several nanoseconds, while the delayed fluorescence is brought about by slow T1 → S1 → S0 transition with a τ of longer than microseconds, where T1 is the triplet excited state, S1 is the singlet excited state, and S0 is the singlet ground state. The detailed emission processes by PL and EL processes of TADF materials are described in Figure 2. In the PL process, only singlet excitons are generated by photoexcitation and the singlet excitons decay radiatively (fluorescence) or nonradiatively (internal conversion) to S0, and some singlet excitons are converted into triplet excitons by an intersystem crossing (ISC) process. The triplet excitons are either converted into singlet excitons again by a reverse intersystem crossing (RISC) process or wasted by an internal conversion process. The up-converted triplet excitons decay radiatively by singlet emission or nonradiatively by internal conversion. In the EL process of TADF materials, both singlet and triplet excitons are formed by hole and electron injection. The singlet excitons decay radiatively or decay nonradiatively, or are transformed into triplet excitons by ISC. The triplet excitons

Figure 1. Recent progress of external quantum efficiency of red, green, and blue thermally activated delayed fluorescent organic light-emitting diodes.

and the EQE of the TADF OLEDs is almost comparable to that of PHOLEDs. Therefore, that this review traces the molecular design strategy of TADF emitters and forecasts future prospects for TADF devices is timely. In this review, we will cover organic-based TADF materials that have been developed over the last couple of years. We will mostly focus on TADF material design strategy from the viewpoint of photophysical properties and material characteristics for superior device performance. Donor and acceptor moieties adopted in the molecular design of the TADF emitters will be compared, and future design strategies for the 1947


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Figure 2. Radiative and nonradiative transition process of thermally activated delayed fluorescent emitters by photoluminescence (a) and electroluminescence (b).

where E is the orbital energy, and K is the electron repulsion energy. In the same molecule, the electron arrangement of singlet and triplet states is the same, and the E and K values are also the same in singlet and triplet states. However, the triplet state is stabilized by the presence of unpaired electrons in different orbitals, while the singlet state is relatively destabilized by the presence of paired electrons in the same orbital. The degree of stabilization and destabilization is J, and the singlet excited state and triplet excited state have an energy gap of 2J. The J value is decided by the overlap integral of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the TADF emitters, and a small overlap of the HOMO and LUMO favors a small J value. In other words, the ΔEST can be reduced by decreasing the HOMO and LUMO overlap, which can be accomplished by spatially separating the HOMO and LUMO through donor− acceptor structures or conjugation-breaking molecular structure. In most cases, donor−acceptor structures are employed to obtain the small ΔEST in the TADF emitters. 2.2.2. High PL Quantum Yield. PLQY reflects the efficiency of all radiative and nonradiative transition processes of the TADF emitters, as is presented in Figure 2. Supposing that phosphorescence is ignored, the radiative and nonradiative transitions include internal conversion (IC), prompt fluorescence (PF), intersystem crossing (ISC), reverse intersystem crossing (RISC), and delayed fluorescence (DF). Among these transitions, PF, DF, ISC, and RISC transitions are critical to the EQE of TADF emitters, and those photophysical transitions can be characterized by PLQY and the rate constant. In order to increase the EQE of TADF emitters, the PLQY of PF (ϕPF) and the PLQY of DF (ϕDF) should be maximized because they are related to the internal QE (ηint) of TADF emitters by the following equation.35

can be retransformed into singlet excitons by RISC or be deactivated by internal conversion. On the other hand, the triplet excitons directly generated by charge injection decay nonradiatively or are up-converted into singlet excitons followed by radiative singlet emission or nonradiative internal conversion. In order to obtain high EQE in the TADF devices, singlet emission by the up-converted triplet excitons needs to be maximized because 75% excitons generated by hole and electron injection are triplet excitons. Therefore, the triplet to singlet transformation and accompanying singlet emission processes are critical to the EQE of the TADF devices. As the efficiency of the TADF process is dominated by the delayed fluorescence process, it is crucial to develop TADF materials with high triplet-to-singlet conversion efficiency and high S1 → S0 transition efficiency at the same time. 2.2. Key Parameters of TADF Emitters. For efficient TADF emission in the emitters, there are several physical parameters that need to be optimized, among which the most important parameters are ΔEST and PL quantum yield (PLQY). The ΔEST is related to the up-conversion of a triplet exciton into a singlet exciton, while PLQY are connected with the radiative transition probability. 2.2.1. Small Singlet−Triplet Energy Gap. The most important physical parameter of TADF emitters is ΔEST, which is connected with the rate of reverse intersystem crossing (kRISC) by the following equation. ⎛ ΔE ⎞ kRISC ∝ exp⎜ − ST ⎟ ⎝ kBT ⎠

Small ΔEST increases the kRISC and triplet-to-singlet conversion efficiency, which may increase the EQE of TADF emitters. Therefore, most TADF emitters were designed to have small ΔEST for high EQE in TADF devices. The ΔEST of organic materials, which is defined as the energy gap between singlet energy and triplet energy, is generally dependent on the exchange energy between singlet excited state and triplet excited state because the singlet energy (ES) and triplet energy (ET) are defined by the following equation.32

ηint = nr,SϕPF + nr,SϕDF + nr,T

ϕDF ϕISC

Here, nr,S is the singlet exciton generation efficiency with a value of 0.25, nr,T is the triplet exciton production efficiency with a value of 0.75, and ϕISC is the quantum yield of singlet to triplet intersystem crossing. From this equation, it is clear that high ϕPF and high ϕDF/ϕISC are essential to reach high internal QE in the TADF devices. The ϕPF can be increased by efficient

ES = E + K + J ET = E + K − J 1948


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Chemistry of Materials fluorescence process, and the ϕDF/ϕISC can be enhanced when ΔEST is small, and ϕPF is large as described in the following equation. ϕDF /ϕISC =

1 1+

k nr ϕPF Αexp(−ΔEST / kT )

Both ϕPF and ϕDF/ϕISC should be maximized for high EQE in the TADF devices, but particularly, the ϕDF/ϕISC value is critical to the EQE of the TADF devices because 75% of the excitons generated by EL process are triplet excitons. From this perspective, the PLQY is closely related with the ΔEST. Figure 3. Schematic diagram relating the material parameters, device performances, and material design methods.

3. MOLECULAR DESIGN OF TADF EMITTERS TADF emitters have been developed to satisfy the basic key requirements of small ΔEST and high PLQY which are closely associated with light-emitting properties. Other than the two parameters, full width at half-maximum (FWHM) of the emission spectrum and stability of the materials can also be included as key requirements. As the parameters are related to each other, the TADF emitters were designed to optimize the parameters at the same time with little sacrifice of other factors. Many studies revealed that donor−acceptor design is the best approach to develop TADF emitters, and current emitters are constructed based on the donor−acceptor molecular design with a large separation of the HOMO and LUMO.26,35,36 However, the simple donor and acceptor based molecular design suffers from a broad light emission spectrum caused by charge transfer (CT) emissive state and sometimes short lifetime by instability of the chemical structure. The broad emission spectrum is related to color purity of the devices, and the instability of the molecular structure is associated with the lifetime of the devices. Therefore, engineering of the simple donor−acceptor design has been accommodated to be free from those issues. The material parameters mentioned above are linked with device characteristics such as EQE, color purity, lifetime, and efficiency roll-off. The EQE is dominated by PLQY, color purity is decided by FWHM, lifetime relies on material stability, and efficiency roll-off depends on τ of delayed fluorescence governed by ΔEST. In order to apply the TADF emitters in display applications, high EQE, good color purity, long lifetime, and little efficiency roll-off must be satisfied in the TADF devices, although good color purity determined by narrow FWHM is not required for the lighting applications. Therefore, molecular design strategy in this work will be explained based on the emitter characteristics interconnected with the device characteristics. A schematic diagram relating the material parameters, device performances, and material design methods is shown in Figure 3. 3.1. Donor and Acceptor Moieties of TADF Emitters. In order to manage the photophysical parameters, the donor and acceptor moieties should be properly selected. Strong donors and acceptors localize the HOMO and LUMO on the donor and acceptor moiety, respectively, which lowers the singlet energy of the TADF emitters with little effect on the triplet energy by spatial separation of the HOMO and LUMO. On the other hand, weak donors and acceptors induce less significant HOMO and LUMO localization, which results in a small decrease of singlet energy. Therefore, strong donor and acceptor moieties are superior to weak donor and acceptor moieties to decrease emission energy of TADF emitters.

There have been many donor and acceptor moieties applied to the design of TADF emitters, and they have different donor and acceptor strengths. The donor and acceptor strengths can be estimated from the HOMO and LUMO of each emitter. Deep HOMO implies weak electron donor strength, while shallow HOMO indicates strong electron donor strength. In the case of acceptor moieties, deep LUMO means strong acceptor strength, while shallow LUMO suggests weak acceptor strength. Typically, donor moieties are chosen from carbazole, benzofurocarbazole, thienocarbaozle, indolocarbazole, bicarbazole, amine, diamine, acridan, phenoxazine, phenothiazine, and phenazine, whereas acceptor moieties are selected from diphenyl sulfone, aromatic ketone, triazine, benzonitrile, phthalonitrile, triazole, oxadiazole, thiadiazole, benzothiazole, benzooxazole, quinoxaline, anthraquinone, and heptazine. Molecular simulation data showing the HOMO/LUMO distribution and HOMO/LUMO levels of the donor and acceptor moieties are given in Table 1 and Table 2. The molecular calculation results of the donor and acceptor moieties were obtained using the B3LYP 6-31G* basis set embedded in Gaussian 09 software. From the HOMO of the donors, the electron donating character of the donor moieties can be estimated, and the electron accepting character of the acceptor moieties can be judged by the LUMO of the acceptors as stated above. Among the donor moieties, acridan, phenoxazine, and phenothiazine are examples of strong donor moieties, while carbonitrile derived moieties are examples of strong acceptors. 3.2. Molecular Design for Short Delayed Fluorescence Lifetime. In the light emission process of TADF emitters, fluorescence and delayed fluorescence coexist at a time scale of tens of nanoseconds and microseconds, respectively. The fast decaying fluorescence does not induce nonradiative loss process, but the slow decaying delayed fluorescence involves efficiency reducing nonradiative mechanisms such as triplet− triplet annihilation, triplet−polaron annihilation, and so on because electrons would stay in triplet excited state for a long time in the microsecond range. Therefore, TADF emitters need to be constructed to minimize τ for delayed fluorescence. The material design approach shortening the τ for delayed fluorescence is the molecular design to reduce ΔEST as the kRISC governing the τ is increased by reducing ΔEST. There have been two main approaches to manage the ΔEST of TADF emitters, which were to intensify donor/acceptor strength in the molecule and to distort the donor and acceptor from a linker. Both methods were quite successful for small ΔEST and short τ. 1949


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Table 1. HOMO and LUMO Calculation Results of Donor Moieties of Thermally Activated Delayed Fluorescent Emitters

1950


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Chemistry of Materials Table 1. continued

3.2.1. Management of Donor and Acceptor Strengths. 3.2.1.1. Management of Donor and Acceptor Moieties. One simple way of decreasing ΔEST is to introduce strong donors and acceptors to construct the TADF emitters because they increase CT character of the materials and stabilize the singlet energy. In the case of donors, acridan, phenoxazine, phenazine, and phenothiazine were popular as the strong donor, while CN, heptazine, and CN substituted heteroaromatics were common as strong acceptors. Short τ for delayed fluorescence below 5 μs was mostly reported in the TADF emitters utilizing the strong donors and acceptors. Several acridan donor based TADF emitters could reach a few microseconds lifetime range by strong donor strength of acridan. DMAC-DPS is a well-known blue TADF emitter with a short τ of 3.1 μs.36,37 Although weakly electron accepting diphenylsulfone was an acceptor of DMAC-DPS, the strong donor character of acridan decreased the τ. Similar compounds like DMTDAc and CzAcSF also provided short τ of 1.2 and 5.6 μs, respectively.38−40 The TADF material design based on acridan and phenoxaborin also offered short τ below 5 μs.41 Phenoxaborin was employed as an acceptor unit of pure blue TADF emitters because of the good electron-accepting character of the boron atom included in the phenoxaborin. The boron atom of phenoxaborin was protected from the base or oxygen by a bulky triisopropylphenyl group in the final molecular design. The weak electron-accepting nature of the phenoxaborin

moiety rendered it to be combined with strong electrondonating moieties derived from acridan, which produced compounds 4, 5, and 6.41 All compounds decreased ΔEST up to 0.01 eV and τ for delayed fluorescence up to 1.6 μs. In the case of 5 and 6, acridan derived spirofluorene and spiroxanthene were donor moieties and exhibited donor strength as strong as that of acridan. Analogously, acridan based indoloacridan and Si acridan donors produced TrzIAc and DTPDDA which showed ΔEST/τ of 0.06 eV/1.6 μs and 0.14 eV/2.3 μs, respectively.42,43 The acridan donors were also linked with phthalonitrile to deliver DMAC-PN with ΔEST/τ of 0.06 eV/1.6 μs and with pyrazinedicarbonitrile to yield Ac-CNP with ΔEST/τ of 0.09 eV/3.3 μs.44,45 Although acridan was one of the most powerful donor moieties for short τ, a series of donor moieties of phenazine, phenothiazine, and phenoxazine were even stronger than acridan in terms of donor strength. The order of donor strength was phenazine > phenothiazine > phenoxazine > acridan according to the electron donating ability of N, S, O, and C in the donor moieties. As the donor strength increased, the singlet energy was lowered, ΔEST was reduced, and τ was shortened. Examples of phenazine donor containing TADF emitters are DHPZ-2BTZ, DHPZ-2TRZ, and DHPZ-2BN which showed ΔEST/τ of ∼0.00 eV/1.0 μs, 0.00 eV/0.1 μs, and 0.10 eV/1.88 μs, respectively.46 A very short τ less than 1.0 μs was observed because of the very strong donor character of the phenazine moiety. Analogously, the PPZ-DPS emitter also 1951


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Table 2. HOMO and LUMO Calculation Results of Acceptor Moieties of Thermally Activated Delayed Fluorescent Emitters

1952


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Chemistry of Materials Table 2. continued

1953


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Chemistry of Materials offered small ΔEST/τ of 0.08 eV/1.0 μs.36 However, PLQY of the phenazine type emitters was rather low by the low emission energy originated nonradiative loss process. It was used only in the design of red and green TADF emitters due to strong donor character, but it was not commonly used in the TADF structure because of the negative effect on PLQY. Phenothiazine and phenoxazine are stronger donor moieties than acridan but weaker donor moieties than phenazine. Therefore, they were in between phenazine and acridan in terms of material characteristics such as singlet energy, ΔEST, τ, and PLQY. Several derivatives of the phenothiazine and phenoxazine donors were reported as fast decaying TADF emitters, which include PXZ−PN with ΔEST/τ of 0.16 eV/ 1.4 μs and PTZ−PN with ΔEST/τ of 0.06 eV/1.1 μs.44 The trend of ΔEST/τ of TADF emitters followed the order of donor strength. Strong phenothiazine type PTZ−PN exhibited smaller ΔEST and shorter τ than phenoxazine type PXZ−PN. The same tendency was noticed in other TADF emitters possessing phenoxazine or phenothiazine. For instance, Px-CNP (ΔEST/τ of 0.04 eV/1.5 μs) with a phenoxazine donor showed reduced ΔEST and τ compared to that of Ac-CNP (ΔEST/τ of 0.09 eV/3.3 μs) with an acridan donor.45 Exactly the same trend was delivered in the Ac-VPN/Px-VPN and PXZ-TRZ/PTZ-TRZ emitters.45,47−49 Other than the materials mentioned above, several phenoxazine or phenothiazine type compounds provided short τ less than 5 μs.36,48,50,51 The carbazole based donor moiety, 3,3′-bicarbazole, was a strong donor moiety reducing ΔEST and τ of TADF emitters. Carbazole itself is a weak donor moiety, but the interlinked 3,3′-bicarbazole acts as a strong donor moiety because one extra carbazole unit adds donor characteristics. 33TCzPN built on the 3,3′-bicarbazole platform is a symbolic TADF emitter showing ΔEST/τ of 0.11 eV/2.35 μs.52 Considering that the TADF emitter with a similar molecular structure based on carbazole has a τ value longer than 10 μs, the strong donor strength of 3,3′-bicarbazole was a key factor for the short delayed fluorescence lifetime of 33TCzPN. The use of strong acceptors was also as successful as the strong donors to manipulate the ΔEST and τ of TADF emitters. Strong electron acceptors, 1,4-bis(phenylsulfonyl)benzene and 1,3-bis(phenylsulfonyl)benzene, delivered a 1 μs range lifetime in the DTC-pBPSB and DTC-mBPSB emitters.53 Donor moieties of DTC-pBPSB and DTC-mBPSB were t-butylcarbazole, but strong electron accepting power of the 1,4-bis(phenylsulfonyl)benzene and 1,3-bis(phenylsulfonyl)benzene acceptors afforded small ΔEST of 0.05 and 0.24 eV, and short τ of 1.23 and 1.16 μs, respectively. Boron derived acceptor units, trimesityl boron and pyridyl pyrrolide boron, also played a role of strong acceptor in the TADF emitters. Three TADF emitters, PXZ-Mes3B, 2DAC-Mes3B, and DAC-Mes3B, were trimesityl boron type compounds utilizing the electron deficiency of trimesityl boron moiety.54 Delayed fluorescence lifetime was not reported, but ΔEST was smaller than 0.1 eV in all trimesityl boron derivatives. Boron complexes having the pyridyl pyrrolide electron deficient units, PrFPCz, PrFCzP, and PrFTPA, worked similarly to the trimesityl boron materials by realizing ΔEST smaller than 0.1 eV and τ less than 5 μs.55 Particularly, the pyridyl pyrrolide type boron complexes achieved a delayed fluorescence lifetime utilizing weak carbazole donors, demonstrating a strong acceptor character of the boron based acceptor units. 3.2.1.2. Management of the Number of Donor Moieties. Donor and acceptor character of the TADF emitters can be

controlled by the donor and acceptor strength of each donor and acceptor itself as explained in section 3.2.1.1, and it can also be managed by the number of donors or acceptors. Addition of more donors or acceptors increases the donor and acceptor character of the molecules and has the same effect of using strong donors and acceptors in the molecular structure. Therefore, multiple donor based emitters were diversely developed to minimize ΔEST and τ. An early TADF emitter applying the concept of multiple donors was 4CzIPN which has four carbazole donor moieties.26 The four carbazole donors strengthened donor character and CT character by distortion of the donors by steric hindrance, resulting in reduced singlet energy, ΔEST and τ. Compared with ΔEST (0.09 eV) and τ (166 μs) of a similarly designed 2CzPN emitter, the ΔEST and τ were significantly reduced to 0.083 eV and 5.1 μs by the four carbazole donors.56 The same effect was observed in the similarly designed 4CzTPN, 4CzTPN-Me, and 4CzTPN-Ph with four carbazole donors.26,57 Motivated by the multiple donor approach of 4CzIPN, various TADF emitters were designed to have several carbazole type donors. 4CzIPN derived compounds, t-4CzIPN and m-4CzIPN, were green TADF emitters with ΔEST/τ values of 0.05 eV/2.9 μs and 0.01 eV/2.6 μs, respectively.58 In place of the carbazole donor of 4CzIPN, t-butylcarbazole and methylcarbazole were donors of t-4CzIPN and m-4CzIPN, which further accelerated the delayed fluorescence by strong donor strength. Introduction of five carbazole type donors applied in the development of 5CzBN(5CzCN) and 5TCzBN(5CzCN) was in line with the multiple donor approach.59 Five donor moieties strengthened the donor strength, reducing the ΔEST/τ values to 0.22 eV/3.7 μs and 0.17 eV/3.4 μs in the 5CzBN(5CzCN) and 5TCzBN emitters, respectively. As observed in other results, delayed fluorescence facilitating the effect by t-butylcarbazole brought about the relatively short τ in the 5TCzBN. Even though absolute τ was not short, the comparison of DCzTrz and TCzTrz also supported the multidonor plan as a TADF quickening method.60,61 3.2.2. Distortion between Donors and Acceptors. Strong CT character is beneficial to the small ΔEST and τ, which is enabled by the distorted backbone structure between donor and acceptor. One of the efficient molecular design approaches for the distorted molecular structure was to connect the donor and acceptor via the ortho-position of a phenyl linker because steric hindrance between the donor and acceptor increased the dihedral angle. Three materials, oBFCzTrz, mBFCzTrz, and pBFCzTrz, were model compounds verifying the effectiveness of the ortho-linking concept for molecular distortion.62 It was demonstrated that oBFCzTrz was better than mBFCzTrz and pBFCzTrz to reduce both ΔEST and τ. The ΔEST of oBFCzTrz was reduced from 0.19 eV of mBFCzTrz and 0.30 eV of pBFCzTrz to 0.002 eV, which also resulted in short τ of 5.4 μs in the oBFCzTrz emitter compared to about 30 μs of mBFCzTrz and pBFCzTrz. The short τ of oBFCzTrz dramatically improved efficiency roll-off of the TADF devices, resulting in less than 15% reduction of the EQE even at 1,000 cd/m2 from the maximum values. Steric hindrance induced CT character increase was also demonstrated in compound 7 with a 1,3,6,8-tetramethylcarbazole donor.41 The two methyl units at 1 and 8 positions of carbazole enforced steric hindrance between a phenyl linker and the carbazole donor, which arranged the donor perpendicular to the phenyl linker and intensified CT character in the 7 emitter. Therefore, small ΔEST and τ values of 0.01 eV 1954


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Figure 4. Molecular design approaches and chemical structures of representative materials for short delayed fluorescence lifetime. ΔEST and τ are indicated in parentheses.

and 3.49 μs were obtained in the 7 emitter. Similar donor distortion was also a contributor to small ΔEST and τ in many TADF emitters possessing acridan, phenoxazine, and phenothiazine derived donor moieties because they align vertically to the phenyl linker. Molecular design methods for short τ and representative materials in each design approach category are summarized in Figure 4. 3.3. Molecular Design for High PLQY. There have been many molecular designs to enhance the PLQY of the TADF emitters, and many compounds were reported as high PLQY emitters. Although the chemical structures are different, the high PLQY emitters can be classified into three categories according to the design method. The three categories are phenyl linker type design, HOMO dispersing design, and dual emitting core design. 3.3.1. Phenyl Linker Type Design. In early work reporting TADF emitters, phenyl linker free design was popular because the necessity of the phenyl linker was not understood. However, the pioneering work of the Adachi group recommended the use of a phenyl linker in the molecular structure for high PLQY through HOMO and LUMO overlap to a large extent. In the early TADF emitters like PIC-TRZ, PIC-TRZ2, CC2TA, and CzT, the PLQY of the emitters was below 70% because of extensive HOMO and LUMO isolation which hindered light absorption and emission processes.34,35,63−66 Therefore, the concept of phenyl linker design was proposed to manage the HOMO and LUMO separation for high PLQY. The concept underlying the phenyl linker design is that the phenyl linker reduces the HOMO and LUMO isolation

because both the HOMO and LUMO can be extended to the aromatic phenyl linker. The HOMO and LUMO extension to the phenyl linker is desired for high oscillator strength reflecting light absorption, which eventually can lead to high PLQY in the TADF emitter. The first phenyl linker included TADF emitter is 4CzIPN which attained high PLQY over 90% both in solution and film. In the case of 4CzIPN, two electron withdrawing CN units and four electron donating carbazole units were attached to the phenyl linker, which led to high PLQY over 90% and high EQE above 20% by HOMO/LUMO overlap at the phenyl linker from molecular orbital calculation.67−70 After the successful demonstration of the phenyl linker concept, many TADF emitters were built by attaching donor and acceptor moieties to the phenyl linker, and they confirmed the superiority of the phenyl linker design in terms of PLQY. One of the best performing red TADF emitter, HAP-3TPA, was developed by attaching a heptazine acceptor and t-butyl modified aromatic amine donors to the phenyl linker.71 As a red TADF emitter, the HAP-3TPA emitter exhibited high PLQY of 95% and EQE of 17.5%. One of the best green emitters, DMAC-Trz, showing high PLQY of 90% and EQE of 26.5% also has a core structure with a diphenyltriazine acceptor and an acridan derived donor at the para-position of a phenyl linker.72 In the same way, the best blue emitting spiroacridan-triazine hybrid, spiroAc-Trz,73 and blue emitting TCzTrz compounds were also built on the phenyl linker design platform.61 Although the phenyl linker was essential in the chemical structure of high PLQY TADF emitters, there is a drawback to consider for optimized TADF characteristics. In the linker 1955


Review

Chemistry of Materials based design, ΔEST of TADF emitters becomes rather large because of increased singlet energy and decreased triplet energy by the phenyl linker. The singlet energy is increased in the phenyl linker based structure because of weakened CT character by less HOMO/LUMO separation. Whereas, the triplet energy is reduced by the conjugation extending effect of the phenyl linker. Therefore, the ΔEST is increased, which accompanies side effects like long τ and efficiency roll-off in the devices. The high singlet energy issue can be avoided by chemical designs strengthening CT character such as the use of strong donors and acceptors, distorted backbone structure, and combination of the two designs. Examples of the first plan are HAP-3TPA having a strong t-butyl modified diphenylamine donor, DPA-TRZ, with a diphenylamine modified diphenylamine donor, and 2DAC-Mes3B possessing a diphenylamine modified carbazole.54,71,74 Examples of the second plan are 2CzPN, BFCz-2CN, and BTCz-2CN which have donor moieties at the ortho-position of the phenyl linker.75 Compounds in the third category are DMAC-DPS, PXZ-DPS, 3ACR-TRz, PXZQ, and DTPDDA which are constructed utilizing acridan or phenoxazine type donors.36,43,54,76,77 In particular, the second and third plans can also overcome the low triplet energy issues of the phenyl linker based compounds. Therefore, many TADF emitters with both high PLQY and small ΔEST have the chemical structure classified into the second and third categories of the phenyl linker designs. 3.3.2. HOMO Dispersing Design. In addition to the linked implemented design, the HOMO dispersing molecular design was effective for high PLQY in the TADF emitters. The idea of the HOMO dispersing design is that the uniform distribution of the HOMO would increase HOMO/LUMO overlap between donors and acceptors in the backbone structure. The increased HOMO/LUMO overlap would increase oscillator strength and improve PLQY of the TADF emitters. The HOMO dispersing approach was effectively integrated in the design of 4CzIPN with four HOMO managing carbazole donor moieties.26 Compared to DCzIPN with two carbazole units, 4CzIPN achieved much higher PLQY of 94% by the HOMO distributing function of the four carbazole donors.78 The high PLQY brought about high EQE above 20% in the TADF device for the first time, and further device work enhanced the EQE to above 30%. On the basis of the same idea, TCzTrz and TmCzTrz with three carbazole type donors were explored as high PLQY compounds advanced from DCzTrz with two carbazole donors.61 Both emitters exhibited 100% PLQY upgraded from 43% of DCzTrz, manifesting the PLQY enhancing role of the additional carbazole unit by delocalizing the HOMO. One of the highest EQE (25.0%) of the blue TADF device was reported in the TCzTrz by the high PLQY. Five carbazole based donor implemented 5CzBN(5CzCN) and 5TCzBN devices also confirmed the PLQY increasing role of the five donors by uniform HOMO dispersion.59,79 EQE values of the 5CzBN(5CzCN) and 5TCzBN devices were 16.7% and 21.2%, respectively. In addition to the HOMO delocalization by multidonors, wide HOMO dispersion by a bulky donor structure also delivered high PLQY in the TADF emitters. 2a, 2b, and 2c compounds were emitters with the HOMO dispersing donor element instead of carbazole.35 Two 3,6-diphenylcarbazole substituted carbazole, 3,6-dicarbazolylcarbazole, and 3-carbazolylcarbazole were donors of 2a, 2b, and 2c compounds. The HOMO dispersing ability of the three emitters was in the order of 2a > 2b > 2c, which coincided with the order of PLQY

of 2a (100%) > 2b (95%) > 2c(93%). Therefore, the EQE of the TADF emitter doped devices was also in the order of 2a (20.6%) > 2b (16.8%) > 2c (14.6%). Two diphenylamine modified carbazoles at 3 and 6 positions also spread the HOMO over the large donor moiety, delivering high PLQY of 100% and EQE of 29.6% in the DACT-II emitter derived from the diphenyltriazine acceptor.80 The same donor was utilized in the design of the 2DAC-Mes3B emitter and provided high PLQY of 100% and EQE 21.6% in the green devices.54 The DACQ emitter was also constructed using the same donor and gave high PLQY of 84%, although the EQE was rather low.76 One diphenylamine substituted carbazole was the HOMO spreading donor in the DAC-Mes3B and m-ATP-CDP emitters,50,54 but it was not as efficient as the corresponding two diphenylamine substituted carbazole. Encouraged by the high PLQY of the two diphenylamine modified carbazole type emitters, two diphenylamine modified diphenylamine was implemented as a building block of DPA-TRZ with 100% PLQY.74 The HOMO dispersing effect of the diphenylamine type donor was proposed as the origin of the high PLQY, but only moderate EQE of 13.8% was obtained. HOMO spreading was also accomplished by a bicarbazole donor employed in the development of 26IPNDCz and 35IPNDCz.81 In the two TADF emitters, the PLQY was below 80%, and EQE was below 10% due to the improper selection of acceptors. However, the EQE was dramatically increased in the BPBCz (23.3%) and TrzBCz (23.6%) emitters having benzophenone and diphenyltriazine as acceptors in combination with the bicarbazole donor, respectively.82 Even further enhancement of the PLQY and EQE was achieved in the 33TCzPN and 33TCzTTrz emitters which realized high PLQY of 87%.52 Particularly, the 33TCzTTrz could accomplish EQE of 25.0% as a green TADF emitter.83 3.3.3. Dual Emitting Core Design. Another molecular design for high PLQY was dual emitting core design which has two TADF emitter units in one molecule. The hypothesis of the dual emitting core design is that two emitter units would enhance light absorption and emission of emitters compared to one emitter unit. The first dual emitting core based TADF emitter was DDCzIPN which was prepared by direct coupling of two DCzIPN emitter units.84 As intended, the absorption coefficient was increased, the PLQY was also enhanced from 0.67 of DCzIPN to 0.91, and EQE of the device was improved from 16.4% of DCzIPN to 18.9% by the coupled molecular structure. However, the emission spectrum of DDCzIPN was shifted to long wavelength due to the increased extent of conjugation. To avoid the color shift problem of the original dual emitting core design, a molecular structure coupling two TADF emitters via a donor moiety was suggested, which could manage the emission spectra of the dual emitting core emitters. This concept was proven in two series of compounds derived from dicyanobenzene and diphenyltriazine acceptors. Dicyanobenzene type compounds with different interlinked positions among carbazole units, 33TCzPN, 34TCzPN, and 44TCzPN, showed different emission spectra depending on the interlinked position.52 The 33TCzPN with two carbazole units linked via the 3-position of each carbazole exhibited a red-shifted emission spectrum due to the small dihedral angle between two carbazole units. In other compounds, the large dihedral angle suppressed the spectral change of the compounds from the single emitting core compound. Analogous results were also obtained in the diphenyltriazine type 23TCzTTrz, 34TCzTTrz, and 33TCzTTrz emitters.83 In all dual emitting core type TADF 1956


Review


Figure 5. Molecular design approaches and chemical structures of representative materials for high PLQY. The PLQY of each emitter is shown in parentheses.

emitters, the PLQY was increased due to improved light absorption, which resulted in high EQE in the TADF devices. Molecular design methods for high PLQY and representative materials in each design approach category are summarized in Figure 5. 3.4. Molecular Design for Narrow Emission. TADF emitters are differentiated from traditional fluorescent emitters in that the origin of fluorescent emission is intramolecular CT singlet excited state rather than local singlet excited state. In general, the CT excited state can be generated in various forms with different energies, and the CT based TADF emitters may have various CT excited states. Therefore, the CT based TADF emitters show very broad emission spectrum originating from the CT singlet excited state. The broad emission spectrum can be advantageous in the lighting applications, but it should be avoided in the display applications which require high color purity. Currently, OLEDs are being used in display products rather than lighting products, indicating that TADF emitters with a narrow emission spectrum should be developed. Molecular design methods for the narrow emission spectrum can be classified into two approaches. One method is to freeze the donor−acceptor based core structure by fused structure or sterically hindered structure. The other method is to adopt a rigid acceptor structure to minimize vibrational motion of the TADF emitters. Although the two strategies were successful in narrowing the emission spectrum, the first method was much better than the second method because the first approach can fix overall molecular structure, and the second approach can locally rigidify the molecule. The broad emission spectrum by CT emission is related to the rotation between donor and acceptor moieties, indicating that restricted rotation between donor and acceptor moieties

may sharpen the emission spectrum of TADF emitters. An efficient molecular design approach to prevent the molecular rotation was to adopt a polycyclic aromatic backbone structure. DABNA-1 and DABNA-2 were pure blue TADF emitters showing a small fwhm less than 30 nm.85 Electron deficient boron and electron rich sp3 nitrogen were included in the core structure for small ΔEST, but they were interconnected in a polycyclic aromatic structure to prohibit the molecular motion of the donor−acceptor based core structure. The rigidity of the polycyclic core structure and suppression of rotation between donor and acceptor units were responsible for the narrow emission spectra of the DABNA-1 and DABNA-2 TADF devices. The sharp EL emission in the DABNA-1 and DABNA-2 devices led to pure blue emission with a color coordinate of (0.13,0.09) and (0.12,0.13), respectively. In addition, the DABNA-2 device exhibited high EQE of 20.2%, which is one of the best EQEs of deep blue emitting TADF emitters. Large steric hindrance based molecular design was also effective to realize narrow emission in TADF emitters. CzBPCN was the blue TADF emitter designed to have large steric hindrance at the central core structure by bulky carbazole units in order to prevent molecular rotation of the backbone structure.86 The CzBPCN emitter was compared with CNBPCz with CN units instead of carbazole units at the central core, which proved that the steric hindrance induced rigidified molecular structure was responsible for the small fwhm (48 nm) of CzBPCN. The CzBPCN device showed deep blue emission color with a color coordinate of (0.14,0.12) due to the narrow emission spectrum. EQE of the CzBPCN device was 14.0%. In addition to the molecular design approach suppressing molecular rotation, a molecular design approach introducing a 1957


Review


materials possessing donor and acceptor moieties among triazine, benzonitrile, and carbazole derived chemical moieties. Among the TADF emitters, 4CzIPN is the most stable green-emitting material which realized a long lifetime of 1380 h at 1,000 cd/m2.93 The 4CzIPN emitter is made up of chemically stable two CN acceptor moieties and four carbazole donor moieties in order to secure stability in the TADF devices. Several lifetime studies comparing the lifetime of 4CzIPN and phosphorescent Ir(ppy)3 emitter were performed, and they revealed that 4CzIPN is comparable to Ir(ppy)3 in terms of lifetime.94,95 In the case of 4CzIPN, the lifetime was optimized at high doping concentration because of strong electron trapping by 4CzIPN unlike the Ir(ppy)3 phosphorescent emitter. The narrow emission zone close to the electron transport layer of the 4CzIPN device was found to be broadened in the highly doped devices by facilitated electron transport through the 4CzIPN emitters, resulting in a long lifetime in the highly doped 4CzIPN devices.96 The CN and cabazole combined material design was also adopted to develop stable blue-emitting 5CzCN(5CzBN) blue TADF compounds.79 Instead of two CN and four carbazole units of 4CzIPN, one CN and five carbazole unit merged 5CzCN(5CzBN) could behave as a blue TADF emitter due to weakened acceptor character. As the 5CzCN(5CzBN) emitter has a molecular structure similar to that of 4CzIPN, it was also stable in the long term stability test under constant driving conditions. The 5CzCN(5CzBN) TADF emitter was compared with a blue-emitting Ir(dbi)3 phosphorescent emitter and survived longer than Ir(dbi)3 up to 50% of initial luminance. A derivative of 5CzCN(5CzBN), 5TCzBN, was even better than 5CzCN(5CzBN) in the lifetime test, although the reason for the improved lifetime is not clear. At the moment, the best lifetime reported in the literature is 770 h at 500 cd/m2 from the 5TCzBN device.59 Another type of molecular design for long lifetimes is to merge a triazine acceptor with carbazole derived donor moieties. DDCzTrz is a representative blue TADF emitter constructed using the triazine and carbazole derivatives. Planar structure of a triphenyltriazine moiety was proposed as the molecular structure stabilizing element of DDCzTrz in addition to the stable carbazole moieties. The DDCzTrz outperformed the Ir(dbi)3 triplet emitter at the same lifetime test condition. In spite of the blue-emitting color coordinate of (0.15,0.22), up to 80% of initial luminance lifetime of 52 h at 500 cd/m2 was achieved in the DDCzTrz device. However, triplet excitons could not be completely harvested in the DDCzTrz device, and further lifetime work is needed using an optimized device structure which can fully utilize all triplet excitons for emission. Other triazine and carbazole joined compounds such as oBFCzTrz, mBFCzTrz, pBFCzTrz, and TrzBCz were also studied as long-lasting blue TADF emitters of blue TADF devices.62,82 Other than the results described, lifetime data of TADF devices were rarely reported because many acceptor moieties

rigid acceptor moiety was also useful to decrease fwhm of TADF devices, although it was not as good as the rotation managing molecular design. One of the representative rigid acceptor moieties for sharp emission is heptazine which was used as the acceptor moiety of red-emitting HAP-3TPA. The heptazine acceptor has a heteroaromatic type core structure with all C = N bonds interconnected. Therefore, the vibrational motion of the heptazine core is limited, and overall molecular vibration of the HAP included compounds would be suppressed. A red-emitting HAP-3TPA showed a small fwhm of about 80 nm with an EQE of 17.5%. Although the fwhm of HAP-3TPA was not as small as an absolute value, it was relatively small compared to that of other orange or red TADF emitters. In the case of red TADF emitters, strong CT character broadened the emission spectrum, resulting in fwhm close to or higher than 100 nm.26,46,87,88 Similarly, a modified version of diphenylsulfone, DMTDAc, could also reduce the fwhm of deep blue TADF emitters.38 A well-known TADF emitter with a diphenylsulfone acceptor, DMAC-DPS, was a high efficiency deep blue emitter providing high EQE above 20%, but the EL spectrum of the DMAC-DPS device was rather broad.89−92 In order to reduce the fwhm of DMAC-DPS while preserving the high EQE performance, the diphenylsulfone acceptor was tied by a dimethylcarbon linker because the fused structure would disallow molecular motion between sulfone and phenyl unit. Although this design method was not as effective as the whole molecular motion suppressing design method, the fwhm was improved to 65 nm compared to 72 nm of DMAC-DPS. Representative TADF materials with narrow emission spectrum are summarized in Figure 6. 3.5. Molecular Design for Stability. Lifetime is one of the most important device performances of OLEDs because short lifetime devices cannot be used in practical applications. Therefore, the TADF emitters should also be designed to withstand degradation during the operation of OLEDs. However, not many studies reported the molecular design rules of TADF emitters for long lifetimes. Only several works investigated the lifetime of TADF devices. From a chemical structure point of view, only stable moieties are allowed in the design of TADF emitters for operational stability. During device operation, the TADF emitters are exposed to positive polaron, negative polaron, and excitons, so they should be stable under charged states. The stability of the molecules can be judged by bond dissociation energy (BDE) under positive polarons or negative polarons. Among the donor moieties, aromatic amine or acridan derived donors have low BDE under negative polarons, but carbazole type donors have relatively high BDE. Among the acceptor moieties, diphenylsulfone or diphenylphosphine oxide type acceptors have small BDE values, but diphenyltriazine or benzonitrile type acceptors have large BDE values. Therefore, TADF emitter developments for long-term stability were directed to design and synthesize

Figure 6. Chemical structures of TADF materials for narrow emission spectra. The fwhm of each emitter is shown in parentheses. 1958


Review


Figure 7. Chemical structures of stable TADF materials. Lifetime and initial luminance for lifetime measurements are also indicated in parentheses.

of Korea funded by the Ministry of Science, ICT, and Future Planning and development of red and blue OLEDs with external quantum efficiency over 20% using delayed fluorescent materials funded by MOTIE.

such as diphenylsulfone, benzophenone, and arylboron are not chemically stable. Moreover, many donor moieties derived from acridan are not applicable in the stable TADF emitter design due to the instability of the acridan moiety. Examples of stable materials are summarized in Figure 7.

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ABBREVIATIONS DMAC-DPS 10,10′-(sulfonylbis(4,1-phenylene))bis(9,9dimethyl-9,10-dihydroacridine) DMTDAc 2,7-bis(9,9-dimethylacridin-10(9H)-yl)-9,9dimethyl-9H-thioxanthene 10,10-dioxide CzAcSF 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine 4 9,9-dimethyl-10-(10-(2,4,6-triisopropylphenyl)-10H-dibenzo[b,e][1,4]oxaborinin3-yl)-9,10-dihydroacridine 5 10-(10-(2,4,6-triisopropylphenyl)-10Hdibenzo[b,e][1,4]oxaborinin-3-yl)-10Hspiro[acridine-9,9′-fluorene] 6 10-(10-(2,4,6-triisopropylphenyl)-10Hdibenzo[b,e][1,4]oxaborinin-3-yl)-10Hspiro[acridine-9,9′-xanthene] TrzIAc 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7,7,13,13-tetramethyl-7,13-dihydro5H-indeno[1,2-b]acridine DTPDDA 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline DMAC-PN 4,5-bis(9,9-dimethylacridin-10(9H)-yl)phthalonitrile Ac-CNP 5,6-bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)pyrazine-2,3-dicarbonitrile DHPZ-2BTZ 5,10-bis(4-(benzo[d]thiazol-2-yl)phenyl)4a,5,10,10a-tetrahydrophenazine DHPZ-2TRZ 5,10-bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-4a,5,10,10a-tetrahydrophenazine DHPZ-2BN 4,4′-(phenazine-5,10-diyl)dibenzonitrile PPZ-DPS 10,10′-(sulfonylbis(4,1-phenylene))bis(5phenyl-5,10-dihydrophenazine) PXZ−PN 4,5-di(10H-phenoxazin-10-yl)phthalonitrile PTZ−PN 4,5-di(10H-phenothiazin-10-yl)phthalonitrile Px-CNP 5,6-bis(4-(10H-phenoxazin-10-yl)phenyl)pyrazine-2,3-dicarbonitrile Ac-VPN 4,4″-bis(9,9-dimethylacridin-10(9H)-yl)[1,1′:2′,1″-terphenyl]-4′,5′-dicarbonitrile Px-VPN 4,4″-di(10H-phenoxazin-10-yl)-[1,1′:2′,1″terphenyl]-4′,5′-dicarbonitrile PXZ-TRZ 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-phenoxazine PTZ-TRZ 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-phenothiazine

4. SUMMARY AND OUTLOOK Red, green, and blue TADF emitters for high-efficiency and long-lifetime TADF OLEDs have been developed over the last couple of years, and significant progress in device performance was made by designing and synthesizing new TADF emitters satisfying photophysical property requirements such as high PLQY, a short τ, small FWHM, and stability. It is true that each material parameter was improved by devising chemical structures overcoming those issues, but no material is available satisfying the requirements at the same time. Each parameter was optimized separately, which accompanied by sacrifice of other parameters. Therefore, future material development should be directed to design materials which can fulfill all the requirements of TADF emitters. The main issues discussed above are interrelated to each other and simultaneous consideration of the design criteria is essential in the molecular design of TADF emitters because each issue cannot be solved independently. The ideal molecular design for the TADF emitters would be a molecular structure with stable donor and acceptor moieties for stability, and the donor and acceptor moieties should be rigid moieties for small fwhm. Furthermore, the stable and rigid donor and acceptor moieties should be linked via an aromatic linker for high PLQY. In addition, strong electron donating power and electron withdrawing power by the donor and acceptor either through strong donors and acceptors or through several donors and acceptors are also essential for short τ in the ideal molecular design. Although the overall device performances of the TADF OLEDs are not as good as those of PHOLEDs, it is expected that the ideal design may enhance the EQEs, efficiency roll-off, color purity, and lifetimes of the TADF OLEDs to the same levels as those of PHOLEDs in the near future.

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AUTHOR INFORMATION

ORCID

Jun Yeob Lee: 0000-0002-7677-0605 Author Contributions §

Y.I. and M.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program (2016R1A2B3008845) through the National Research Foundation 1959


Review

Chemistry of Materials 6,6′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)bis(4-(9H-carbazol-9-yl)isophthalonitrile) DTC-pBPSB 1,4-bis((4-(3,6-ditert-butyl-9H-carbazol-9yl)phenyl)sulfonyl)benzene DTC-mBPSB 3,6-ditert-butyl-9-(3-((3-((4-(3,6-ditertbutyl-9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)sulfonyl)phenyl)-9H-carbazole PXZ-Mes3B 10-(4-(dimesitylboryl)-3,5-dimethylphenyl)10H-phenoxazine 2DAC-Mes3B 9-(4-(dimesitylboryl)-3,5-dimethylphenyl)N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6diamine DAC-Mes3B 9-(4-(dimesitylboryl)-3,5-dimethylphenyl)N,N-diphenyl-9H-carbazol-3-amine PrFPCz 5,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,3-bis(trifluoromethyl)-5H-pyrrolo[1′,2′:3,4][1,3,2]diazaborolo[1,5-a]pyridin-6-ium-5uide PrFCzP 5,5-bis(9-phenyl-9H-carbazol-3-yl)-1,3-bis(trifluoromethyl)-5H-pyrrolo[1′,2′:3,4][1,3,2]diazaborolo[1,5-a]pyridin-6-ium-5uide PrFTPA 5,5-bis(4-(diphenylamino)phenyl)-1,3-bis(trifluoromethyl)-5H-pyrrolo[1′,2′:3,4][1,3,2]diazaborolo[1,5-a]pyridin-6-ium-5-uide 4CzIPN 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile 2CzPN 4,5-di(9H-carbazol-9-yl)phthalonitrile 4CzTPN 2,3,5,6-tetra(9H-carbazol-9-yl)terephthalonitrile 4CzTPN-Me 2,3,5,6-tetrakis(3,6-dimethyl-9H-carbazol-9yl)terephthalonitrile 4CzTPN-Ph 2,3,5,6-tetrakis(3,6-diphenyl-9H-carbazol-9yl)terephthalonitrile t-4CzIPN 2,4,5,6-tetrakis(3,6-ditert-butyl-9H-carbazol9-yl)isophthalonitrile m-4CzIPN 2,4,5,6-tetrakis(3,6-dimethyl-9H-carbazol-9yl)isophthalonitrile 5CzBN(5CzCN) 3-(3-(tert-butyl)-9H-carbazol-9-yl)-2,4,5,6tetra(9H-carbazol-9-yl)benzonitrile 5TCzBN 3-(3-(tert-butyl)-6-isopropyl-9H-carbazol-9yl)-2,4,5,6-tetrakis(3,6-ditert-butyl-9H-carbazol-9-yl)benzonitrile DCzTrz 9,9′-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3phenylene)bis(9H-carbazole) TCzTrz 9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)tris(9H-carbazole) oBFCzTrz 5-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole mBFCzTrz 5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole pBFCzTrz 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole 7 1,3,6,8-tetramethyl-9-(10-(2,4,6-triisopropylphenyl)-10H-dibenzo[b,e][1,4]oxaborinin-3-yl)-9H-carbazole PIC-TRZ 12,12′-(6-([1,1′-biphenyl]-4-yl)-1,3,5-triazine-2,4-diyl)bis(11-phenyl-11,12dihydroindolo[2,3-a]carbazole) PIC-TRZ2 12-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-phenyl-5,12-dihydroindolo[3,2-a]carbazole CC2TA 9,9″-(6-phenyl-1,3,5-triazine-2,4-diyl)bis((9H-3,9′-bicarbazole)) CzT 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole

HAP-3TPA

33TCzPN

DPA-TRZ BFCz-2CN BTCz-2CN PXZ-DPS 3ACR-TRz PXZQ DCzIPN TmCzTrz 2a 2b 2c DACT-II DACQ m-ATP-CDP 26IPNDCz 35IPNDCz BPBCz TrzBCz 33TCzTTrz DDCzIPN 34TCzPN 44TCzPN 23TCzTTrz 34TCzTTrz DABNA-1 DABNA-2

CzBPCN

1960

4,4′,4″-(1,3,3a-1,4,6,7,9-heptaazaphenalene2,5,8-triyl)tris(N,N-bis(4-(tert-butyl)phenyl)aniline) N 1 -(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-N 1 -(4-(diphenylamino)phenyl)N4,N4-diphenylbenzene-1,4-diamine 4,5-bis(5H-benzofuro[3,2-c]carbazol-5-yl)phthalonitrile 4,5-bis(5H-benzo[4,5]thieno[3,2-c]carbazol5-yl)phthalonitrile 10,10′-(sulfonylbis(4,1-phenylene))bis(10H-phenoxazine) 2,4,6-tris(4-(9,9-dimethylacridin-10(9H)yl)phenyl)-1,3,5-triazine 10-(4-(quinoxalin-2-yl)phenyl)-10H-phenoxazine 4,6-di(9H-carbazol-9-yl)isophthalonitrile 9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)tris(3,6-dimethyl-9Hcarbazole) 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-3,3″,6,6″-tetraphenyl-9′H9,3′:6′,9″-terbenzo[b]indole 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′H-9,3′:6′,9″-terbenzo[b]indole 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-3,9′-bicarbazole 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6-diamine N3,N3,N6,N6-tetraphenyl-9-(4-(quinoxalin-2yl)phenyl)-9H-carbazole-3,6-diamine 9,9′-(dibenzo[f,h]quinoxaline-7,10-diyl)bis(N,N-diphenyl-9H-carbazol-3-amine) 2,2′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)diisophthalonitrile 5,5′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)diisophthalonitrile bis(4-(9′-phenyl-9H,9′H-[3,3′-bicarbazol]9-yl)phenyl)methanone 9′,9‴-((6-phenyl-1,3,5-triazine-2,4-diyl)bis(4,1-phenylene))bis(9-phenyl-9H,9′H-3,3′bicarbazole) 9,9′-bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H,9′H-3,3′-bicarbazole 3,3′,5,5′-tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-2,2′,6,6′-tetracarbonitrile 6,6′-(9H,9′H-[3,4′-bicarbazole]-9,9′-diyl)bis(4-(9H-carbazol-9-yl)isophthalonitrile) 6,6′-(9H,9′H-[4,4′-bicarbazole]-9,9′-diyl)bis(4-(9H-carbazol-9-yl)isophthalonitrile) 9,9′-bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H,9′H-2,3′-bicarbazole 9,9′-bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H,9′H-3,4′-bicarbazole 5,9-diphenyl-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene 9-([1,1′-biphenyl]-3-yl)-N,N,5,11-tetraphenyl-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-3-amine 4,4′,6,6′-tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-3,3′-dicarbonitrile DOI: 10.1021/acs.chemmater.6b05324 Chem. Mater. 2017, 29, 1946−1963

Review

Chemistry of Materials CNBPCz Ir(ppy)3 Ir(dbi)3 DDCzTrz

■

Exciton-Confining Structure for Reduced Efficiency Roll-Off. Adv. Mater. 2008, 20, 4189−4194. (18) Wang, X.; Gong, S.-L.; Song, D.; Lu, Z.-H.; Wang, S. Highly Efficient and Robust Blue Phosphorescent Pt(II) Compounds with a Phenyl-1,2,3-triazolyl and a Pyridyl-1,2,4-triazolyl Chelate Core. Adv. Funct. Mater. 2014, 24, 7257−7271. (19) Choy, W. C. H.; Chan, W. K.; Yuan, Y. Recent Advances in Transition Metal Complexes and Light-Management Engineering in Organic Optoelectronic Devices. Adv. Mater. 2014, 26, 5368−5399. (20) Hohenleutner, A.; Schmidbauer, S.; Vasold, R.; Joosten, D.; Stoessel, P.; Buchholz, H.; König, B. Rapid Combinatorial Synthesis and Chromatography Based Screening of Phosphorescent Iridium Complexes for Solution Processing. Adv. Funct. Mater. 2012, 22, 3406−3413. (21) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S. Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems. Adv. Mater. 2009, 21, 4418−4441. (22) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Highly Phosphorescent Bis-Cyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes. J. Am. Chem. Soc. 2001, 123, 4304−4312. (23) Yook, K. S.; Lee, J. Y. Organic Materials for Deep Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2012, 24, 3169−3190. (24) Schmidbauer, S.; Hohenleutner, A.; König, B. Chemical Degradation in Organic Light-Emitting Devices: Mechanisms and Implications for the Design of New Materials. Adv. Mater. 2013, 25, 2114−2129. (25) Verbeeck, J.; Tian, H.; Van Tendeloo, G. How to Manipulate Nanoparticles with an Electron Beam? Adv. Mater. 2013, 25, 1114− 1117. (26) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234−238. (27) Parker, C. A. Advances in Photochemistry; Wiley & Sons, Inc.: Hoboken, NJ, 1964. (28) Parker, C. A.; Hatchard, C. G. Triplet-singlet emission in fluid solutions; Phosphorescence of eosin. Trans. Faraday Soc. 1961, 57, 1894−1904. (29) Jablonski, A. Efficiency of Anti-Stokes Fluorescence in Dyes. Nature 1933, 131, 839−840. (30) Jablonski, A. About the mechanism of photo-luminescence of dye phosphors. Eur. Phys. J. A 1935, 94, 38−46. (31) Lewis, G. N.; Lipkin, D.; Magel, T. T. Reversible photochemical processes in rigid media. A study of the phosphorescent state. J. Am. Chem. Soc. 1941, 63, 3005−3018. (32) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Sausalito, CA, 1978; pp98−100. (33) Bohne, C.; Abuin, E. B.; Scaiano, J. C. Characterization of the triplet-triplet annihilation process of pyrene and several derivatives under laser excitation. J. Am. Chem. Soc. 1990, 112, 4226−4231. (34) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes. Appl. Phys. Lett. 2011, 98, 083302. (35) Hirata, S.; Sakai, Y.; Masui, K.; Tanaka, H.; Lee, S. Y.; Nomura, H.; Nakamura, N.; Yasumatsu, M.; Nakanotani, H.; Zhang, Q.; Shizu, K.; Miyazaki, H.; Adachi, C. Highly efficient blue electroluminescence based on thermally activated delayed fluorescence. Nat. Mater. 2015, 14, 330−336. (36) Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence. Nat. Photonics 2014, 8, 326−332. (37) Zhang, Q.; Tsang, D.; Kuwabara, H.; Hatae, Y.; Li, B.; Takahashi, T.; Lee, S. Y.; Yasuda, T.; Adachi, C. Nearly 100% Internal Quantum Efficiency in Undoped Electroluminescent Devices Employing Pure Organic Emitters. Adv. Mater. 2015, 27, 2096−2100.

4,4′,5,5′-tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-2,2′-dicarbonitrile tris[2-phenylpyridinato-C2,N]iridium(III) tris[1-(2,4-diisopropyldibenzo[b,d]furan-3yl)-2-phenyl-1H-imidazole) iridium(III) 9,9′,9″,9‴-((6-phenyl-1,3,5-triazine-2,4diyl)bis(benzene-5,3,1-triyl))tetrakis(9Hcarbazole)

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Molecular Design Strategy of Organic Thermally Activated Delayed

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