Recent progress of green thermally activated delayed fluorescent emitters

ABSTRACT Pure organic-based thermally activated delayed fluorescent (TADF) emitters have been studied for the last five years because of their potential as high-efficiency emitters comparable to phosphorescent emitters. Although the initial external quantum efficiency (EQE) of the TADF emitters was much lower than that of phosphorescent emitters, the current EQE of the TADF organic light-emitting diodes (OLEDs) is quite similar to that of phosphorescent OLEDs. In particular, the EQE of the green TADF OLEDs is already over 30% with the help of the new molecular design fully harvesting triplet excitons of the TADF emitters for light emission through the up-conversion process. In this work, the progress of the device performances of the TADF OLEDs was studied by reviewing the green TADF emitters that had been developed in the last five years.


Introduction
Thermally activated delayed fluorescent (TADF) emitters are becoming very popular because of their high external quantum efficiency (EQE) due to their efficient radiative transition process assisted by the up-conversion process of triplet excitons. In common fluorescent emitters, the triplet excitons generated by the carrier injection process are useless because of their non-radiative nature due to the prohibited triplet-to-singlet radiative transition based on the spin selection rule. The non-radiative triplet excitons, however, can be converted to singlet excitons in the TADF emitters through the reverse intersystem crossing process induced by the weak spin-orbit coupling and the small singlet-triplet energy gap ( E ST ) in specially designed donor-acceptor structures [1]. Therefore, all singlet and triplet excitons can be converted to photons for light emission, and enable the maximum theoretical efficiency in the TADF devices [2,3].
There has been much progress in the EQE of the red, green, and blue TADF devices for the last five years, and the most significant advances were made in the green TADF devices because of the relatively easy molecular design of the green TADF emitters. The development of the red TADF emitters was relatively difficult due to the exciton loss mechanism of the energy gap law, while the blue TADF emitters suffer from a large E ST due to the weak donor and acceptor strength of the building units. In this work, the chemical structures and device performances of the green TADF emitters were covered based on the literature data reported for the last couple of years to provide a guideline for the development of high-efficiency TADF organic light-emitting diodes (OLEDs) as the device performances of the green TADF emitters are quite similar to those of the green phosphorescent emitters. The relationship between the chemical structures and the various material parameters is discussed, and their effect on the device performances is also described.

Molecular design of green TADF emitters
The green TADF emitters generally have peak emission wavelengths of around 500 nm and are the most widely developed emitters among the red, green, and blue emitters because of their high EQE and easy molecular design. They are potentially advantageous for realizing a high EQE compared to that of a red TADF emitter because the non-radiative decay process can be reduced by a high emission energy according to the energy gap law [4]. In the case of the red TADF emitters, a small singlet energy of the red TADF emitters brings about non-radiative decay and a reduced EQE of the red TADF devices. For obtaining a high EQE, the green TADF emitters are also better than the blue TADF emitters because the TADF emission can be easily harvested and optimized by various host materials. In the case of the blue TADF emitters, the limited availability of high-triplet-energy host materials makes it difficult to fully activate and optimize the TADF emissions of the blue emitters. Therefore, the best EQE of the green TADF OLEDs (31.2%) was much higher than that of the red (17.5%) and blue (25.0%) TADF OLEDs [5][6][7].
A general design rule underlying the molecular backbone structure of the green TADF emitters is to combine the donor and acceptor moieties in several different ways. They can be directly linked or connected through an aromatic linker for a high photoluminescence quantum yield (PLQY) and a small E ST , which are the most important material characteristics of the TADF emitters.
The basic design platform of the green TADF emitters was not different from that of the red or blue TADF emitters, and the simple management of the degree of conjugation and the donor-acceptor strength could control the emission color. For example, the para-connection rather than meta-connection of the aromatic units and the increases in the number of donor moieties could manage the color from blue to green. The green TADF emitters in this work cover the wavelength range from 480 to 560 nm, which includes the bluish-and yellowishgreen emitters.

cyano (CN)-modified TADF emitters
The most well-known green TADF emitter is 2,4,5,6tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), with four carbazole donor units and a 1,3-dicyanobenzene acceptor [1]. Three isomers with 1,2-dicyanobenzene (4CzPN), 1,3-dicyanobenzene (4CzIPN), and 1,4dicyanobenzene (4CzTPN) were compared, and a high EQE (19.2%) was achieved in the 4CzIPN by widely dispersing the highest occupied molecular orbital (HOMO) over the donor moieties, which is helpful for a high PLQY. A high PLQY of above 90% and a relatively short excited-state lifetime of 5.1 μs for delayed emission were reported using the 4CzIPN emitter. After the demonstration of a high EQE close to 20% using the organic 4CzIPN TADF emitter for the first time, several works optimizing the device structure of the 4CzIPN TADF OLEDs followed and reported an improved EQE surpassing the EQE obtained by Adachi et al. Several bipolar host materials with a triplet energy high enough for both the singlet and triplet harvesting of 4CzIPN were proven to be better than the carbazoletype host materials and provided EQEs higher than 20% [8][9][10][11][12][13][14][15][16][17]. In particular, 3-(3-(carbazole-9-yl)phenyl) pyrido [3 ,2 :4,5]furo [2,3-b]pyridine (3CzPFP) was the best host material of 4CzIPN and realized the best EQE of 31.2% in the 4CzIPN devices [6]. A mixed host of holeand electron-transport-type host materials also worked effectively as the host of 4CzIPN [17][18][19][20]. Several mixed host systems made up of carbazole-type hole transport hosts and phosphoine-oxide or pyridine-type electron transport hosts were developed through the proper selection of host materials based on the energy levels, carrier transport properties, and triplet energy [17,18]. All the mixed hosts provided a high EQE of above 20%, and the highest EQE of the 4CzIPN device that was obtained using the mixed host was 29.6% [18].
In addition to its high EQE and versatile processing in different fabrication processes, 4CzIPN could provide long-term stability in the green devices [25]. The 4CzIPN emitter was evaluated as a stable green emitter compared with the conventional green phosphorescent emitter, and exhibited a longer lifetime than the phosphorescent emitter, which confirmed that the TADF emitters could replace the current phosphorescent emitters in practical applications.
Another approach that was employed for the utilization of the CN-based acceptor moieties was to apply a CN-modified pyridine moiety as the acceptor unit of the donor-acceptor structure to strengthen the accepting character. 2,6-Di(9H-carbazol-9-yl)-4-phenylpyridine-3,5-dicarbonitrile (CPC) is a green emitter with a 4phenylpyridine-3,5-carbonitrile moiety as the acceptor and two carbazole units as the donor [33]. The strong electron acceptor decreased the E ST up to 0.04 eV, and increased the EQE up to 21.2%, demonstrating the superiority of the 4-phenylpyridine-3,5-carbonitrile as the acceptor of the TADF material design.
Although several TADF emitters with CN-based cyanobenzene or dicyanobenzene acceptors have been reported, they could not reach the EQE level of 4CzIPN. Therefore, a new design approach involving a dualemitting core design to enhance the device performances of the CN-type TADF emitters was developed by coupling two TADF emitters [34]. The main purpose of the dual-emitting core design was to intensify the absorption of the emitters so as to gain a high PLQY for the TADF emitters. Increases in the absorption coefficient and PLQY were observed through the coupling of two TADF emitters (3,3 ,5,5 -tetra(9H-carbazol-9-yl)-[1,1biphenyl]-2,2 ,6,6 -tetracarbonitrile (DDCzIPN)) [34], and a high EQE of 18.9% was reported compared to 16.4% for each TADF unit [35]. Additionally, the emission wavelength was shifted to a longer wavelength through the extension of the conjugation as two aromatic moieties were directly coupled. Therefore, the dual-emitting core design was effective at controlling the emission color and at enhancing the light absorption and the PLQY.
The chemical structures, photophysical parameters, and device performances of the CN-modified acceptorbased green TADF emitters are presented in Tables 1 and 2.
After the initial literatures about the triazine derivatives, great progress was achieved in the device performances by modifying the triazine acceptor with an acridine-type donor moiety. 10-(4-(4,6-Diphenyl-1,3,5triazin-2-yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-TRZ) is one of the best-performing triazine derivatives for the green TADF OLEDs [52,53]. The modification of the triazine moiety with the acridine donor effectively produced a green emitter with a small E ST of 0.05 eV, a short excited-state lifetime of 3.6 μs, and an accompanying high PLQY of 0.83 owing to the strong donor power of the acridine moiety. A strong CT character was observed due to the large dihedral angle between the acridine and the acceptor moiety, and the strong donor character of the acridine. A very high EQE of 26.5% was achieved using the DMAC-TRZ emitter, and even the non-doped DMAC-TRZ device exhibited a high EQE of 20% because of the hindered intermolecular interaction by the perpendicularly oriented acridine moiety. Analogously, the indenoacridine-type moiety was also effective as the donor moiety of the triazine derivative (5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl)-7,7,13,13-tetramethyl-7,13-dihydro-5H-indeno [1,2-b]acridine (TrzIAc)), which provided a high EQE of 20.9% [53]. The indenoacridine donor shifted the emission spectrum to a long wavelength because of the aromatic fluorene moiety. Additionally, the solubilityenhancing effect of the acridine moiety through the perpendicular geometry enabled the development of a soluble triazine compound (2,4,6-tris(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-1,3,5-triazine (3ACR-TRZ)) [54]. The three-acridine-included backbone structure resulted in an E ST of 0.015 eV and a high PLQY of 0.98. The final soluble device provided a high EQE of 18.6%, which is one of the best EQE values of the soluble TADF OLEDs. The chemical structures, photophysical parameters, and device performances of the triazine-acceptorbased green TADF emitters are presented in Tables 3  and 4.
The overall progress of the EQE of the green TADF emitters is summarized in Figure 1. Year Figure 1. Progress of the EQE of green TADF devices. Adopted with permission from reference [77]. Copyright 2017, American Chemical Society.

Summary and outlook
As described above, the green TADF emitters were more efficient than the red TADF emitters, and the green TADF OLEDs already reached the EQE level of the green phosphorescent organic light-emitting diodes (PHOLEDs) by engineering TADF emitters and host materials for the TADF emitters. The main reason for this is the design versatility of the green TADF emitters, which permits the donor and acceptor moieties for the green TADF emitter to be diversely selected based on various combinations, such as strong donor-weak acceptor, weak donor-strong acceptor, and moderate donor-moderate acceptor. The other reason is the minimal emission energy loss because all the emissions were within the visible wavelength range. The infrared emission loss of the red emitters and the UV emission loss of the blue TADF emitter can be avoided in the green emitters, which contributes to the high EQE of the green device. Although a high EQE comparable to that of PHOLEDs was achieved, only several donor and acceptor moieties were applied in the design of the green TADF emitters, and further development of the green TADF emitters for a high EQE and a long lifetime is needed. Particularly, a molecular design guaranteeing a stable lifetime must be developed because few emitters were reported as stable emitters. Among the green emitters, only 4CzIPN and 5CzBN could behave as stable emitters because of the intrinsic instability of the common strong donor moieties, such as acridine, phenoxazine, and phenothiazine, and of the typical acceptor moieties, such as the boron derivatives, sulfone derivatives, ketone derivatives, pyrazine, and pyridine. Therefore, the green emitters should possess a molecular structure with stable donors mostly derived from carbazole, an aromatic linker, and with stable acceptors mostly derived from CN-modified aromatics or triazine-based moieties, for both a high EQE and a long lifetime.