Triazine-Acceptor-Based Green Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

High-efficiency thermally activated delayed fluorescence (TADF) is leading the third-generation technology of organic light-emitting diodes (OLEDs). TADF emitters are designed and synthesized using inexpensive organic donor and acceptor derivatives. TADF emitters are a potential candidate for next-generation display technology when compared with metal-complex-based phosphorescent dopants. Many studies are being conducted to enhance the external quantum efficiencies (EQEs) and photoluminescent quantum yield of green TADF devices. Blue TADF reached an EQE of over 35% with the support of suitable donor and acceptor moieties based on a suitable molecular design. The efficiencies of green TADF emitters can be improved when an appropriate molecular design is applied with an efficient device structure. The triazine acceptor has been identified as a worthy building block for green TADF emitters. Hence, we present here a review of triazine with various donor molecules and their device performances. This will help to design more suitable and efficient green TADF emitters for OLEDs.


Introduction
Organic light-emitting diodes (OLEDs) and the application of organic materials in emitting technology have attracted much attention from industrial and research communities since 1987. The advantages of OLED technology, such as its light weight, image quality, high contrast, fast response time, thin film, and wide-view angle, have made it a potential candidate in commercial applications instead of liquid crystal displays (LCDs) [1][2][3][4][5]. Moreover, OLED displays can be fabricated on foldable and bendable substrates, thus making them a leading type of next-generation display. OLEDs have received considerable attention as an energy-efficient technology because they do not require any backlighting support [6][7][8]. OLED technology has developed from single-to multilayer devices across three generations of dopant materials. Multilayer OLED devices consist of several layers between an anode and a cathode, including a hole-injection layer (HIL), a hole-transporting layer (HTL), an electron-blocking layer (EBL), an emission layer (EML), a hole-blocking layer (HBL), an electron-transporting layer (ETL), and an electron-injection layer (EIL). The emission layer is made of two components, namely, host and dopant materials. The dopant material, at a suitable doping concentration, is usually doped with a high-triplet energy host material to support effective energy balance and emit colors by a proper charge recombination [9][10][11][12][13][14][15][16][17][18][19].
The spin rule explains the singlet and triplet emission possibilities of OLED dopants, where 25% are only responsible for singlet emission while 75% are from the triplet state. First-generation fluorescent emitters harvest only singlet emission with an internal quantum efficiency (IQE) of 25%. The remaining current efficiencies of acridine-donor-based DMAC-TRZ and TRZ-DDPAc were 66.8 and 62.8 cd/A, respectively [81][82][83][84].
When we compare the device efficiencies of various acceptor-based green TADF emitters, the triazine acceptor with suitable donor moieties enhances the device efficiencies and photoluminescent quantum yield (PLQY). In this review, we focus on triazine-based green TADF emitters and their device characteristics. As a moderate acceptor, triazine is an interesting derivative for green TADF emitters. Triazine-based green TADF emitters are depicted in Figures 1-6, and their photophysical properties and device performances are summarized in Tables 1 and 2, respectively.

Results and Discussion
The heterocyclic triazine acceptor is a well-known moiety for green TADF emitters due to its stable and moderate electron acceptability. Many studies have reported that efficient green TADF emitters were developed by replacing various donor moieties in different positions and suitable device structures, especially the host material.
Lee et al. reported another molecule with a disubstituted bicarbazole donor derivative at the second and fourth positions of the triazine acceptor. 9,9 -(6-phenyl-1,3,5-triazine-2,4-diyl)bis((9H-3,9 -bicarbazole)) (CC2TA) ( Figure 1) showed a low energy difference of 0.05 eV between the singlet and triplet levels, which was supported by the considerable separation between the donor and acceptor units. A photoluminescent quantum yield of 62% was recorded, while delayed fluorescence was observed at 22 µs. This OLED device was constructed using a double emission layer with host materials such as mCP and DPEPO. The double-layered host materials were responsible for opposite charge transportation, and a thin layer of DPEPO was employed to block excitons at the interface between the emission layer and the electron-transporting layer. The device exhibited an external quantum efficiency of 11% and an emission of 490 nm [87].
showed low PLQYs and device efficiencies compared with TCzTrzDBF. Changing the acceptor attached position and reducing the amount of carbazole did not reveal any interesting efficiency enhancements. However, a dibenzofuran linker can suppress the nonradiative mechanism when compared with the presence of a phenyl linker moiety [90,91]. Carbazole and its derivatives with a triazine acceptor were the subject of an interesting study on device performances. The monosubstituted indolocarbazole donor moiety PIC-TRZ2 showed a wellseparated frontier molecular orbital distribution compared with disubstituted indolocarbazole PIC-TRZ2, which helped to increase the EQE from 5.3% to 12.5%. However, these two molecules did not have any phenyl linker or spacer molecule between the donor and the acceptor. A bicarbazole donor and triazine acceptor without any phenyl linker showed the opposite performance, and disubstituted CC2TA revealed better performances (11%) than the monosubstituted CzT molecule (6%). Symmetrical molecules of DPA-TRZ and DACT-II with a phenyl linker unit exhibited better device properties. Carbazole with diphenyl amine at the third and sixth positions (DACT-II) enhanced the device quantum efficiency up to 29.6%, while diphenyl amine, at the third and sixth positions of diphenylamine (DPA-TRZ), showed a low efficiency of 13.8%. So, carbazole with a diphenylamine donor at the third and sixth positions resulted in a more interesting effect with the triazine acceptor than a similar molecular design with diphenylamine donor derivatives. A IDCzTrzDBF molecule was constructed with a furan linker between an indolocarbazole donor and a triazine acceptor, but this linker moiety and substituted position did not have a successful effect on the external quantum efficiency. A furan linker moiety attached to a symmetrical donor of carbazole with third-and sixthposition-substituted phenyl carbazole (TCzTrzDBF) showed better performance. When the number of carbazole donor moieties was increased and attached to the phenyl group at meta and para positions (TmCzTrz), the result was an EQE over 25%. Overall, a carbazole donor containing a symmetrical structure, along with free rotating substitutions at the third and sixth positions, and a number of carbazole donors attached through meta and para substitution further helped to achieve high EQEs for green TADF emitters compared with carbazole-based rigid donor derivatives of indolocarbazole.
The indeno-acridine strong-donor-based 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7,7,13,13tetramethyl-7,13-dihydro-5H-indeno[1,2-b]acridine (TrzIAc) molecule ( Figure 3) was reported to have a PLQY of 97%. Delayed fluorescence was observed at 1.6 μs, with a singlet-triplet energy difference of 0.06 eV. The rigid donor molecule enhanced the thermal stabilities of TrzIAc. OLED device performances were noticed when 20 wt % was doped with mixed hosts of mCP and TPBI. An EQE of 20.9% was recorded, which was higher than that of acridine-donor-based TrzAc (17.7%). The indeno-acridine donor moiety not only enhanced the thermal stability but also improved device efficiencies with green color emission (511 nm) [61]. Carbazole and its derivatives with a triazine acceptor were the subject of an interesting study on device performances. The monosubstituted indolocarbazole donor moiety PIC-TRZ2 showed a well-separated frontier molecular orbital distribution compared with disubstituted indolocarbazole PIC-TRZ2, which helped to increase the EQE from 5.3% to 12.5%. However, these two molecules did not have any phenyl linker or spacer molecule between the donor and the acceptor. A bicarbazole donor and triazine acceptor without any phenyl linker showed the opposite performance, and disubstituted CC2TA revealed better performances (11%) than the monosubstituted CzT molecule (6%). Symmetrical molecules of DPA-TRZ and DACT-II with a phenyl linker unit exhibited better device properties. Carbazole with diphenyl amine at the third and sixth positions (DACT-II) enhanced the device quantum efficiency up to 29.6%, while diphenyl amine, at the third and sixth positions of diphenylamine (DPA-TRZ), showed a low efficiency of 13.8%. So, carbazole with a diphenylamine donor at the third and sixth positions resulted in a more interesting effect with the triazine acceptor than a similar molecular design with diphenylamine donor derivatives. A IDCzTrzDBF molecule was constructed with a furan linker between an indolocarbazole donor and a triazine acceptor, but this linker moiety and substituted position did not have a successful effect on the external quantum efficiency. A furan linker moiety attached to a symmetrical donor of carbazole with third-and sixth-position-substituted phenyl carbazole (TCzTrzDBF) showed better performance. When the number of carbazole donor moieties was increased and attached to the phenyl group at meta and para positions (TmCzTrz), the result was an EQE over 25%. Overall, a carbazole donor containing a symmetrical structure, along with free rotating substitutions at the third and sixth positions, and a number of carbazole donors attached through meta and para substitution further helped to achieve high EQEs for green TADF emitters compared with carbazole-based rigid donor derivatives of indolocarbazole.
Further development of acridine-donor-based 2,4,6-tris(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-1,3,5-triazine (3ACR-TRZ) TADF emitters ( Figure 6) for solution-processable OLEDs was reported by Wada et al. 3ACR-TRZ showed a high PLQY of 98%, which was higher than that of DMAC-TRZ. The increased number of acridine donor molecules helped to reduce the energy gap between the singlet and triplet states to 0.015 eV, and a slightly longer delayed fluorescence was recorded at 6.7 µs. The OLED device was fabricated with a 16 wt % emitter doped with CBP host material. The EQE was 18.6%, which was higher than that of the phenoxazine-based three site molecule Tri-PXZ-TRZ. The dimethyl acridine donor provided good properties as well as easy solution processability [82,94,96].
Moreover, the selection of host materials, adjacent layers, and doping concentrations is important to ensure the effectiveness of the device. Among the above-reported triazine-based green TADF emitters, 20 wt % doped emitters of TrzIAc and DCzmCzTrz showed EQEs of 20.9% and 21.3%, respectively, and an emission layer thickness of 25 nm. The 30 wt % doped BFAcTrz, TmCzTrz, and TRZ-DDPAc exhibited quantum efficiencies of 20.4%, 25.5%, and 27.3%, respectively, and had the same emission layer thickness of 25 nm. A DACT-II-based device showed better device properties at a low doping concentration of 6%, but the thickness of the emission layer was reported to be 40 nm, and we believe that the greater thickness of the host material (CBP) supported an effective energy flow to achieve an EQE of 29.6%. DMAC-TRZ showed good device characteristics (EQE of 26.5%) and employed an 8% doping (20 nm) concentration and hole-blocking layer (DPSS). The acridine-based 3ACR-TRZ was 16 wt % doped with CBP as the host material, and the emission layer thickness was as high as 55 nm, but the device could not reach an EQE over 19%. So, for device optimization, using various doping concentrations and host materials is crucial to obtain an effective device. Host materials play a major role in device performance as they are responsible for supplying energy to the emission layer. At the same time, host materials control the charge recombination of collected electrons and holes from the cathode and anode, respectively. The choice of host material depends on the triplet energy of the dopant material, and high triplet energy host materials dope with dopant to establish an effective device.

Conclusions
Triazine-acceptor-based green TADF emitters with suitable donor derivatives and host materials have shown great performance in terms of device efficiency. The EQEs were over 29%, which were higher than those of any red TADF emitters. Still, many improvements are needed in the molecular design to achieve a high efficiency. Host materials play a major role in device efficiency by supporting effective energy transfer to the dopant. Moreover, a proper doping concentration also enhances device performance. Triazine has exhibited good withdrawing characteristics and a suitable donor moiety connecting the appropriate position, which should result in a highly efficient and stable molecular design for green TADF emitters.