Emitting Materials for Organic Light

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Recent Progress in High-Efficiency Blue-Light-Emitting Materials for Organic Light-Emitting Diodes Yirang Im, Seong Yong Byun, Ji Han Kim, Dong Ryun Lee, Chan Seok Oh, Kyoung Soo Yook, and Jun Yeob Lee* external quantum efficiency (EQE), lifetime, and driving voltage. In particular, EQE enhancement and lifetime extension to a commercialization level were key achievements enabling application of OLEDs in various products. At first, only fluorescent emitters and devices were developed, but the low efficiency issue of fluorescent emitters drove development of high-efficiency emitters, such as phosphorescent emitters or delayed fluorescent emitters because these provide fourfold higher EQE than common fluorescent emitters. The first OLED product used red, green and blue fluorescent emitters, but in the active matrix OLED product the red fluorescent emitter was replaced by a phosphorescent emitter.[20] Current OLED products use red and green phosphorescent emitters, and blue fluorescent emitters. Therefore, the efficiency of blue devices is lower than that of red and green devices. Additionally, the lifetime of blue fluorescent devices is shorter than that of red and green phosphorescent devices. The low EQE problem of blue fluorescent devices prompted researchers to consider blue phosphorescent emitters as replacements for the traditional blue fluorescent emitters. Blue phosphorescent emitters exhibit high EQE of almost 30%, relative to the 10% of the common fluorescent emitters.[22–26] However, the lifetime of blue phosphorescent OLEDs (PhOLEDs) is too short for use in practical applications. As an alternative to phosphorescent emitters, a thermally activated delayed fluorescent (TADF) emitter has been investigated.[18,27–33] Although the lifetime of blue TADF devices is short, TADF OLEDs are candidate high-efficiency blue OLEDs. Therefore, TADF OLEDs and PhOLEDs may compete in the development of high-efficiency and long lifetime blue OLEDs. Additionally, a singlet harvesting technology of common fluorescent emitters by Forster energy transfer, TADF sensitized fluorescence is emerging as an approach to realizing high-efficiency blue OLEDs.[34–40] In this work, recent progress in blue emitters for high EQE and long lifetime OLEDs is reviewed based on the literature published after 2010 because the future competitiveness of OLEDs will be determined by the device performance of blue OLEDs. We describe recent results regarding high-efficiency and long-lifetime blue emitters by classifying them into fluorescent emitters, phosphorescent emitters and TADF emitters. The three types of blue emitter are compared in terms of

Organic light-emitting diodes (OLEDs) are increasingly used in displays replacing traditional flat panel displays; e.g., liquid crystal displays. Especially, the paradigm shifts in displays from rigid to flexible types accelerated the market change from liquid crystal displays to OLEDs. However, some critical issues must be resolved for expansion of OLED use, of which blue device performance is one of the most important. Therefore, recent OLED material development has focused on the design, synthesis and application of highefficiency and long-life blue emitters. Well-known blue fluorescent emitters have been modified to improve their efficiency and lifetime, and blue phosphorescent emitters are being investigated to overcome the lifetime issue. Recently, thermally activated delayed fluorescent emitters have received attention due to the potential of high-efficiency and long-living emitters. Therefore, it is timely to review the recent progress and future prospects of high-efficiency blue emitters. In this feature article, we summarize recent developments in blue fluorescent, phosphorescent and thermally activated delayed fluorescent emitters, and suggest key issues for each emitter and future development strategies.

1. Introduction Since the first discovery of organic electroluminescence (EL) from tris-(8-hydroxyquinoline) aluminum, much effort has been devoted to improve the device performance of organic light-emitting diodes (OLEDs).[1–21] In particular, the development of material and device engineering technology commercialized organic light-emitting diodes (OLEDs) for small and large panel applications. Small OLED panels are used for the displays of mobile phones, while large OLED panels are popular as high performance displays for televisions and signage. Furthermore, OLEDs are penetrating all flat panel display applications, such as flexible displays, transparent displays, mirror displays and so on. It is expected that OLEDs will become the dominant technology in the display market. The successful commercialization of OLEDs as displays was made possible by progress in device performance, such as Y. Im, S. Y. Byun, J. H. Kim, D. R. Lee, C. S. Oh, Prof. K. S. Yook, Prof. J. Y. Lee School of Chemical Engineering Sungkyunkwan University 2066, Seobu-ro, Jangan-gu Suwon, Gyeonggi 440–746, South Korea E-mail: [email protected]

DOI: 10.1002/adfm.201603007 1603007 (1 of 24)

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2. Basic Emission Mechanism of Blue Fluorescence, Phosphorescence and Delayed Fluorescence Blue emitters can be classified into three types: fluorescent emitters, phosphorescent emitters and delayed fluorescent emitters.[41] Fluorescent emitters utilize only singlet excitons for light emission from a singlet excited state to a ground state and the theoretical maximum internal QE by EL process is 25% according to spin statistics. Typically, strylbenzene, anthracene, and perylene compounds have been used as blue fluorescent emitters.[7,42–47] Although the internal QE of the fluorescent device is low, aromatic-based backbone structures stabilize the compounds and allow blue devices to have a relatively stable lifetime. Blue phosphorescent emitters take advantage of the radiative transition process from a triplet excited state to a singlet ground state. By hole and electron injection from charge transport layers to the emitting layer, singlet excitons and triplet excitons are generated at a ratio of 1:3. The singlet excitons are converted into triplet excitons by intersystem crossing and all excitons can radiate by phosphorescence. Ideal phosphorescent emitters can exhibit 100% internal QE because all singlet and triplet excitons can be converted into photons by radiative transition. Ir-based organometallic compounds have been investigated as phosphorescent emitters.[48–50] Although the EQE of blue PhOLEDs is high, the weak chemical bond between Ir and ligands destabilizes the Ir emitters and results in a short lifetime.[51,52] Additionally, small bond dissociation energy of high triplet energy host materials for blue PhOLEDs was also suggested as the lifetime shortening parameter.[53,54] Blue delayed fluorescent emitters harvest singlet emission by a reverse intersystem crossing process (TADF) or triplet-triplet fusion (TTF) process.[18,51,52,55–62] In conventional fluorescent emitters, triplet excitons are lost by non-radiative transition, but can be converted into singlet excitons by small singlet-triplet energy gap driven up-conversion (TADF) or triplet-triplet collision induced reverse intersystem crossing. The former process is called E-type delayed fluorescence because the phenomenon was reported from Eosin for the first time, and the latter process P-type delayed fluorescence because its origin is Pyrene.[63,64] Complete triplet to singlet up-conversion followed by singlet transition can result in 100% internal QE in TADF devices, while perfect triplet-triplet fusion followed by triplet to singlet down-conversion can allow 62.5% internal QE in the TTF device. Donor–acceptor type compounds are popular as TADF emitters and pyrene- or anthracene-type compounds are examples of TTF emitters.[29,65–70] TTF emitters are between conventional fluorescent emitters and phosphorescent emitters in terms of QE and are similar to fluorescent emitters in terms

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Yirang Im received her B. S. Degree (2012) and M. S. Degree (2014) from the Department of Polymer Science and Engineering at Dankook University, Korea. She is now a Ph.D candidate at the School of Chemical Engineering of Sungkyunkwan University. Her major research areas are the synthesis of organic electronic materials for phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes. Chan Seok Oh received his B. S. Degree (2012) and M. S. Degree (2014) from the Department of Polymer Science and Engineering of Dankook University. He is now a Ph.D candidate at the School of Chemical Engineering of Sungkyunkwan University. His main research topics are the synthesis and the device application of organometallic compounds and themally activated delayed-fluorescene-emitting materials for high efficiency and long-lifetime organic light-emitting diodes. Prof. Jun Yeob Lee received his Ph.D degree from the Seoul National University, Korea in 1998. After a postdoc at Rensselaer Polytechnic Institute (1998∼1999) he joined Samsung SDI and developed active matrix organic light-emitting diodes for 6 years. After that, he worked as a professor at the Department of Polymer Science and Engineering of Dankook University and he is now a professor at the School of Chemical Engineering of Sungkyunkwan University since 2015. His main research areas are the synthesis of organic electronic materials and the development of novel device structures for organic electronic devices.

of lifetime. In contrast, TADF emitters are at present similar to phosphorescent emitters in terms of QE and lifetime.[67,71] In the case of the TADF emitters, synergetic oxidation process was proposed as the lifetime limiting factor.[53] The emission processes of the various blue emitters are shown in Figure 1. As explained above, traditional fluorescent emitters are not appropriate as high-efficiency blue emitters and current blue



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material design, photophysical properties and device performance. In particular, the merits and demerits of the three types of emitter are explained and strategies for realization of highefficiency and long-lifetime blue OLEDs discussed. The blue emitters covered in this paper are the emitters which can reach CIE y coordinate below 0.40 although deep blue emitters for display application require CIE y value below 0.15.


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Figure 1. Light emission process of TTF, phosphorescent, and TADF emitters.

fluorescent emitters are TTF-type delayed fluorescent emitters. Therefore, high-efficiency blue emitters will be categorized into TTF fluorescent emitters, phosphorescent emitters and TADF emitters in this work. In addition, fluorescent emitters having hybrid local and charge-transfer (HLCT) excited state will also be covered as an emerging technology for blue emission.

3. TTF-Type Fluorescent Emitters Blue fluorescent materials have been developed for more than 20 years and various chromophores were introduced for blue emission in fluorescent emitters.[72–76] At first, common blue fluorescent emitters without delayed components were generally investigated, but delayed fluorescent type emitters are being studied because of their high-efficiency mediated by the triplet exciton contribution to singlet emission. For example, the best TTF device showed a high EQE of 14.8% using a deep blue fluorescent emitter, which is superior to the ∼5% of common blue fluorescent emitters.[23] Therefore, TTF-type blue fluorescent emitters have potential as high-efficiency blue emitters. The most common TTF-type blue fluorescent emitters are anthracene and pyrene type compounds because the emission wavelengths of those compounds falls in the blue emission region and the photoluminescence (PL) quantum yield is as high as 100%.[59] Anthracene or pyrene chromophores were modified with auxochromores like diphenylamine or were coupled for better light-emitting properties because they were TTF inducing core structures. Anthracene-based TTF-type emitters have been widely studied following the detailed physical analysis of the TTF process by Kondakov.[62,77] Early works achieved less than 10% EQE with TTF blue devices, but recent studies reported >10% EQE using anthracene derivatives as the blue emitter or host of the TTF devices having a conventional fluorescent device structure of indium tin oxide (ITO)/hole injection layer (HIL)/hole transport layer (HTL)/emitting layer (EML)/electron transport layer (ETL)/electron injection layer (EIL)/Al.[78] Common HTL, ETL and host materials were generally adopted in the device development, but high triplet energy charge transport materials were selected in the best EQE TTF devices, indicating that triplet harvesting charge transport materials are effective in the TTF devices. As the triplet excitons contribute to the light emission in the TTF mechanism, triplet 1603007 (3 of 24)


exciton confining device structure was effective to enhance the EQE. One of the best-performing TTF blue emitters is 1-(10-(4-methoxyphenyl)anthracen-9-yl)-4-(10-(4-cyanophenyl) anthracen-9-yl)benzene (BD3), which has a bianthracene-based donor–acceptor structure.[61] Diphenylanthracene-type backbone structures were coupled and each diphenylanthracene unit was modified with an electron-donating methoxy and electron-withdrawing CN separately to control the charge balance in the emitting layer, which resulted in the highest occupied molecular orbital (HOMO) of -6.01 eV and the lowest unoccupied molecular orbital (LUMO) of -3.03 eV. PL quantum yield was 0.82 in the donor–acceptor type structure and doped and non-doped blue devices developed using the BD3 dopant exhibited high EQE values of 12.0% and 4.2%, respectively, by the high PL quantum yield. Particularly, the color coordinates of the BD3 device was (0.15, 0.06), which fell in the deep blue emission color. From the time-resolved electroluminescence response of the BD3 device, it was confirmed that the abnormally high EQE of the BD3 device was due to delayed fluorescence by the TTF mechanism. The donor–acceptor structure was mainly responsible for the high EQE because similar compounds without such a donor–acceptor structure functioned poorly in the devices. Anthracene-type materials were also effective as the TTFinducing host material of blue fluorescent dopants. 2,2′-bis(10phenylanthracen-9-yl)-9,9′-spirobifluorene (Spiro-FPA), which has a spirobifluorene core substituted with phenylanthracene, and 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) were investigated as host materials for harvesting triplet excitons of the dopant material by the TTF process.[60] A transient electroluminescence experiment clarified the contribution of the TTF to light emission by blue fluorescent devices. The EQE of the Spiro-FPA and MADN devices was >40% higher than that of the non-anthracene based device. This work demonstrated that triplet exciton activation by TTF in the host material enhances the EQE of blue fluorescent devices due to energy transfer from the singlet exciton of the host to the singlet exciton of the blue dopant. Pyrene-based materials are also popular as TTF-type blue emitters and TTF hosts of blue fluorescent emitters. One efficient pyrene compound is 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl) pyrene (DMPPP), which has two pyrene units linked via a dimethylphenyl linker to prevent excimer formation.[22,59] The DMPPP material is itself a good blue emitter with a high PLQY of 0.85, and it is an effective host material of a blue fluorescent emitter. As the host material of 2-(styryl)triphenylbenzene (TS) derivatives, DMPPP increased the PLQY of TS-based blue emitters and showed proper HOMO/LUMO energy level of -5.80/-2.50 eV and high singlet energy of 3.20 eV for efficient carrier injection and energy transfer. Delayed fluorescence in the TS doped DMPPP device was confirmed by transient electroluminescence of the doped film, although the origin of the TTF was unclear. It was proposed that both the host and dopant functioned as the TTF material. Among the TS derivatives, (E)N,N-diphenyl-4-(4-(triphenylen-2-yl)styryl)aniline (TSTA) was the best blue emitter, affording a high QE of 10.2% and current efficiency of 12.3 cd/A.[59] The pyrene based TTF emitter was also proven as long lifetime blue emitter by affording 8,000 h half lifetime at 1,000 cd/m2 in addition to 11.9% EQE.[78] It was


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Device structure

Ref.

ITO/TAPC (40 nm)/CBP:BD3 (20 nm)/B3PyPB (40 nm)/LiF (1nm)/Al (80 nm)

[61]

ITO/TAPC (60 nm)/ SBTF:CN-SBAF (15 nm)/TmPyPB (25 nm)/LiF (1 nm)/Al (100 nm)

[23]

ITO/mCP:ReO3(4 wt%) (30 nm)/mCP (10 nm)/mCPPO1:(HFP)2Ir(mpic) (30 nm)/TSPO1 (10 nm/mCP:Rb2CO3(8 wt%) (30 nm)/Al (100 nm)

[93]

BD3 CN-SBAF (HFP)2Ir(mpic)

(F2CF3CH3ppy)2Ir(pic-N-oxide) ITO/2-TNATA (30 nm)/TAPC (30 nm)/TCTA (10 nm)/mCP:doapnt (20 nm)/UGH:dopant (10 nm)/BAlq (40 nm)/LiF (1 nm)/Al (100 nm) ITO/PEDOT:PSS (60 nm)/TAPC (30 nm)/DBTAcCz:Ir(dbi)3 (25 nm)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (200 nm)

Ir(dbi)3

ITO (90 nm)/HATCN (5 nm)/ TAPC (35 nm)/26DCzPPy:dopant (10 nm)/Tm4PyPB (50 nm)/Liq (2 nm)/Al (80 nm)

[25]

ITO/HATCN (10 nm)/NPD (40 nm)/TAPC (10 nm)/26mCPy:PtON1 (25 nm)/DPPS (40 nm)/LiF/Al

[115]

ITO/HATCN (10 nm)/NPD (40 nm)/TAPC (10 nm)/TAPC:PO15:PtON7-dtb (25 nm)/PO15 (10 nm)/BmPyPB (30 nm)/LiF/Al

[88]

ITO/HATCN (5 nm)/NPB (30 nm)/TCTA (10 nm)/mCBP:5TCzBN (30 nm)/DpyPA: Liq (30 nm)/LiF (0.5 nm)/Al (150 nm)

[128]

fac-Ir(mpim)3 PtON1 PtON7-dtb 5TCzBN TCzTrz

[81] [107]

ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/DPEPO:TCzTrz (25 nm)/TSPO1 (5 nm)/TPBi (20 nm)/LiF (1 nm)/Al (200 nm)

[29]

SpiroAC-TRZ

ITO/MoO3 (1 nm)/TAPC (50 nm)/mCP (10 nm)/mCPCN: SpiroAC-TRZ (20 nm)/3TPYMB (50 nm)/LiF (0.5 nm)/Al (150 nm)

[132]

DMAC-DPS

ITO/MoO3 (6 nm)/NPB (70 nm)/mCP (10 nm)/DPETPO:DMAC-DPS (20 nm)/DPETPO (10 nm)/BPhen (30 nm)/LiF (1 nm)/Al

[58]

ITO/NPB (30 nm)/mCP (20 nm)/CzPS: DCBPy (30 nm)/DPEPO (5 nm)/TmPyPb (60 nm)/LiF (1 nm)/Al (100 nm)

[57]

DCBPy

proposed that high triplet energy and deep HOMO of HTL were main reasons for the high EQE and long lifetime although clear supporting data were not provided. In addition to anthracene- and pyrene-type materials, recent research has revealed that benzoanthracene-type blue emitters behave as TTF emitters. The benzoanthracene-derived TTF emitter N6,N9-bis(4-cyanophenyl)-N3,N9diphenylspiro[benzo[de]anthracene-7,9′-fluorene]-3,9-diamine (CN-SBAF) provided a high EQE of 14.8% in a deep blue device with color coordinates of (0.14, 0.11).[23] This is the best EQE achieved by a TTF-type blue fluorescent device to date. The TTF behavior was confirmed by the delayed component of transient PL and transient electroluminescence. However, the main reason for the exceptionally high EQE of the CN-SBAF device is unclear. In the optimization of the device performances, the device stack structure of the TTF device was different from common fluorescent devices in that high triplet energy hole transport and electron transport materials were used for triplet exciton harvesting for TTF process. For example, the device structure of the CN-SBAF device was ITO/TAPC (60 nm)/ SBTF:CN-SBAF (15 nm)/TmPyPB (25 nm)/LiF (1 nm)/Al (100 nm). Similar device stacks were applied in the optimization of the BD3 emitter for both singlet and triplet exciton confinement as shown in Table 1. Chemical structures of TTF-type blue emitters are shown in Figure 2 and material and device performances are listed in Table 2. The EQE of the TTF device has improved dramatically from 30% in PhOLEDs.[26,90] Since the first report of FIrpic as a blue triplet emitter,[5,91] many arylpyridine-type derivatives with different substituents and ancillary ligands have been demonstrated to be suitable as highefficiency blue phosphorescent emitters. For high PL quantum yield, the Ir emitter design with a phenylpyridine ligand having F based functional unit modified phenyl moiety as the aryl group is desired. Development of arylpyridine-type emitters has focused on improving the color purity of blue PhOLEDs. To improve the color purity of blue PhOLEDs, two approaches to modifying the arylpyridine-type ligands have been used.[81,82,92,93] One approach was to add an additional electronwithdrawing unit to the HOMO-determining aryl unit of the ligand. For example, a CN functional group was attached to the difluorophenyl unit of FIrpic to shift the emission color to the deep-blue region.[94] The other approach was to modify the LUMO controlling pyridine unit with an electron-donating moiety, such as a methyl or trimethylsilyl unit. Sometimes, the two approaches were used in combination to produce a deep-blue emission color. Tris[(3,5-difluoro-4-cyanophenyl)pyridine] iridium(III) (FCNIr) and bis((3,5-difluoro-4-cyanophenyl)pyridine) iridi um(III) picolinate (FCNIrpic) are deep-blue emitters with two F units at the 2- and 4- positions and an extra CN unit at the 3- position of the phenyl unit of the phenylpyridine ligand.[95,96] The strong electron-withdrawing character of the CN unit deepened the HOMO of the emitters, which blue-shifted the emission color. A high EQE of 25.0% and a deep blue color coordinate with a y of 2.7 eV) HTL, ETL and host materials were used to prohibit triplet exciton quenching of phosphorescent emitters. Moreover, bipolar host materials rather than hole type or electron type host material were preferred for carrier balance. The same method was applied in the design of deep-blue triplet emitters of bis(1-(2,6-difluoro-3-(4-methylpyridin-2-yl) phenyl)-2,2,2-trifluoroethanone) iridium(III) picolinate ((TF)2 Ir(pic)), bis(1-(2,6-difluoro-3-(4-methylpyridin-2-yl)phenyl)-2,2,2trifluoroethanone) iridium(III) 2-(3-(trifluoromethyl)-1H-1,2,4triazol-5-yl)pyridine ((TF)2Ir(fptc)), bis(1-(2,6-difluoro-3-(4-methylpyridin-2-yl)phenyl)-2,2,3,3,4,4,4-heptafluorobutan-1-one) iridium(III) picolinate ((HF)2Ir(pic)), bis(1-(2,6-difluoro-3-(4methylpyridin-2-yl)phenyl)-2,2,3,3,4,4,4-heptafluorobutan-1-one)) iridium(III) 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine


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electron-withdrawing substituent of the phenyl unit. In the case of (F2CF3CH3ppy)2Ir(pic-N-oxide), the combination of an electron-withdrawing CF3 unit in the phenyl and an electrondonating methyl unit in the pyridine had a synergistic effect on the emission color. A deep-blue color coordinate of (0.15, 0.20) was evident, together with a high EQE of 23.3%. Ideal molecular design of the phenylpyridine type Ir emitters for high EQE is schematized in Figure 3. As an alternative to introducing a HOMO-deepening substituent for deep-blue emission, a difluoropyridine was adopted in place of difluorobenzene as the HOMOgoverning moiety. As the pyridine itself is an electrondeficient moiety, the difluoropyridine deepened the HOMO of the triplet emitter more than the corresponding difluorobenzene. The first difluoropyridine-based triplet emitter was tris(2′,4′-difluoro-2,3′-bipyridine) iridium(III) (Ir(dfpypy)3), which has 2′,6′-difluoro-2,3′-bipyridine as the ligand.[100] After the first demonstration of a deep-blue color using fluorine-modified bipyridine ligand-based Ir emitters, several publications have reported deep-blue Ir emitters derived from 2′,6′-difluoro-2,3′-bipyridine. Ancillary ligand engineering and t-butyl substitution of pyridine for a wide HOMO-LUMO gap were investigated using bis(2′,4′-difluoro-2,3′-bipyridinato)(3-trifluoromethyl5-(2-pyridyl) pyrazolate) iridium(III) (Ir(dfpypy)2(fppz)), bis(2′,4′-difluoro-2,3′-bipyridinato) (3-(trifluoromethyl)-5-(4t-butylpyridyl) pyrazolate) iridium (III) (Ir(dfpypy)2(fpbpz)), bis(4-(tert-butyl)-2′,6′-difluoro-2,3′-bipyridinato) 3-trifluoromethyl-5-(2-pyridyl) pyrazolate iridium (III) (Ir(dfpybpy)2 (fppz)), and bis(4-(tert-butyl)-2′,6′-difluoro-2,3′-bipyridinato) 3-(trifluoromethyl)-5-(4-t-butylpyridyl) pyrazolate iridium (III) (Ir(dfpybpy)2(fpbpz)).[101] Pyridine pyrazolate was the ancillary ligand of the Ir triplet emitters and deep-blue emission from the four Ir emitters was observed. An auxiliary color purifying and aggregation suppressing effect was mediated by the t-butyl substituent. The color coordinate

Figure 3. Design approach of high efficiency phosphorescent and TADF emitters.

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((HF)2Ir(fptc)), bis(2-(2,4-difluoro-3-(perfluoropropyl)phenyl)4-methylpyridine) iridium(III) picolinate ((HFP)2Ir(pic)), bis(2(2,4-difluoro-3-(perfluoropropyl)phenyl)-4-methylpyridine) iridium(III) 4-methylpicolinate ((HFP)2 Ir(mpic)), and bis(2-(2,4difluoro-3-(perfluoropropyl)phenyl)-4-methylpyridine) iridium(III) 3-(trifluoromethyl)-5-(2-pyridyl)-1,2,4-triazolate ((HFP)2 Ir(fptz)).[92,93] A trifluoromethyl carbonyl was the additional electron-withdrawing unit of (TF)2Ir(pic) and (TF)2Ir(fptc), heptafluoropropyl carbonyl was the additional substituent of (HF)2Ir(pic) and (HF)2Ir(fptc), and heptafluoropropyl was the electron-withdrawing modifier of (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz). The F derived substituents shifted the HOMO level to the range of –5.90 ∼ –6.20 eV range, which was well matched with the energy levels of various host materials. In all triplet emitters, the y color coordinate was 30

>30

500

N/A

0.15 ± 0.05, 0.15 ± 0.05

0.15 ± 0.05, 0.30 ± 0.05

0.15 ± 0.05, 0.10 ± 0.05

High efficiency

Moderate stability

Deep blue color

Deep blue color

High efficiency

Color coordinate Merits

Easy energy level matching with host Demerits

Short lifetime

Poor color purity Difficult energy level matching with host

Low efficiency Short lifetime Difficult energy level matching with host

a)Lifetime

at 1000 cd m−2.

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PtON7

PtON1

Figure 5. Chemical structure of the Pt triplet emitters.

bridged with oxygen to the same carbazolylpyridine ligand also achieved a high forward viewing EQE of 25.2% with a CIE of (0.15, 0.13). Although the lifetime test results of the PtON7 and PtON1 devices were provided, the instability of the charge transport materials used in the device structure made it difficult to judge the lifetime level of these Pt-based blue triplet emitters. The encouraging device results of PtON1 and PtON7 inspired further modification of the Pt compounds, which produced one of the best performing deep-blue emitters, [6-(1,3-dihydro-3-methyl-2H-imidazol-2-ylidene- κ C 2)-4-tertbutyl-1,2-phenylene-κC1]oxy[9-(4-tert-butyltpyridin-2-yl-κN)-9Hcarbazole-1,2-diyl-κC1] platinum(II) (PtON7-dtb).[88] PtON7-dtb is a t-butyl modified version of PtON7 developed by attaching a t-butyl group to the pyridine unit of the carbazolylpyridine ligand to increase the emission energy of the carbazolylpyridine unit. It was proposed that the energy separation between the interlinked carbene ligand and carbazolylpyridine ligand minimized the influence of the carbazolylpyridine ligand on the emission process of PtON7-dtb. Therefore, a very narrow emission spectrum with a FWHM of 19 nm was demonstrated in the PtON7-dtb emitter. Device optimization using a mixed host yielded deep-blue PhOLEDs with a high EQE of 24.8%, deep blue CIE of (0.148, 0.079), and narrow FWHM of 29 nm. This is the best device performance of deep-blue PhOLEDs reported to date. Molecular structures, material characteristics and device data of Pt compounds are presented in Figure 5 and Table 5. As described above, the tetradentate-type Pt complexes demonstrated promising device results as deep-blue triplet emitters in terms of EQE, color coordinate and suppressed excimer formation. However, there is still a stability issue with the tetradentate-type Pt complexes, although a recent report suggested a stable operational lifetime of red PhOLEDs.[116] Therefore, further study of the lifetime of blue Pt complexes is warranted.

TADF emitters are differentiated from phosphorescent emitters in that the origin of light emission is electronic transition from a singlet excited state to a ground state, compared to electronic transition from a triplet excited state to a ground state in phosphorescent PtON7-dtb emitters. Both emitters can harvest all singlet and triplet excitons generated in the emitting layer. In the case of the TADF emitters, reverse intersystem crossing from a triplet excited state to a singlet excited state activated by the small singlet-triplet energy gap (ΔEST) dominates the light-emitting performance of the devices.[41] Therefore, a molecular design approach to decrease the ΔEST of emitters by separating the HOMO and LUMO was used to develop TADF emitters.[66] In most cases, the HOMO and LUMO separation was demonstrated by proper selection of donor and acceptor moieties in the donor–acceptor type molecular structure, which enabled a high EQE of 25% for blue TADF devices.[29,69] Although the EQE of the best blue TADF OLEDs was lower than that of blue PhOLEDs, the EQE of TADF OLEDs is gradually catching up with the 30% EQE of blue PhOLEDs. Various donor and acceptor moieties were introduced to the backbone structure for extensive HOMO and LUMO separation, and TADF emitters can be categorized into several types according to the acceptor: cyanobenzene, diphenylsulfone, aryltriazine, arylboron and others.[117–119] The acceptor strength of the acceptors used in blue TADF emitters is in the order cyanobenzene > aryltriazine > arylboron > diphenylsulfone.[120] This indicates that cyanobenzene-type acceptors can be linked to weak donors such as carbazole, but diphenylsulfone acceptors should be combined with a strong donor such as acridine or aromatic amines for blue emission. Aryltriazine or arylboron acceptors can be joined with moderate donors such as carbazolylcarbazole or diphenylaminocarbazole. In addition to HOMO-LUMO separation by the donor–acceptor structure, the HOMO-LUMO overlap is also essential for high PL quantum yield in blue TADF emitters, which was enabled by inserting a phenyl linker between the donor and acceptor. Insertion of the phenyl linker induced a HOMO-LUMO overlap in the phenyl linker and dramatically increased the PL quantum yield of the emitters.[29,121] Therefore, most blue TADF emitters were designed based on donor-linker-acceptor type molecular building block. To date, cyanobenzene, aryltriazine, diphenylsulfone, and arylboron type emitters have achieved a high EQE of 20% in

Table 5. Photophysical properties and device performances of Pt triplet emitters. Photophysical property

PtON7

Device performance

HOMO (eV)

LUMO (eV)

ET (eV)

ΦPL

–

–

2.87

Color index

0.07/0.89a)

23.7

0.15, 0.14

[115]

a)

25.2

0.15, 0.13

[115]

24.8

0.148, 0.079

[88]

PtON1

–

–

2.82

0.71/0.82

PtON7-dtb

–

–

–

0.7/0.91a)

a)Doped

Ref.

EQEmax (%)

PMMA film.

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5. TADF Emitters

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blue devices, but lifetimes have been reported only in cyanobenzene and aryltriazine-type compounds because other molecules are unstable during lifetime evaluation. In order to evaluate the blue TADF emitters, a device stack with a high triplet energy HTL, ETL and host was used for high EQE by preventing triplet exciton quenching of the TADF emitters as shown in Table 1. Especially, the host material for deep blue TADF emitters was typically DPEPO with a very high triplet energy above 3.0 eV because other host materials could not be matched with the triplet energy of deep blue TADF emitters.

5.1. Cyanobenzene-Type Emitters Cyanobenzene-type TADF emitters are the most widely studied delayed fluorescent emitters to achieve both a high QE and long lifetime. The strong electron-withdrawing character of the CN unit allows the use of a weak carbazole donor for blue emission, and all cyanobenzene derivatives possess CN units as the electron acceptor and carbazole units as the electron donor. The strong CT character due to the strong CN acceptor and weak carbazole donor induced facile reverse intersystem crossing for high QE, and stable chemical structure of triple-bonded CN and aromatic carbazole stabilized the molecule to yield a long lifetime. In fact, a high QE close to 20% and blue TADF OLEDs with the longest lifetime were demonstrated using cyanobenzene-type emitters. The first cyanobenzene-based blue emitter was 4,5-di(9Hcarbazol-9-yl)phthalonitrile (2CzPN), which has a phthalonitrile acceptor and two carbazole units as the donor.[41,122] As a TADF emitter, 2CzPN showed a small ΔEST of 0.09 eV and a high PL quantum yield of 0.89 in the doped film, but the delayed fluorescent lifetime was long (166 µs). Initially, the EQE of the 2CzPN device was only 8.0%, but was increased to 15.8% by optimizing the host materials of the 2CzPN emitter.[123] A bipolar host material was better than a carbazole-type host material, and a high QE and a sky-blue color coordinate of (0.16, 0.29) was observed in the 2CzPN device. Although the 2CzPN emitter was a highly efficient emitter from the PL quantum yield, the long delayed fluorescence lifetime could not reduce the efficiency roll-off of the device, which motivated the development of 4,5-bis(5H-benzo[4,5]thieno[3,2-c]carbazol5-yl)phthalonitrile (BFCz-CN) and 4,5-bis(5H-benzofuro[3,2c]carbazol-5-yl)phthalonitrile (BTCz-2CN) emitters.[124] The design of BFCz-CN and BTCz-CN emitters was different from that of 2CzPN in that the donor moiety was switched from carbazole to sterically hindered benzofurocarbazole (BFCz-CN) or benzothienocarbazole (BTCz-2CN). The bulky benzofurocarbazole and benzothienocarbazole donors were distorted from the phthalonitrile plane and intensified the CT character of the compounds, resulting in a short delayed fluorescent lifetime and high PL quantum yield. The distortion-induced strong CT character may shorten the excited state lifetime of delayed emission. The long delayed fluorescent lifetime of the 2CzPN material was shortened by changing the acceptor from phthalonitrile to isophthalonitrile. The isophthalonitrile-based 4,6-di(9Hcarbazol-9-yl)isophthalonitrile (DCzIPN) emitter reduced the ΔEST to 0.05 eV and delayed the fluorescence lifetime.[125,126] 1603007 (13 of 24)


Moreover, the color coordinate of the DCzIPN device was blue-shifted from (0.16, 0.29) of the 2CzPN device to (0.17, 0.19) of the DCzIPN device. The EQE of the DCzIPN device (16.4%) was higher than that of the 2CzPN device. Therefore, the isophthalonitrile acceptor was appropriate for blueemitting TADF emitters. Following this work, several compounds with isophthalonitrile acceptors were reported as blue emitters. 6,6′-(9H,9′H-[3,4′-bicarbazole]-9,9′-diyl)bis(4-(9Hcarbazol-9-yl)isophthalonitrile) (34TCzPN) and 6,6′-(9H,9′H[4,4′-bicarbazole]-9,9′-diyl)bis(4-(9H-carbazol-9-yl)isophthalonitrile) (44TCzPN) were highly efficient blue TADF emitters with a dual emitting core structure.[68] Two DCzIPN units were coupled through the 3, 4′- or 4, 4′-positions of carbazole in the two emitting cores, yielding the 34TCzPN and 44TCzPN emitters, respectively. The coupling of the two emitting cores increased the absorption coefficient of the molecule without a large red-shift of the emission spectra due to the large dihedral angle between the two carbazole units connecting the two emitting cores. The increased light absorption enhanced the EQE of the blue device, which resulted in a high EQE of 21.8% in the 34TCzPN device. The emission spectra were found in the short wavelength using 44TCzPN as an emitter instead of 34TCzPN. Therefore, the dual emitting core approach is useful to enhance the EQE of TADF devices. Similarly, 5,5′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)diisophthalonitrile (35IPNDCz) and 2,2′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl) diisophthalonitrile (26IPNDCz) were developed as blue emitters, but their EQE was below 10% despite a moderate PL quantum yield of 0.72.[127] Benzonitrile is also popular as a cyanobenzene-derived acceptor in blue TADF emitters. Several benzonitrile-derived blue TADF emitters were reported, and the basic material design platform was benzonitrile substituted with multiple carbazole donor moieties. As the acceptor strength of the benzonitrile moiety was weaker than that of phthanlonitrile or isophthalonitrile, multiple donors were introduced to the core structure to increase the donor strength for small ΔEST. 2,3,5,6-tetra(9Hcarbazol-9-yl)benzonitrile (4CzBN), 3-(3-(tert-butyl)-9H-carbazol-9-yl)-2,4,5,6-tetra(9H-carbazol-9-yl)benzonitrile (5CzBN, 5CzCN), 2,3,5,6-tetrakis(3,6-di-tert-butyl-9H-carbazol-9-yl) benzonitrile (4TCzBN) and 3-(3-(tert-butyl)-6-isopropyl-9Hcarbazol-9-yl)-2,4,5,6-tetrakis(3,6-di-tert-butyl-9H-carbazol-9-yl) benzonitrile (5TCzBN) are representative blue TADF emitters derived from the benzonitrile acceptor.[65,128] The ΔEST values of the four emitters ranged from 0.16 eV to 0.30 eV and the PL quantum yield was 0.49 to 0.86. The increase in the number of donor moieties decreased the ΔEST and enhanced the PL quantum yield of the emitters. In addition, the use of t-butyl modified carbazole instead of carbazole optimized the photophysical parameters for efficient TADF emission. The delayed fluorescence lifetime of the four compounds was below 4.0 µs due to the multiple donor-aided considerable donor strength. The four emitters were doped in a 3,3-di(9H-carbazol-9-yl) biphenyl (mCBP) matrix for device evaluation; the 5TCzBN device exhibited the best performance as evidenced by an EQE of 21.2%. As expected from the photophysical parameters of the four emitters, the use of more and stronger donor moieties enhances the EQE of the device, although the emission color was shifted to a longer wavelength. Furthermore, the 5TCzBN


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5.2. Aryltriazine-Type Emitters Aryltriazine acceptor-based compounds are popular as blue TADF emitters. Aryltriazine is between benzonitrile and dicyanobenzene in terms of acceptor strength and can be used to build blue-emitting backbone structures in combination with carbazole donors. As aryltriazine is not a strong acceptor, multiple carbazole-based donor units such as carbazolylcarbazole, bicarbazole, and triscarbazole are used as donors for efficient delayed fluorescence. The intrinsic stability of the triazine and carbazole moieties makes the aryltriazine class one of the best performing TADF materials in terms of EQE and lifetime, along with the cyanobenzene material class. The best EQE of an aryltriazine-emitter based device is 25.0%, which is the highest reported to date.[29,69] The first blue-emitting aryltriazine-type material was 9,9″-(6-phenyl-1,3,5-triazine-2,4-diyl)bis((9H-3,9′-bicarbazole)) (CC2TA), which was based on a phenyltriazine acceptor and carbazolylcarbazole donor without a phenyl linker.[130] The direct coupling of the donor and acceptor allowed HOMO and LUMO separation to yield a small ΔEST of 0.06 eV, but little overlap of HOMO and LUMO results in a relatively low EQE of 11.0%. An advanced molecular design connecting the donor and acceptor using a phenyl linker increased the device performance of aryltraizine-type emitters. Three compounds known as 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)3,3″,6,6″-tetraphenyl-9′H-9,3′:6′,9″-terbenzo[b]indole (2a), 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′H-9,3′:6′,9″terbenzo[b]indole (2b), and 9-(4-(4,6-diphenyl-1,3,5-triazin2-yl)phenyl)-9H-3,9'-bicarbazole (2c) have a diphenyltriazine acceptor, a phenyl linker and a carbazolylcarbazole-type donor as the building blocks of the emitters.[66] Compound 2a was

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superior to 2b and 2c because of the marked donor strength of the diphenylcarbazolylcarbazole donor. The ΔEST of 2a was 0.10 eV and the PL quantum yield was 100%. The PL quantum yield of 2b and 2c was >90%, suggesting the diphenyltriazineand carbazolylcarbazole-based material designs to have good light-emitting performance. The final EQE of the 2a device was 20.6%, although the color coordinate (0.19, 0.35) was in the sky-blue range. Analogous material design was implemented in the subsequent development of high-EQE TADF emitters. 9,9′-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9Hcarbazole) (DCzTrz) and 9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin2-yl)benzene-1,2,3-triyl)tris(9H-carbazole) (TCzTrz) emitters were constructed based on the same material design platform.[29,67] In place of the carbazolylcarbazole donor, two carbazole (DCzTrz) or three carbazole (TCzTrz) units were attached to the phenyl linker. The increase in the number of carbazole units decreased ΔEST due to the enhanced donor strength and enhanced the EQE due to the large overlap of HOMO and LUMO. The multiple donor-derived material design allowed HOMO and LUMO overlap as well as HOMO and LUMO separation. The EQE of the TCzTrz device was 25.0%, which is the highest of all blue TADF devices. The uniform distribution of the extensively overlapping HOMO and LUMO was responsible for the high EQE of the TCzTrz device. Similarly, 9,9′,9″,9″′-((6-phenyl-1,3,5-triazine-2,4-diyl)bis(benzene5,3,1-triyl))tetrakis(9H-carbazole) (DDCzTrz) with four carbazole units behaved as a high EQE emitter.[67] Moreover, the DDCzTrz emitter exhibited a longer lifetime than the common Ir(dbi)3 triplet emitter because of the chemical stability of the donor and acceptor moieties. This work demonstrated that blue TADF emitters have potential as high EQE and stable blue emitters and could be competitive with phosphorescent emitters. Bicarbazole was used as a donor moiety paired with an aryltriazine acceptor. 3,3′-Bicarbazole was included in the chemical structure of blue emitters, but the strengthened donor character due to the coupled carbazole units led to greenishblue rather than blue emission. 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′-phenyl-9H,9′H-3,3′-bicarbazole (BCzT) is a greenish-blue emitter with a high PL quantum yield of 0.95 and a high EQE of 21.7%.[32] Another 3,3′-bicarbazole compound, 9,9′-bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)9H,9′H-3,3′-bicarbazole (33TCzTTrz), was superior to BCzT in terms of higher absorption and PL quantum yield due to a dual emitting core design.[69] The EQE of the 33TCzTTrz device was 25.0%. However, 2,3′-bicarbazole and 3,4′-bicarbazole performed poorly as donors of aryltriazine-based emitters due to the weak donor strength originating from the isolated HOMO distribution and the distortion of the two donors, respectively. Dibenzoazasiline or indoloacridine-type donors were also utilized in blue TADF emitters with an aryltriazine acceptor.[33,131] Currently, aryltriazine-type blue emitters show promising results as high-efficiency blue emitters as evidenced by a 100% PL quantum yield and 25% QE.[29,69] Particularly, 37% EQE was recently demonstrated using a triazine type TADF emitter although the CIE y-coordinate was 0.43.[132] Moreover, the lifetime of aryltriazine-type emitters is comparable to that of phosphorescent emitters.[67,133] Therefore, the aryltriazine-type



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emitters also behaved as stable blue TADF emitters and showed a lifetime of 770 h at 500 cd m−2, which suggested that the carbazole- and benzonitrile-based molecular designs ensure the stability of blue TADF emitters due to their chemical stability. The same result was reported in other work, and the 5CzCN emitter functioned well as a soluble blue TADF emitter. The EQE of the solution-processed 5CzCN device reached 18.7%, comparable to that of a vacuum-processed 5CzCN device. As the soluble TADF emitters based on the benzonitrile acceptor, F-modified 2,4,6-tri(9H-carbazol-9-yl)-3,5-difluorobenzonitrile (3CzFCN) and 2,3,4,6-tetra(9H-carbazol-9-yl)-5-fluorobenzonitrile (4CzFCN), also showed a high EQE of 20% due to the F-mediated increase in solubility.[129] The F unit both increased the solubility and decreased the ΔEST of the soluble TADF emitters. Other than dicyanobenzene- and benzonitrile-type TADF emitters, a pyridinedicarbonitrile-type 2,6-di(9H-carbazol9-yl)-4-phenylpyridine-3,5-dicarbonitrile (CPC) has also been reported.[30] Although few cyanobenzene-type TADF emitters have been reported, they are some of the best performing blue TADF emitters in terms of EQE and lifetime. Thus, the benzonitrile family shows promise as blue TADF emitters due to their small ΔEST and long-term stability. Therefore, an advanced molecular design utilizing a benzonitrile acceptor would further improve the device performance of blue TADF emitters.

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materials are candidates for use in high-QE and long-lifetime blue emitters. However, the excited state lifetime of the aryltriazine-type emitters is relatively long and serious efficiency roll-off is frequently observed. Therefore, a modified molecular design that reduces the excited state lifetime is required for practical applications.

5.3. Diphenylsulfone-Type Emitters Diphenylsulfone acceptor-derived materials were investigated as high-EQE blue TADF emitters by linking the acceptor with a strong donor such as acridine due to the weak acceptor strength of diphenylsulfone. In fact, only acridine functioned well as the donor of diphenylsulfone-based emitters.[18,58,70,134–136] Carbazole-type donors were ineffective in diphenylsulfone compounds, which limited the development of diphenylsulfone-type TADF emitters because acridine reduces the lifetime of the device. Furthermore, the instability of the diphenylsulfone acceptor hampers research into diphenylsulfone-type compounds. 10,10′-(sulfonylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS) is the most well-known diphenylsulfone-type blue emitter, and exhibits a 20% EQE, short excited state lifetime of 3.1 µs, and blue color with a y color coordinate below 0.20. The strong electron donor character of acridine reduced the ΔEST to 0.09 eV, which influenced the short excited lifetime of DMAC-DPS. Several groups optimized the EQE of the DMAC-DPS device and doping of DMAC-DPS in the 2,2′,4-tris(di(phenyl)phosphoryl)-diphenylether (DPETPO) host produced a blue TADF OLED with a high EQE of 23.0%.[58] DMAC-DPS could also function as a non-doped emitter; indeed the non-doped DMAC-DPS device attained a high EQE of 19.5% due to suppression of intermolecular inter actions.[134] The perpendicular orientation of the acridine moiety by steric hindrance of hydrogen was the main reason for the weak intermolecular interactions and little aggregation-induced quenching behavior. An asymmetrically designed 10-(4-((4-(9H-carbazol-9-yl) phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) emitter having one acridine and one carbazole instead of two acridine units performed similarly to DMACDPS.[70] The PL quantum yield of 0.71 in solution, delayed fluorescence lifetime of 5.6 µs and ΔEST of 0.14 eV of CzAcSF realized a high EQE of 21.7% in a blue device. CzAcSF could also be used as a host material harvesting singlet excitons of a common fluorescent 2,5,8,11-tetra-tert-butylperylene (TBPe) dopant material by a Forster energy transfer process. A fluorescent TBPe device demonstrated a high EQE of 18.1% by triplet harvesting of the CzAcSF host by a TADF process. This work proposed the feasibility of applying a TADF emitter as the host material of a common fluorescent emitter for a blue fluorescent device with a high EQE. Another important diphenylsulfone derivative is 2,7-bis(9,9dimethylacridin-10(9H)-yl)-9,9-dimethyl-9H-thioxanthene 10,10-dioxide (DMTDAC), which overcomes the broad emission spectrum issue of TADF emitters.[137] The broad emission spectrum of emitters can be narrowed by rigidifying the molecular structure,[138] and this approach was successful in the design of 1603007 (15 of 24)


the DMTDAC emitter. The rigidity of the DMTDAC emitter was donated by 9,9-dimethyl-9H-thioxanthene 10,10-dioxide, which had a diphenylsulfone acceptor instead of diphenylsulfone. The interlocking of the two phenyl units linked to the sulfone unit prohibits molecular motion of the acceptor unit, which induced a narrow emission spectrum of the DMTDAC emitter. The full width at half maximum of the DMTDAC device was 65 nm, which was less than the 72 nm of DMAC-DPS with an untied diphenylsulfone acceptor. In addition, the EQE of the device was 19.8% and the color coordinate was (0.15, 0.13) because of a high PL quantum yield of 100%, short delayed fluorescence lifetime of 1.2 µs and no singlet-triplet energy splitting. In fact, the DMTDAC showed the photophysical properties required for blue TADF emitters, although it is not of practical use due to poor stability. Other than the three materials introduced above, several disulfone-type emitters were synthesized using carbazoletype donors, but the photophysical properties and device performance were not satisfactory because of the weak acceptor strength of the diphenylsulfone moiety.[27,139,140] Diphenylsulfone acceptor-based blue emitters were acceptable as high-efficiency and deep-blue emitting emitters when the diphenylsulfone acceptor was substituted for acridine donors. However, they are unacceptable as long-lifetime emitters because of the low bond dissociation energy of CS bonds under polaron formation. Therefore, the diphenylsulfone-based molecular design should be used as the material design to achieve a high EQE, but should be avoided as the design platform of stable blue emitters.

5.4. Other TADF Emitters Cyanobenzene, aryltriazine, and diphenylsulfone-type emitters are typical blue TADF emitters. Other high-efficiency emitters include arylboron and arylketone emitters, although diazatriphenylene, diazafluorene, and oxadiazole materials were also developed as blue emitters.[55,57,117–119,141–145] Arylboron-type emitters take advantage of the electron deficiency of the boron atom, which results in a small ΔEST and high PL quantum yield. As the boron atom easily forms complexes with other electron-donating moieties, it is generally protected by bulky alkyl units. A phenoxaborin acceptor-based emitter modified with acridine delivered a high EQE of 20% and a dimethylphenyl-protected boron emitter with a diphenylamine-modified carbazole donor afforded a high EQE of 21.6%, but no lifetime result was reported due to the instability of the arylboron moiety. Recently, a novel boron based TADF emitter, 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 (DABNA-2), was reported as a deep blue emitter with a very narrow FWHM of 28 nm, CIE y-value below 0.13, and maximum EQE of 20.2%.[138] Multiple resonance effect in the rigid polycyclic building block was responsible for the small FWHM and deep blue color. This is the best FWHM value reported in the TADF emitters until now. Arylketone-type emitters are built on a benzophenone-type acceptor moiety, and bis(4-(9H-carbazol-9-yl)phenyl)methanone (Cz2BP) and bis(4-(9H-[3,9′-bicarbazol]-9-yl)phenyl)methanone


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the singlet excitons transfer energy to the fluorescent emitter by Forster energy transfer process. As the emission process of the fluorescent emitter is Forster energy transfer, electrons of the fluorescent emitter are excited to excited state and then return to ground state by radiative transition process without any loss of excitons. Theoretically, 100% internal QE can be reached if there is no Dexter energy transfer process and direct charge trapping by the fluorescent dopant. Although the loss process is inevitable in the real device, device engineering of the TADF sensitized device realized high EQE of 18.1% in the TADF sensitized fluorescent device. At first, 10-phenyl-10H, 10′H-spiro[acridine-9,9′-anthracen]10′-one (ACRSA) TADF dopant was applied as the sensitizer of 2,5,8,11-tetra-tert-butylperylene (TBPe) fluorescent dopant and enabled fabrication of blue fluorescent device with a high EQE of 13.4% with a color coordinate of (0.17, 0.30).[40] As an initial QE of the TADF sensitized device, the QE was high. Moreover, the lifetime was also extended by the TADF sensitizer. However, incomplete triplet to singlet conversion of the ACRSA TADF emitter and poor energy transfer limited the EQE and color purity of the TADF sensitized fluorescent device. Dramatic improvement of the device performances was made using a CzAcSF TADF emitter as the sensitizer.[149] Relatively short wavelength emission and efficient up-conversion of CzAcSF compared to those of ACRSA facilitated the sensitizing process and enhanced the device characteristics of the TBPe device. A fluorescent TBPe device demonstrated a high EQE of 18.1% by triplet harvesting of the CzAcSF host by a TADF process. It was found that the increase of CzAcSF doping concentration and decrease of TBPe doping concentration could enhance the QE of the blue fluorescent devices. The high sensitizer doping concentration reduced charge trapping by TBPe and low TBPe doping concentration limited Dexter energy transfer and charge trapping by TBPe. This work could provide a guideline to maximize the EQE of the TADF sensitized fluorescent device. The same CzAcSF host material could also work as the host of TBPe without any other host material as the non-doped CzAcSF possessed TADF character.[150] The EQE of the CzAcSF:TBPe device was 15.4%, which was similar to that of non-doped CzAcSF device, indicating complete Forster energy transfer from the TADF host to the fluorescent emitter. This work proposed the feasibility of applying a TADF emitter as the host material of a common fluorescent emitter for a blue fluorescent device with a high EQE. Similarly, TADF type exciplex was also effective as the host of blue fluorescent dopant.[147] Although not many TADF sensitized fluorescent emitter systems are available, they are promising as the high efficiency blue emitters because of potential as high EQE and long lifetime emitters. Ideally, they can achieve high efficiency of TADF sensitizer and long-term stability of the fluorescent emitter if stable TADF materials are secured.

5.5. TADF Sensitized Fluorescent Emitters TADF emitters can be applied as emitters by themselves, but they can also play a role of harvesting singlet excitons of common fluorescent emitters by Forster energy transfer process. In this process, the emitting layer consists of host, TADF sensitizer, and common fluorescent emitter. The singlet excitons of the TADF emitters are harvested by up-conversion and

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5.6. HLCT-Type Emitters HLCT type emitters have emerged as potentially efficient fluorescent emitters by harvesting singlet and triplet excitons through hybrid excited state of local excited state and CT excited state.[151–154] It has been proposed that origin of light emission



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(CC2BP) are representative blue emitters.[55] Blue emission was produced using the two emitters, but the EQE of the blue device was below 15% because of a low PL quantum yield. The weak electron-accepting character of benzophenone was responsible for the low EQE of the device, which was resolved by replacing benzophenone with phenylpyridinylmethanone. Substitution of one phenyl unit of benzophenone with pyridine strengthened the acceptor character, which improved the photophysical parameters of the compounds for efficient TADF emission. (2,5-di(9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone (DCBPy) and (3,5-di(9H-carbazol-9-yl)phenyl)(pyridin4-yl)methanone (mDCBP) are phenylpyridinylmethanonetype compounds with high QE values of 24.0% and 18.4%, respectively.[57,144] In other reports, the TADF characteristics were realized in an intermolecular charge transfer complex known as exciplex.[146–148] Since the paper explaining TADF behavior of the exciplexes, blue exciplexes were also considered as a kind of TADF emitters. However, it was difficult to design the blue exciplexes because only a few donor and acceptor type molecules meeting the requirement of high triplet energy and appropriate donor or acceptor strength are available. ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T) was the most widely used acceptor molecule, and it generated blue intermolecular charge transfer complex with carbazole type donor molecules. An exciplex of 4,4′-bis(9carbazolyl)-2,2′-dimethylbiphenyl (CDBP) and PO-T2T was the best performing blue exciplex enabling 13.0% EQE as a blue emitter with a CIE coordinate of (0.17, 0.29).[147] Although the EQE of the blue exciplex was moderate, this result open a path of utilizing the exciplex as a kind of blue TADF emitter. Chemical structures, photophysical properties and device characteristics of TADF emitters are displayed in Figure 6 and Table 6. The device performances of the TADF devices were all gathered from devices with the TADF emitting layer sandwiched between high triplet energy exciton blocking layers for exciton confinement. The device structure of the TADF devices in Table 1 was quite similar to that of the PhOLEDs because triplet excitons should be harvested for light emission. From the chemical structures of the blue TADF emitters, an ideal design of TADF emitters is to have a donor–acceptor structure with a phenyl linker between the donor and the acceptor, and to have a HOMO dispersing donor whether the HOMO dispersion is from multiple donors or single donor. Dual emitting core based structure also was proven to be efficiency enhancing strategy. In addition, the key requirement of the donor and acceptor is stability resistant to decomposition under positive or negative polaron state. Schematic diagram of the high efficiency TADF molecules is presented in Figure 3.


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Figure 6. Chemical structure of the TADF emitters.

1603007 (17 of 24)



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Photophysical property

2CzPN

HOMO (eV)

LUMO (eV)

ES/ET (eV)

–5.80

–3.0

2.60/2.51

Device performance

Ref.

ΦPL (solution/film)

EQEmax (%)

Color index

0.47/0.89 a)

b)

15.8

0.16, 0.29

[123]

c)

BTCz-2CN

–6.17

–3.58

2.63/2.46

0.29 , 0.94 /0.85

11.8

–

[124]

BFCz-2CN

–6.19

–3.58

2.59/2.46

0.25a), 0.95b)/0.85c)

12.1

–

[124]

DCzIPN

–6.40

–3.88

2.77/2.72

–/67

16.4

0.17, 0.19

[125]

34TCzPN

–6.30

–3.52

3.02/2.86

0.66

21.8

0.17, 0.29

[68]

44TCzPN

–6.38

–3.54

3.05/2.84

0.61

19.5

0.16, 0.23

[68]

35IPNDCz

–

–

3.0/2.86

0.15a), 0.5b)/0.58c)

9.2

487 nme)

[127]

26IPNDCz

–

–

2.9/2.84

0.12a), 0.72b)/0.72c)

9.6

501 nme)

[127]

4CzBN

–5.73

–2.87

3.00/2.70

0.49

10.6

0.17, 0.20

[128]

5CzBN

–5.55

–2.74

2.90/2.68

0.70

16.7

0.22, 0.40

[128]

5CzCN

–6.29

–3.44

2.95/2.79

0.08a,c), 0.71b,c)/0.61a,c), 0.81b,c)

19.7

–

[65]

f)

18.7

0.17, 0.27

[65]

4TCzBN

–5.48

–2.73

2.86/2.62

0.68

16.2

0.16, 0.22

[128]

5TCzBN

–5.45

–2.74

2.77/2.60

0.86

21.2

0.21, 0.41

[128]

CPC

–6.25

–3.47

2.77/2.73

0.50

21.2

0.20, 0.35

[30]

3CzFCN

–6.38

–3.56

3.06/3.00

0.13a,c), 0.76b,c)/0.34a,c), 0.74b,c)

17.8f)

0.16, 0.19

[129]

4CzFCN

–6.31

–3.49

2.97/2.91

0.16a,c), 0.81b,c)/0.57a,c), 1.0b,c)

20.0f)

0.16, 0.25

[129]

–5.90

–2.60

–

0.62

11 ± 1

–

[130]

2a

–

–

–

–/1.0

20.6 ± 1.8

0.19, 0.35

[66]

2b

–

–

–

–/0.95

16.8 ± 1.7

0.17, 0.27

[66] [66]

CC2TA

c)

–

–

–

–/0.93

14.6 ± 1.0

0.18, 0.28

DCzTrz

–5.88

–2.86

2.89/2.64

0.43

17.8

0.15, 0.16

[67]

DDCzTrz

–6.01

–2.90

2.80/2.53

0.66

18.9

0.16, 0.22

[67]

TCzTrz

–

–

2.96/2.80

17/43a,c)

25

0.18, 0.33

[29]

BCzT

–

–

–

0.96/–

21.7

492 nme)

[32]

–5.74

–3.21

3.01/2.76

–/0.21a,c), 1.0b,c)

25.0

0.23, 0.42

[69] [18,58]

2c

33TCzTTrz DMAC-DPS

–5.92

–2.92

3.00/2.91

0.80/0.80

23.0

0.16, 0.21

CzAcSF

–5.89

–3.00

2.80/2.66

0.71/–

20.7

0.16, 0.21

[70]

DMTDAC

–6.10

–3.35

3.10/3.10

0.96a,c)/1.0a,c)

19.8

0.15, 0.13

[137]

Cz2BP

–5.74

–2.64

3.10/2.89

0.21/0.55

8.1

0.16, 0.14

[55]

CC2BP

–5.65

–2.63

3.02/2.88

0.38/0.73

14.3

0.17, 0.27

[55]

DCBPy

–

–

2.87/2.84

0.88c)

24

0.17, 0.36

[57]

mDCBP

–5.72

–2.71d)

3.0/2.94

0.90

18.4

0.16, 0.25

[144]

DABNA-2

–5.38

–2.75

2.64/2.47

0.90

20.2

0.12, 0.13

[138]

a)without

N2; b)under N2; c)in the host films; d)HOMO – ES; e)EL wavelength; f)solution device.

is local singlet excited state rather than CT excited state, which is advantageous for narrow emission spectrum, blue-shift of the emission wavelength, and short excited state lifetime. The HLCT mechanism is differentiated from the TADF mechanism in that the final emissive excited state of HLCT process is local excited state compared to CT excited state of TADF process. On the other hand, the HLCT mechanism can be grouped into the same category of TADF in that reverse intersystem crossing process from triplet excited state to singlet excited state is required for full harvesting of excitons and CT excited state is involved in the exciton converting process. The CT state of the

Adv. Funct. Mater. 2017, 27, 1603007

HLCT mechanism contributes to facilitate reverse intersystem crossing by small ΔEST and the local excited state increases radiative transition probability of the emitters. Therefore, the HLCT type emitters can possess the advantages of conventional fluorescent emitters and TADF emitters ideally. A schematic picture of the HLCT process is described in Figure 7. In order to develop the HLCT compounds, a specific mole cular design approach is required to generate the hybrid excited state. For local excited state emission, a chromophore should be included and a donor–acceptor structure should be the main backbone structure for CT state mediated intersystem



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Table 6. Photophysical properties and device performances of TADF emitters.


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Figure 7. Schematic illustration of the HLCT process.

crossing. Therefore, the selection of efficient chromophore and engineering of molecular parameters such as dihedral angle, donor strength, and acceptor strength are critical to the hybrid excited state formation. For example, too large dihedral angle or too small dihedral angle between donor and acceptor moieties leads to only CT excited state or only local excited state rather than hybrid excited state. Therefore, most HLCT type emitters were constructed to have a choromophore in a moderately twisted donor–acceptor backbone structure. The merits of blue-shifted emission spectrum and narrow emission spectrum in addition potentially high efficiency of the HLCT mechanism based emitters encouraged the development of blue HLCT emitters. Since the first demonstration of enhanced fluorescent emission by the CT state assisted emission process, several blue emitters were reported.[158–161] Initial blue fluorescent material identified as the HLCT emitter was N,N-diphenyl-4′-(1-phenyl-1H-phenanthro[9,10-d]imidazol2-yl)biphenyl-4-amine (TPA-PPI) with a twisted donor–acceptor structure.[161] PPI was a chromophore as well as an acceptor and TPA was a donor of the molecule. Twisted structure allowed CT excited state formation in the TPA-PPI molecule. Both CT and local excited state character was confirmed by solvatochromic experiment, which contributed to high EQE above 5.0% in the deep-blue emitting TPA-PPI device without any doping. Similarly designed N-phenyl-4′-(1-phenyl-1H-phenanthro[9,10-d] imidazol-2-yl)-N-(4′-(1-phenyl-1H-phenanthro[9,10-d]imidazol2-yl)-[1,1′-biphenyl]-4-yl)-[1,1′-biphenyl]-4-amine (TPA-2PPI) was also synthesized as a modified version of TPA-PPI for better thermal stability and device characteristics.[153] Instead of one PPI unit in the TPA-PPI, two PPI units were included in the TPA-2PPI, which improved efficiency roll-off behavior of the devices without damaging the maximum EQE. Another modified version of TPA-PPI is diphenyl-[3′-(1-phenyl-1Hphenanthro[9,10-d]imidazol-2-yl)-biphenyl-4-yl]-amine (mTPAPPI) which has meta- linked donor–acceptor structure in place of para- linked donor–acceptor structure of TPA-PPI. The meta- linkage increased the singlet energy by reduced degree of conjugation and shifted the emission wavelength to 404 nm. Color coordinate of the mTPA-PPI device was (0.16, 0.05) and maximum EQE was 3.3%. The meta- linkage was favorable for deep blue emission, but had negative effect on the EQE of the device.[162] Similar conclusion could also be made in the experiment comparing 3′-(phenanthren-9-yl)-N,N-diphenyl-5′(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1′-biphenyl]1603007 (19 of 24)


4-amine (m-PTPAPI) and 4′-(1-(4-(phenanthren-9-yl)phenyl)1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1′biphenyl]-4-amine (p-PTPAPI) possessing the same PPI and triphenylamine moieties.[163] The PPI moiety was also applied in other HLCT type material design and produced 2,2′-(sulfonylbis([1,1′-biphenyl]-4′,4diyl))bis(1-phenyl-1H-phenanthro[9,10-d]imidazole) (PMSO) with diphenylsulfone as an acceptor built on a molecular platform of donor–acceptor-donor.[154] In the case of PMSO, the PPI moiety played a role of a donor not an acceptor. Efficient deep blue device with a EQE of 6.8% and CIE of (0.15, 0.08) was derived using the PMSO emitter doped in the host material. The device performances of non-doped PMSO device were similar to those of the TPA-PPI device. Tris(4-phenanthrene-phenyl)amine (TPA-PA) and tris(4anthracene-phenyl)amine (TPA-AN) which have phenanthrene and anthracene as the acceptor as well as chromophore were also HLCT type emitters constructed based on twisted donor– acceptor backbone structure.[152] “Hot exciton” channel in highly lying excited state was proposed as exciton converting path to explain high calculated singlet emission fraction above 25%. However, the EQE of the TPA-PA and TPA-AN devices was rather lower than that of the PPI derived emitters. Other than these, 4-(2-(4-(diphenylamino)phenyl)-1H-phenanthro[9,10-d]imidazol1-yl)benzonitrile (TPMCN) and 4-[2-(4′-diphenylaminobiphenyl4-yl)-phenanthro[9,10-d]imidazol-1-yl-benzonitrile (TBPMCN) were also HLCT mechanism based emitters.[155–157] Chemical structures, photophysical properties, and device performances of the HLCT type emitters are described in Figure 8 and Table 7. As described above, several HLCT type emitters were investigated as a new type of exciton harvesting material by reverse intersystem crossing channel. Although the EQE of the HLCT type emitters is not as high as that of TADF devices, they are promising as high efficiency blue emitters due to merits over TADF emitters such as short excited state lifetime, narrow emission spectrum, good color purity and low efficiency rolloff. Until now, only several materials were reported and exact design rule or mechanism was not clarified yet. Therefore, further study about molecular design to maximize the triplet to singlet exciton up-conversion is essential in future development.

6. Future Perspectives Recent effort to develop high-efficiency blue emitters has resulted in an increase in the EQE of blue OLEDs from the 20% of phosphorescent and TADF OLEDs. Therefore, these three technologies may compete for use in high-efficiency blue OLEDs due to merits and demerits of each technology as summed up in Table 8. However, each technology has challenges to be overcome to replace current blue fluorescent OLEDs. TTF-type blue fluorescent OLEDs are the most promising blue OLED technology for practical applications due to their long lifetime despite a lower efficiency compared to TADF and phosphorescent-type OLEDs. The maximum EQE of the TTFtype OLEDs is limited to about 15% from theoretical calculation, but there is still some room for improvement because


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Figure 8. Chemical structure of the HLCT emitters.

outcoupling-enhancing technologies such as horizontal dipole orientation can be used in TTF-type OLEDs by molecular engineering of TTF-type emitters. The EQE of TTF-type blue OLEDs may be increased to >20%, although it will not be comparable to those of TADF and phosphorescent OLEDs. Because TTF-type blue OLEDs are superior to TADF and phosphorescent OLEDs in terms of operational stability, they may be competitive as high-efficiency blue OLEDs if the EQE can be further enhanced without sacrificing the lifetime. In the TTF material design, modification of current anthracene or pyrene chromophores might be effective way of enhancing the EQE and lifetime within short time, but new chromophore design and synthesis may further upgrade the device performances of the TTF blue devices.

Blue PhOLEDs are the leading technology for high-efficiency blue OLEDs because of their superior QE compared to other blue OLEDs. However, the short lifetime of blue PhOLEDs remains to be resolved. There may be several reasons for the short lifetime of blue PhOLEDs, and one critical problem is the weak chemical bonding of the organic ligand with the Ir core. The Ir-N bond is particularly weak and a new ligand structure that stabilizes the Ir-N bond is necessary to produce blue PhOLEDs with a stable lifetime. As phenylimidzole-type Ir emitters are potentially better than other Ir or Pt emitters in terms of lifetime, ligand modification that stabilizes the Ir-N bond of phenylimiazole-based Ir emitters and to blue-shift the emission wavelength should be performed to improve the lifetime. In addition, stable high triplet energy host materials are

Table 7. Material and device performances HLCT type emitters. Photophysical property HOMO (eV)

LUMO (eV)

Device performance

PLmaxa) (nm)

ΦPL (solution/film)

EQEmax (%)

Ref.

Color index

TPMCN

–

–

480

0.078/0.13

–

–

[155]

TBPMCN

–5.20

–2.40

451

0.79/0.40

7.8

0.16, 0.16

[155]

TPA-PPI

–5.22

–2.27

438

0.90/0.90

5.0

0.15, 0.11

[161] [153]

TPA-2PPI

–5.19

–2.29

437

0.85/–

4.91

0.15, 0.11

m-TPA-PPI

–5.23

–2.09

408b)

–

3.33

0.16, 0.05

[162]

m-PTPAPI

–5.30

–2.40

410

0.37

2.39

0.15, 0.05

[163]

p-PTPAPI

–5.30

–2.57

446

0.50

3.71

0.15, 0.15

[163]

b)

PMSO

–5.57

–2.73

–/0.45

6.8

0.15, 0.08

[154]

TPA-PA

–5.22

–2.17

422

0.6/0.7

2.0

0.15, 0.07

[152]

TPA-AN

–5.24

–2.55

468

0.65/0.5

3.0

0.15, 0.23

[152]

a)solution

470

PL; b)film.

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Table 8. Comparison of TTF, phosphorescent and TADF OLEDs. TTF OLED

PhOLED

Theoretical internal QE (%)

62.5

100

100

External QE (%)

30

∼25

∼11 000[164]

20 000[165]

∼300

0.15 ± 0.05, 0.10 ± 0.05

0.15 ± 0.05, 0.30 ± 0.05

0.15 ± 0.05, 0.30 ± 0.05

14.8[75]

33.2[25]

37[132]

Lifetime at 1000 cd m−2 (h) Color coordinate Best EQE of device (%)

essential for long-lifetime blue PhOLEDs. To date, few stable host materials for blue triplet emitters are available, and these are not satisfactory. Without such stable host materials, longlifetime blue PhOLEDs cannot be guaranteed because host degradation reduces the lifetime of the devices. To minimize degradation of the host materials, mixed hole-type and electron-type hosts need to be developed, as for green PhOLEDs. As mixed hosts are also advantageous in terms of EQE and efficiency roll-off of PhOLEDs, stable mixed hosts comprising high triplet energy hole-type and electron-type hosts are essential for producing blue PhOLEDs with a stable lifetime. Therefore, ideal blue PhOLEDs with a long lifetime and high EQE would have a chemically stabilized triplet emitter and a mixed host in the emitting layer. TADF OLEDs have a very short history as high-efficiency blue OLEDs, but rapid progress in device performance has been made in the last several years. In fact, the EQE and lifetime of blue TADF OLEDs are catching up with those of blue PhOLEDs. However, several challenges remain to be overcome for practical applications. The EQE of the TADF OLEDs should be enhanced and the efficiency roll-off issue must be resolved. The two issues related to efficiency behavior are associated with both TADF emitters and hosts. To maximize the EQE of TADF devices, TADF emitters with a PL quantum yield of 100% and hosts that fully harvest TADF emission are essential. A few blue TADF emitters have achieved a close to 100% PL quantum yield, but no host that fully utilizes all excitons of the TADF emitters is available. High-triplet-energy host materials such as bis[2-(diphenylphosphino)phenyl]ether oxide were reported to enable complete exciton conversion into photons, but these host materials suffer from exciton quenching by triplet-triplet annihilation and triplet-polaron annihilation due to their unipolar charge transport properties. Therefore, the development of bipolar or mixed hosts can be a solution to the high EQE issue of the blue TADF OLEDs in combination with high PL quantum yield TADF emitters. Bipolar or mixed hosts can also resolve the efficiency roll-off problem by reducing the triplet-related quenching processes if TADF emitters with a short excited state lifetime below 1.0 µs become available. Therefore, the solution to the low EQE and efficiency roll-off of the TADF OLEDs is to combine the TADF emitter with a short excited state lifetime and bipolar or mixed host in the emitting layer. In particular, the mixedhost approach can also enhance the lifetime of TADF OLEDs, as for the lifetime of PhOLEDs. From the viewpoint of the stability of TADF and phosphorescent emitters, pure organictype TADF emitters are superior to phosphorescent emitters because π electron-based covalent bonds are more stable than 1603007 (21 of 24)


TADF OLED

Ir-N bonds.[51] Therefore, blue TADF OLEDs could be competitive with blue PhOLEDs as next-generation high-efficiency blue OLEDs. In the design of the TADF emitters, donor, linker, and acceptor combined molecular design is essential and the donor should be chosen from multiple donor structure or the HOMO dispersing donor for high EQE. Additionally, rigid acceptor is preferred for narrow emission spectrum to relieve the stress of the host material. Among the three blue OLED technologies, TTF-type blue OLEDs would likely dominate initially, but TADF OLEDs or PhOLEDs may replace TTF OLEDs in the long run if the lifetime issue of the TADF OLEDs or PhOLEDs can be overcome through material and device engineering. In future, key topics of materials research should be the design and synthesis of stable TADF emitters, stable phosphorescent emitters and stable high triplet energy mixed hosts.

Acknowledgements This work was supported by Basic Science Research Program (NRF2016R1A2B3008845) and Nano Material Research Program (NRF2016M3A7B4909243) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning. Received: June 16, 2016 Revised: September 13, 2016 Published online: February 20, 2017

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