Highly Twisted DonorâAcceptor Boron Emitter and High Triplet Host

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Highly Twisted Donor-Acceptor Boron Emitter and High Triplet Host Material for Highly Efficient Blue Thermally Activated Delayed Fluorescent Device Dae Hyun Ahn, Hyuna Lee, Si Woo Kim, Durai Karthik, Jungsub Lee, Hyein Jeong, Ju Young Lee, and Jang Hyuk Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00931 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Applied Materials & Interfaces

Highly Twisted Donor-Acceptor Boron Emitter and High Triplet Host Material for Highly Efficient Blue Thermally Activated Delayed Fluorescent Device Dae Hyun Ahn, † Hyuna Lee, † Si Woo Kim, † Durai Karthik, † Jungsub Lee, ‡ Hyein Jeong, ‡ Ju Young Lee,* † and Jang Hyuk Kwon*†

†Organic

Optoelectronic Device Lab. (OODL), Department of Information Display,

Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul, Republic of Korea.

‡Display

Research Center, Samsung Display Co., 1, Samsung-ro, Giheung-gu, Yonginsi, Gyeonggi-do, Republic of Korea.

KEYWORDS : OLED, TADF, deep blue, high efficiency, high triplet host

ABSTRACT

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New highly efficient thermally activated delayed fluorescence (TADF) dopant materials (PXB-DI, PXB-mIC) for blue organic light-emitting diode are reported. These materials were designed by combining highly conjugated rigid ring donor moieties and boron acceptor with highly twisted configuration to have high TADF performance and minimized self-quenching properties. In addition, a new high triplet energy and hole transport type host material, PPBI, is also reported. This host represents deeper blue color owing to keeping original spectra of emitters. Fabricated blue TADF device with PXB-mIC in PPBI host exhibited max EQE of 12.5% with CIE of (0.15, 0.08) which is near National Television System Committee blue color. The blue TADF device performances of PPBI host was compared with electron transport type 2,8-bis(diphenylphosphine oxide) dibenzofuran (DBFPO) host. The blue TADF device with PXB-DI in DBFPO host exhibited max EQE of 37.4% in sky blue region. This study demonstrates that our molecular design concept of new emitters and host is beneficial for future high efficiency deep blue TADF devices.


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1. INTRODUCTION Over the past few years, thermally activated delayed fluorescence (TADF) materials have been extensively researched for the future organic light-emitting diode (OLED) applications. These TADF materials are highly efficient because of utilization of triplet excitons by reverse intersystem crossing (RISC) from the lowest triplet excited state (T1) to the lowest singlet excited state (S1) and also cost effective by the use of inexpensive pure organic chemicals.1-5 Especially, blue TADF materials are actively researching for the next generation OLED to overcome the current technical barriers such as no good deep blue phosphorescence emitters and low efficiency of fluorescence materials. For the past few years, many researchers have been reported highly efficient TADF materials by using various acceptor and donor moieties. Recently, boron acceptors have been studied as highly efficient TADF materials.6-10 They possess beneficial properties of trigonal planar geometry, p-π* conjugation through a vacant p-orbital, and the π-electronaccepting ability of the trivalent boron moiety in the excited state. Thus, molecular design of TADF materials with rigid and strong acceptor using boron atom is possible.



However, to date, there were few reports that how highly efficient boron emitter could be designed for TADF materials. According to previous report on high photoluminescence quantum yield (PLQY) boron fluorescent materials, boron emitters can greatly maximize their PLQY in the highly twisted structure. Such highly twisted structures with strong electron-donating abilities could give an advantage of induced intramolecular charge transfer (ICT) transition11-12 because of better orbital connection than the planar plane at the excited state. It also gives rise to large Stokes shift resulting in small spectral overlap between the absorption and emission spectra, which could suppress the Förster energy transfer process. In addition, emitter self-quenching can be minimized by reducing the Dexter energy transfer due to increase of molecular distance in the film state. Both of these mechanisms are related with non-radiative process, thus high PLQY can be achieved.13 To design these boron acceptor moieties, boron atom can be utilized as a trivalent or tetravalent structure. Generally, trivalent boron is more favorable as a strong acceptor unit than tetravalent one because tetravalent boron has already donated nonbonding electrons from N-heterocyclic donor such as pyridine. Therefore, trivalent boron moieties have an advantage for the design of good TADF materials.


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For the donor moieties, carbazole and acridine based materials are widely used.14-17 Acridine is a more appropriate TADF donor than carbazole, because it has a strong donor ability and can easily induce a twisted structure between the donor and the acceptor. On the other hand, simple carbazole is not appropriate donor due to weak donor ability and less separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).18 Therefore, recently, carbazole derivative materials such as indolocarbazole (IC) and diindolocarbazole (DI) are suggested as good TADF donors.19 Large electron donor plane of IC and DI can deliver a large spatial HOMO volume to the emitters and thereby leading to high oscillator strength. Moreover, such bulky and rigid ring system can provide high PLQY by reducing vibronic coupling and small ΔEST by the high steric hindrance, which could result in good performance of TADF devices.20-21 Based on these concepts, it is expected that combining the boron based acceptor and bulky and rigid ring donor would show good TADF characteristics. For IC donor materials, design of various IC donor could be possible by switching the indole position.22-23 Among these materials, the IC with the nitrogen attached to the meta-position (mIC) gave the



largest steric hindrance, and DI could also give the similar effect. Therefore, these two donor moieties were implemented for our TADF material design. Meanwhile, selecting the high T1 host is also required for highly efficient blue TADF devices. The host T1 exciton energy should be higher than 2.9 eV, because deep blue TADF emitters generally have at least 2.9 eV of T1 energy. Therefore, phosphine-oxide series materials, such as (oxybis(2,1-phenylene))bis(diphenylphosphine oxide) (DPEPO) and 2,8-bis(diphenylphosphine oxide) dibenzofuran (DBFPO) have widely used as host materials. These materials possess sufficiently high T1 energy over 3.1 eV, which can transfer exciton energy to deep blue TADF emitter. However, these host materials indicate strong electron transport (ET) type and high polarity characteristics. Such ET type characteristic induces exciton recombination zone shift to hole transport layer (HTL) side, and high host polarity can stabilize the charge transfer S1 energy of TADF dopant. Thus, both of characteristics can lead to red shifted spectrum, which results in sky blue characteristic. To overcome these limitations, less polar hole transport (HT) type or bipolar host materials is highly desired. Generally, hole or bipolar type host materials possess


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carbazole moieties or their derivatives. However, they exhibit low T1 energy of about 2.9 eV due to long electronic conjugation. Therefore, by using these insufficient T1 level hosts, external quantum efficiency (EQE) of deep blue TADF devices are lower than phosphineoxide based host devices. To design the highly efficient deep blue OLED devices, a new host material based on a new chemical moiety is required. Thus, we selected 5Hbenzo[d]benzo[4,5]imidazo[1,2-a]imidazole (BI) moiety. Here the effective π-conjugation could be reduced by two sp3-nitrogen atoms while retaining the donor property. Hence, expected high S1 and T1 energies would be achieved using this moiety. In this work, we report two new blue TADF emitter materials, 5,10-diphenyl-15-(10(2,4,6-triisopropylphenyl)-10H-dibenzo[b,e][1,4]oxaborinin-3-yl)-10,15-

dihydro-5H-

diindolo[3,2-a:3',2'-c]carbazole (PXB-DI) and 5-phenyl-12-(10-(2,4,6-triisopropylphenyl)10H-dibenzo[b,e][1,4]oxaborinin-3-yl)-5,12-dihydroindolo[3,2-a]carbazole

(PXB-mIC)

(Figure 1), and a new high T1 host material of 5-(5-(2,4,6-triiso-propylphenyl)pyridin-2-yl)5H-benzo[d]benzo[4,5]imidazo[1,2-a]imidazole (PPBI) (Figure 2). Both emitter materials are composed of rigid boron acceptor and rigid donor with highly twisted donor-acceptor structure. Our molecular design strategy is well correlated with reported well-known



concept for high PLQY and small ΔEST of good TADF emitters. In addition, our molecular design strategy is also combined with high PLQY boron emitter concept. For the deep blue TADF device, a new high T1 host material with HT type and non-polar characteristics is designed with benzoimidazole chemical moiety. Further, a pyridine moiety was attached on BI unit to control LUMO level and host polarity. Finally, the highly tilted and bulky triisopropyl benzene ring was incorporated on pyridine unit to increase the host molecular weight. By using this host material, highly efficient blue devices are achieved with near NTSC blue color coordinates. In addition, we also compared PPBI host properties with an ET type DBFPO host. We found that the device based on DBFPO host exhibited high efficiency but in sky blue region, whereas PPBI exhibited relatively low efficiency but in deep blue region.

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization All reagents for synthesizing the blue TADF dopant materials and host material were purchased from Sigma–Aldrich, TCI (SEJINCI) and used without additional purification.


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1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) was purchased from EM Index. 1,1-bis[(di-4-tolylamino) phenyl] cyclo-hexane (TAPC), and 2,2',2''-(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) were purchased from Jilin OLED Material Tech Co., Ltd. The other materials, 3,5-di(9H-carbazol-9-yl)-N,N-diphenylaniline (DCDPA), 2,8-bis(diphenylphosphine oxide) dibenzofuran (DBFPO) were synthesized by using the previously reported methods.24,

25

For the characterization of photo-physical

properties in solution, all materials were prepared in toluene solution at the concentration of 1 × 10−5 M. The UV-vis absorption spectrum was measured by V-750 Spectrophotometer (Jasco). The solution PL spectrum and low temperature (77 K) PL spectrum were measured by FP-8500 Spectrofluorometer (Jasco). The absolute PLQY values in doped films were measured by connecting an integrating sphere to the same spectrofluorometer. These doped films were also used for transient PL measurement. Transient PL was measured when photon counts were reached until 10,000 in a nitrogen environment using Quantaurus-Tau fluorescence lifetime measurement system (C1136703, Hamamatsu Photonics Co.). Thermal properties of newly synthesized materials were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis



(TGA). The glass transition temperature (Tg) was measured by DSC graph and the decomposition temperatures (Td) of emitters measured by TGA at 5% weight loss. Electrochemical analyses were performed using EC epsilon electrochemical analysis equipment. To measure the cyclic voltammetry (CV) characteristics of TADF emitters, platinum, carbon wire and Ag wire in 0.01 M AgNO3, 0.1 M tetrabutyl ammonium perchlorate (Bu4NClO4), acetonitrile solution were used as counter, working and reference electrodes, respectively. For supporting electrolyte, 0.1 M tetrabutyl ammonium perchlorate in acetonitrile solution was used. Using an internal ferrocene/ferrocenium (Fc/Fc+) standard, the potential values were converted to the saturated calomel electrode (SCE) scale. The optical band-gap was determined from absorption onset. The LUMO level of each material was calculated from both the optical band gap and HOMO level.

2.2. Device Fabrication and Characterization To fabricate OLEDs, Indium-Tin-Oxide (ITO) coated glass substrates (50 nm, sheet resistance of 10 Ω/□,) were sequentially cleaned in ultrasonic bath with acetone, and


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isopropyl alcohol for 10 minutes each, and then rinsed with deionized water. Finally, substrates were dried using nitrogen followed by UV-ozone treatment for 10 minutes. All organic layers and metal cathode were deposited on the pre-cleaned ITO glass by vacuum evaporation technique under a vacuum pressure of ~1×10-7 torr. The deposition rate of all organic layers in was about 0.5 Å/s. Similarly, the deposition rate of LiF and Al were maintained at 0.1 Å/s, 4.0 Å/s, respectively. Finally, all devices were encapsulated using glass cover and UV curable resin inside the nitrogen filled glove box. The OLED area was 4 mm2 for all the samples studied in this work. J-V and L-V characteristics of fabricated OLED devices were measured by using Keithley 2635A SMU and Konica Minolta CS-100A, respectively. EL spectra and CIE 1931 color coordinates were obtained using Konica Minolta CS-2000 spectroradiometer. All measurements were performed in ambient condition.

3. RESULTS AND DISCCUSION 3.1. Computational investigations



Molecular optimization and energy calculation on the HOMO, LUMO, S1, and T1 level was performed using time-dependent density functional theory (TDDFT) with the Gaussian 16 at the B3LYP/6-311G (d) level.26 As explained in introduction, both dopants indicated highly tilted structure between donor and acceptor due to the bulky donor structure. The calculated dihedral angle of PXB-DI and PXB-mIC was 72.3° and 71.3°, respectively. In addition, the out-of-plane structure was found in both materials as shown in Figure 1. Thus, both dopant materials showed well separated HOMO and LUMO orbitals. The PXB-DI has donor with longer conjugation than in the PXB-mIC, thus higher HOMO orbital distribution observed for the former, whereas the LUMO orbitals were almost similar in both materials due to the same acceptor. The HOMO conjugation length of PXB-mIC was shorter than PXB-DI, so low oscillator strength was expected for former. Indeed, the calculated oscillator strength of PXB-DI and PXB-mIC was 0.225 and 0.094, respectively. Even though the overlap of HOMO and LUMO is almost the same, the probability of transition is low for PXB-mIC due to low oscillator strength. The calculated S1 value of PXB-mIC was higher than PXB-DI. In addition, ΔEST of both materials were small due to the well separated HOMO and LUMO. PXB-DI exhibited small ΔEST of 0.09


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eV, as the S1 value of 2.71 eV and the T1 value of 2.62 eV. PXB-mIC showed relatively larger ΔEST of 0.20 eV, as the S1 and T1 values of 2.98 eV and 2.78 eV, respectively (Supporting Information Table S1). The ICT characteristics of both dopants were different due to the different donor, which results in small difference about ΔEST values. However, it is expected that both dopants would exhibit high TADF performances due to small ΔEST. In addition, PXB-mIC could show deeper blue color characteristic than PXB-DI. Further, we calculated the HOMO, LUMO, S1 and T1 level of PPBI host using the same TDDFT method. Firstly, the T1 energy of BI moiety was calculated and compared with that of carbazole. The T1 energy of BI was 3.42 eV and carbazole was 3.23 eV, so we found that BI can be used as a moiety for high T1 host instead of carbazole. Then, molecule 1 and 2 (Figure S1) substituted with different moieties from our target host material (PPBI) were calculated for comparison. Compare to PPBI, molecule 1 is the structure in which pyridine is substituted with a phenyl ring, and molecule 2 is the structure in which an isopropyl groups removed as shown in Figure S1. Compared with molecule 1, PPBI maintains the high T1 level and has relatively low LUMO level, therefore, it could lower electron trapping in dopant. In addition, it showed very low host polarity of 1.25 D,



whereas molecule 1 showed higher polarity of 2.73 D. Compared to molecule 2, PPBI exhibited highly tilted structure with short conjugation length induced by the isopropyl group, which could have an advantage for maintaining high T1 value. Indeed, molecule 2 showed longer conjugation length, which results in low calculated T1 value of 3.08 eV. Further, the HOMO and T1 values of PPBI compared with widely used HT type host 3,3'di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP). The relatively higher HOMO level of PPBI than mCBP suggests that PPBI can act as potential HT type host. Thus, we believed that PPBI would show great HT type, high T1 and non-polar properties, and it could be a suitable host for deep blue emitters. The summarized simulated data are shown in Figure S1.

3.2. Synthesis The starting materials boron acceptor (PXB-Br) and the donor compounds (DI and mIC) were synthesized according to the reported procedures.19,23 The target emitters PXB-mIC and PXB-DI were synthesized by Buchwald-Hartwig cross coupling reaction between boron acceptor and the corresponding donor compounds. The synthetic scheme is shown


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in Supporting Information Scheme S1. The PPBI was synthesized by two step protocol. Firstly, aromatic nucleophilic substitution reaction of imidazoimidazole (BI) and 5-bromo2-fluoropyridine, secondly, Suzuki coupling reaction between the obtained bromo compound (PyBI) and (2,4,6-triisopropylphenyl)boronic acid. The synthetic scheme of PPBI is shown in Supporting Information Scheme S2. All the synthesized materials were thoroughly characterized by NMR and high resolution mass spectrometry (HRMS) techniques. The detailed procedure and structural characterization of the compounds are described in Supporting Information.

3.3. Thermal Properties The thermal stability of the newly synthesized materials are important for the morphological stability and device performances. In this regard, thermal properties of host and two emitters were investigated by DSC and TGA. The Td (5 wt% loss) of PPBI, PXBDI, and PXB-mIC were observed at 283.64 °C, 380.20 °C, and 345.92 °C, respectively. The Tg of PPBI, PXB-DI, and PXB-mIC were observed at 101.26 °C, 193.14 °C, and



158.85 °C, respectively. The high Tg of these materials indicate all materials possess amorphous nature. The detailed properties are summarized in Figure S2.

3.4. Electrochemical and Photophysical Properties The electrochemical properties of two dopant materials were measured as explained in the experimental section. The HOMO and LUMO energies of PXB-DI and PXB-mIC were estimated as -5.54 eV, -5.73 eV and -2.62, -2.63 eV, respectively (Figure S3). The LUMO values were almost same, which were derived from the same acceptor, and the HOMO values were different due to the different donor moieties. Singlet and triplet energies were measured from the onset point of toluene PL and low temperature PL (LTPL) spectrum, respectively. PXB-DI showed deep blue PL peak of 458 nm with small ΔEST of 0.09 eV as shown in Figure 3a. Meanwhile, PXB-mIC showed blue shifted PL due to the shorter conjugation. It showed 438 nm PL peak with relatively large ΔEST of 0.19 eV as shown in Figure 3b. All data of electro- and photo-physical properties were well matched with calculated data.


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The electro- and photo-physical properties of PPBI host material were also measured by the same method used for dopants. PPBI host exhibited wide bandgap of 3.70 eV, and deep blue LTPL as shown in Figure 2b. The high T1 energy of 3.34 eV was measured at the onset point of LTPL. Further, the HOMO level was estimated as -5.71 eV and the LUMO level was -2.01 eV (Figure 2c). Thus, it can serve as perfect HT type high T1 host material. In addition, these results were well matched with our calculation data, and this material could be utilized as a deep blue TADF OLED host. All photo-physical properties data are summarized in Table 1.

3.5. Investigation of PLQY and Exciton Decay Lifetime The PLQY and delayed exciton decay lifetime were measured using 20 wt% doped film in two host materials. 2,8-bis (diphenylphosphine oxide) dibenzofuran (DBFPO) was used as ET type host and PPBI was used as HT type host. The PLQY value of PXB-DI was 97.1% in DBFPO and 78.5% in PPBI, respectively. PXB-mIC showed lower PLQY of 63.1% in DBFPO and 50.6% in PPBI, respectively. The PLQY difference between DBFPO and PPBI was occurred. The high polarity of host can stabilize the singlet CT state, which



resulting in red shift in PL spectrum and increased PLQY.27 DBFPO has higher polarity (5.17 D) than PPBI (1.25 D), therefore, the DBFPO hosted film showed red-shifted spectrum with high PLQY. Due to this characteristic, both of dopant materials showed spectrum peak difference of at least 20 nm as shown in Figure S4. Meanwhile, PXB-mIC exhibited lower PLQY than PXB-DI. This is due to lower oscillator strength of PXB-mIC than PXB-DI, which decreased the PLQY of former. Unfortunately, we found that high PLQY cannot be achieved by only highly twisted structure. Instead, our design concept was investigated by measurement of low selfquenching property. The PLQY variation with different doping concentration was measured (Figure S5). Both materials exhibited the highest PLQY at 20 wt% doping concentration in DBFPO host. Even in 50 wt% doping condition, PXB-DI showed high PLQY of 80.7% and PXB-mIC showed 53.9%. PLQY of PXB-DI was reduced by 16.8% and PXB-mIC was reduced by 15.6%, which means that PLQY values were well maintained even in high doping concentration of 50 wt%. We believed that the selfquenching phenomenon was minimized due to the highly twisted structure.


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The delayed exciton lifetime was also measured at same condition. The delayed PL curves of doped films are shown in Figure S6. The measured delayed exciton lifetimes of PXB-DI were 2.57 μs (DBFPO) and 3.33 μs (PPBI) and PXB-mIC were 3.89 μs (DPEPO) and 4.04 µs (PPBI), respectively. PXB-DI which showed smaller ΔEST indicated shorter delayed lifetime. In addition, delayed component was decreased as change of host polarity, due to the S1 stabilization effect. Here PXB-mIC that has large ΔEST showed much larger change, indicating that reverse intersystem crossing (RISC) was significantly reduced due to the decreased S1 stabilization effect. To analyze these results numerically, various rate constants of two dopants were calculated by using the reported method.28 The singlet radiative decay rate constant (krS), intersystem crossing rate constant (kISC), reverse intersystem crossing rate constant (kRISC) and triplet non-radiative decay rate constant (knrT) in DBFPO host were calculated using prompt PLQY, delayed PLQY, prompt and delayed exciton decay lifetime. The kRISC and knrT of PXB-DI was 1.17×106 s-1 and 1.65×104 s-1, respectively. The kRISC was almost 100 times faster than knrT, so RISC process was dominant, which results in great TADF performance. On other hand, kRISC and knrT of PXB-mIC was 5.22x105 s-1 and 1.26x105



s-1, respectively. Here, kRISC was just 4 times faster than knrT, thus, the difference was much smaller than PXB-DI. Hence, the RISC process was not dominant and non-radiative decay could occur, which results in poor TADF performance. Meanwhile, knrT became larger in PPBI host. The kRISC and knrT of PXB-DI was 6.18×105 s-1 and 9.64×104 s-1, and those of PXB-mIC was 1.03×105 s-1 and 2.06×105 s-1, respectively. PXB-DI still showed more than 6 times faster kRISC, however, PXB-mIC showed the contrary result. Therefore, higher device performance for PXB-DI in DBFPO host would expected. The detailed calculation data is shown in Supporting Information Table S2.

3.6. Device Performances To confirm that the PPBI host is the HT type host, hole only device (HOD) and electron only device (EOD) were fabricated. The HOD structure was ITO (50 nm)/ 1,1-bis[(di-4tolylamino) phenyl] cyclo-hexane (TAPC, 20 nm)/ PPBI (30 nm)/ TAPC (20 nm)/ Al, and the EOD structure was ITO (50 nm)/ 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole) (TPBi, 20 nm)/ PPBI (30 nm)/ TPBi (20 nm)/ LiF (1.5 nm)/ Al. As we


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expected, PPBI showed much faster J-V curve in HOD than that of EOD as shown in Figure S7. Therefore, it was confirmed that PPBI host is strong HT type host. To examine the TADF device performances of these new emitters, firstly, we fabricated blue TADF devices using phosphine oxide-based DBFPO (T1 energy of 3.2 eV) as host. The

fabricated

device

configuration

was

ITO

(50

nm)/

1,4,5,8,9,11-

hexaazatriphenylenehexacarbonitrile (HATCN, 7 nm)/ TAPC (50 nm)/ 3,5-di(9Hcarbazol-9-yl)-N,N-diphenylaniline (DCDPA, 10 nm)/ host: 20 wt% dopant (25 nm)/ DBFPO (5 nm)/ TPBi (30 nm)/ LiF (1.5 nm)/ Al (100 nm) as shown in Figure S8. Here, HATCN was used as a hole injection layer, TAPC and TPBi were used as hole and electron transport layer, respectively. DCDPA and DBFPO were used as triplet exciton blocking (EBL) layers. Both devices exhibited good performances as expected from material photo-physical properties. The device performances are shown in Figure 4 and Figure S9a. The collected data are listed in Supporting Information Table S3. Firstly, PXB-DI device showed very good performances. The maximum current efficiency and EQE was 66.2 cd/A, and 37.4%, respectively, with color coordinates of (0.16, 0.34). Although it showed sky blue



color characteristic, its efficiency result was very excellent. Such high EQE could be ascribed to high PLQY of 97.1% and high kRISC. In addition, it still maintained high EQE over 30% at 1,000 cd/m2, which indicates small efficiency roll-off due to the short delayed exciton lifetime. These EQE was measured under the assumption that PXB-DI emission pattern is Lambertian distribution. Therefore, we additionally measured angular distribution to confirm whether the light emission is stronger in front direction as shown in Figure S10. As a result, the emission pattern almost followed the Lambertian pattern and slightly narrow pattern was indicated only at the high viewing angle. Therefore, such high EQE was proved to be reliable. Secondly, the PXB-mIC device showed lower efficiency than PXB-DI. The maximum current efficiency of PXB-mIC device was 22.1 cd/A, and the maximum EQE was 18.8%. The low efficiency of PXB-mIC device attributed to low PLQY and kRISC. Instead, it indicated blue color characteristic of CIE (0.14, 0.18). Though the efficiency of these devices were good their colors were not satisfactory in blue region. In order to obtain pure blue color we fabricated the device using PPBI host. The device configuration was similar as the DBFPO host. The device performance plots are shown in Figure 5 and Figure S9b. The obtained data are listed in Supporting Information Table


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S3. Both devices exhibited lower efficiencies and blue shifted EL emission when compared to DBFPO hosted devices. The PXB-DI device showed maximum current efficiency and EQE of 37.8 cd/A, and 28.4%, respectively, with color coordinates of (0.14, 0.20). The PXB-mIC device showed lower efficiency than PXB-DI device. The maximum current efficiency was 8.6 cd/A, and the maximum EQE was 12.5%. However, it showed very deep blue color coordinates of (0.15, 0.08) which is near to NTSC blue color. Although the efficiency slightly decreased, the color characteristics were significantly improved in PPBI host devices. Table 2 shows recent material performance and its device EQE with CIE characteristics. Indeed, our device performances are very excellent compared with other reported data as shown in Table 2. Though both of devices exhibited great performances, it is hard to compare directly due to different color characteristic. Thus, the relative current efficiency was calculated by dividing the current efficiency by the y color coordinate. As a result, the relative current efficiency of PXB-DI in DBFPO and PPBI hosts were calculated to be 194.7 cd/A/y, and 189 cd/A/y, respectively. Although the absolute efficiency of the PPBI device is lower than that of DBFPO, current efficiency of PPBI is about equivalent level with DBFPO when the



color is considered together. Similarly, the relative current efficiency of PXB-mIC was also compared in both hosts. DBFPO device was 122.8 cd/A/y, and PPBI device was 107.5 cd/A/y. Both values were lower than PXB-DI, due to the low PLQY and poor TADF performance. However, PPBI device showed similar level with DBFPO device, which is demonstrating the proper host for deep blue TADF devices. The operational lifetime of blue TADF device is very important for real application. Our PXB-DI with DBFPO and PPBI host devices indicated very short operational lifetime under 1 hour. In order to investigate more accurate our PXB-DI emitter, many layer materials were changed to consider better stability of each layer.29-31 All the changed materials are summarized in Figure S11d. Fabricated devices with these materials, the maximum EQE slightly decreased to 23.1% due to relative low T1 of changed materials and its color coordinate was (0.16, 0.32). In this condition, we achieved good LT50 of 125 hours at initial luminance of 1,000 cd/m2. This good device lifetime is responsible for excellent PXB-DI emitter characteristic. The detailed device lifetime is provided in Figure S11.


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4. CONCLUSION In summary, we designed and synthesized two new TADF dopant materials, PXB-DI and PXB-mIC, and a new high T1 host material, PPBI. All materials were designed and evaluated for the highly efficient deep blue TADF devices. For high T1 host, the new electron donor moiety was introduced and combined with other moieties for high torsion angle. As a result, PPBI indicates high T1 value of 3.34 eV with strong HT type and nonpolar characteristic, which is useful for deep blue OLED devices. Our new TADF dopant material design with highly twisted structure exhibited high PLQY and minimized selfquenching effect even at 50 wt% doping concentration. TADF device fabricated using PXB-DI dopant showed very high EQE of 37.4% with sky blue color in DBFPO host, and PXB-mIC dopant showed relative low efficiency but with deep blue color in PPBI host which is near to NTSC blue color. We believe that our molecular design concept for the TADF emitter would be very useful for future OLED materials research.

ASSOCIATED CONTENT



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Supporting Information

This material is available free of charge via the Internet at http://pubs.acs.org.

The detailed synthetic procedure of PPBI, PXB-DI, and PXB-mIC. 1H NMR data,

13C

NMR data, and high resolution mass spectrometry (HRMS) data. Cyclic voltammetry data. Computational data of PPBI, PXB-DI, and PXB-mIC. PLQY variation curve and rate constant table. Device structure and energy diagram. Film PL data and exciton decay lifetime data. Device performance table.

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] (Jang Hyuk Kwon)

* E-mail: [email protected] (Ju Young Lee)

ORCID

Jang Hyuk Kwon: 0000-0002-1743-1486


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Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

This work was supported by National Research Foundation of Korea Grant (No. NRF2016R1A6A3A11930666) and the Industrial Strategic Technology Development Program of MKE/KEIT (10048317). And this work was also supported by Samsung Display Co., Ltd.

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Table 1. Photo-physical and thermal data of used materials.

Material

Bandga

HOMO

LUMO

T1

(nm)

p (eV)

(eV)

(eV)

(ΔEST) C)

345

3.70

5.71

2.01

3.34 PPBI

(0.50)

2.86 PXB-DI

Td ( °Tg ( °

λmaxa

458

2.92

5.54

τpb

τdb

(ns)

(μs)

-

-

-

380.2 193.1

0.971

24.70

2.57

0

b

b

b

0.785

23.15

3.33

c

c

c

345.9 158.8

0.631

26.96

3.89

2

b

b

b

0.506

29.61

4.04

c

c

c

C)

283.6 101.2 4

6

4

2.62 (0.09)

2.92 PXB-mIC

438

3.10

5.73

5

2.63 (0.19)

a Toluene

ΦPL

solution (10-5 M), b DBFPO: 20% doping film, c PPBI: 20% doping film

Table 2. Summary of photo-physical characters of highly efficient deep blue to sky blue TADF materials and electroluminescent properties.


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Materials

PLQY (%)

τd (μs)

ΔEST

EQEmax

EQE1000

(meV) (%)

(%)

CIE

Ref.

PXB-DI

97

2.57

90

37.4

31.7

(0.16, 0.34)

This work

PXB-mIC

63

3.89

190

12.5

5.2

(0.15, 0.08)

This work

DPAC-TRZ

82

2.9

133

25.8

18.3

(0.17, 0.38)

[5]

100

2.1

72

36.7

30.5

(0.18, 0.43)

[5]

98

0.27

20

31.9

20.1

(0.14, 0.18)

[32]

PX-SBA

63

9.7

60

20.8

7.0

(0.16, 0.15)

[33]

PM-SBA

73

23.1

70

29.2

12.5

(0.17, 0.28)

[33]

BCzTrz

96

31.2

40

23.6

5.2

(0.23, 0.42)

[34]

TCzTrz

99

23.6

10

31.8

11.3

(0.20, 0.44)

[34]

PXB-SPACX 56

2.06

60

20.1

18.0

(0.14, 0.16)

[7]

SpiroACTRZ 3DPyMpDTC



Figure 1. Molecular structure and simulation results of (a) PXB-DI and (b) PXB-mIC.

Figure 2. (a) Molecular structure of PPBI, (b) UV-Vis, toluene PL, and LTPL spectra of PPBI, (c) Cyclic voltammetry data of PPBI.


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Figure 3. UV-vis, PL, LTPL spectra of (a) PXB-DI and (b) PXB-mIC in toluene.


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Figure 4. Device performances of DBFPO host devices. (a) Voltage vs. current density, (b) current density vs. EQE, (c) EL spectra.

Figure 5. Device performances of PPBI host devices. (a) Voltage vs. current density, (b) current density vs. EQE, (c) EL spectra.



Table of Contents artwork


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