CN-Modified Host Materials for Improved Efficiency and Lifetime in

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CN modified host materials for improved efficiency and lifetime in blue phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes Sung Yong Byeon, Ji Han Kim, and Jun Yeob Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15502 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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ACS Applied Materials & Interfaces

CN modified host materials for improved efficiency and lifetime in blue phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes Sung Yong Byeon, Ji Han Kim, Jun Yeob Lee* School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 16419, Korea [email protected]

Abstract CN modified host materials, 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3carbonitrile (o-CzCN) and 9-(3-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3carbonitrile (m-CzCN), which can improve the external quantum efficiency and lifetime of both blue phosphorescent and thermally activated delayed fluorescent (TADF) emitters were developed. A molecular design approach to stabilize the molecular structure and reduce the energy gap produced two high triplet energy host materials of o-CzCN and m-CzCN compatible with the phosphorescent and TADF emitters. The new host materials lowered operation voltage, increased quantum efficiency, and elongated lifetime of both phosphorescent and TADF devices. Keywords: blue device, delayed fluorescence, host, efficiency, lifetime

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Introduction Device performances of organic light-emitting diodes (OLEDs) are governed by organic materials stacked within the device structure, but among those the most important are host and dopants 1-5 Even though final emission color and light-emitting characteristics are dominated by the dopant, good light emission performances cannot be realized without the host material, indicating that the host is as important as the dopant.6-8 Therefore, many materials have been explored as the host materials of OLEDs and were commercialized mostly for red and green phosphorescent OLEDs and full color fluorescent OLEDs. However, it was challenging to commercialize blue phosphorescent host materials because of strict criteria such as triplet energy higher than 2.75 eV, stability under device operation, and small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap.9-12 The most well-known host for blue phosphorescent OLEDs (PhOLEDs) is 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) which is regarded as a hole carrying host. In many publications covering blue PhOLEDs, the mCBP host was generally used.13-15 Recently, it is also being applied as the host of blue thermally activated delayed fluorescent (TADF) OLEDs which are candidate of high efficiency blue OLEDs.16-19 However, mCBP has several limitations as the host of blue PhOLEDs or TADF OLEDs due to weak C-N bond in the backbone structure, poor electron accepting character by shallow LUMO, and mismatch of hole/electron carriers.20-22 These drawbacks of mCBP incited high triplet energy host development for PhOLEDs or TADF OLEDs. Many host materials were developed based on several design approaches, but most of the host materials only focused on quantum efficiency (QE) of the devices without considering the lifetime.23-25 Therefore, host for both high efficiency and extended lifetime in blue devices is essential. Especially, universal host materials which are compatible with both PhOLEDs and TADF OLEDs are desired because both devices are promising as next generation high efficiency blue devices.


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In this work, universal host materials for both blue PhOLEDs and TADF OLEDs, 9-(2-(9phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (o-CzCN) and 9-(3-(9-phenyl9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (m-CzCN), were developed for both high QE and improved lifetime. The two host materials worked perfectly in the blue PhOLEDs and TADF OLEDs by improving the QE and lifetime simultaneously. Stabilization of molecular structure and good electron accepting character were proposed as key factors for the superior device performances. This is the first work reporting high QE and stable lifetime in the blue PhOLEDs and TADF OLEDs based on the same device structure for the QE and lifetime measurement.

Results and discussion To overcome the challenging issues of mCBP, a molecular design strategy connecting carbazole with a phenyl unit via 3- position rather than 9- position of carbazole and employing CN modified carbazole instead of carbazole was approached. The first design route was intended to reduce the number of C-N unit in the main backbone unit and the second design route was adopted to improve electron accepting character and electron injection properties. A biphenyl backbone structure was modified with CN modified carbazole via 9- position and 9-phenylcarbazole via 3- position to develop o-CzCN and mCzCN, respectively. The two host materials were built on the same chemical platform except for the linking position of CN modified carbazole to the biphenyl core. Synthesis of o-CzCN and m-CzCN was schematically explained in Scheme 1. More detailed description of the synthesis was added in experimental section. First, triplet energy of the host materials was measured as the two host materials were intended for use in blue PhOLEDs and TADF devices which involved triplet excitons directly or indirectly in the radiative transition process. Fluorescence and phosphorescence spectra for the analysis of the radiation process is presented in Figure 1. Both fluorescence and 3 Environment ACS Paragon Plus


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phosphorescence spectra of o-CzCN were blue shifted compared to those of m-CzCN due to distortion of the backbone structure by steric hindrance. The triplet energy values of o-CzCN and m-CzCN were 2.80 eV and 2.78 eV, respectively. As explained, reduced degree of conjugation by molecular distortion was the origin of the high triplet energy of o-CzCN. The triplet energy of o-CzCN and m-CzCN was quite similar to that of mCBP (2.80 eV). Electronic molecular orbital calculation result by Gaussian 09 software was analyzed to identify the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of o-CzCN and m-CzCN. In the HOMO and LUMO calculation results in Figure 2, phenylcarbazole was a HOMO dispersing block and CN modified carbazole was a LUMO spreading block in the two host materials. There was little difference of the HOMO and LUMO dispersion in the two host materials, indicating that the HOMO and LUMO of the two host materials would be similar. As predicted from the HOMO and LUMO calculation results, the HOMO/LUMO values of o-CzCN and m-CzCN were -6.10/-2.57 eV and -6.07/-2.57 eV from electrochemical analysis (Figure 3), respectively. Compared to the HOMO and LUMO of mCBP, the HOMO was similar, but the LUMO was stabilized by the LUMO dominating CN modified carbazole in the o-CzCN and m-CzCN. From the HOMO and LUMO values, it can be assumed that the o-CzCN and m-CzCN may have better electron accepting character than mCBP. One important parameter in the selection of host material is its bipolar charge transport character which governs hole and electron balance in the emitting layer. The bipolar charge transport character of o-CzCN and m-CzCN was examined by comparing hole and electron current densities of the single charge devices. The current density data of the single charge devices in Figure 4 (a) and Figure 4 (b) projected bipolar character of the o-CzCN and mCzCN. Even though the hole current density was higher than electron current density in the oCzCN and m-CzCN devices, the relative difference between hole and current density of the oCzCN and m-CzCN devices was small compared to that of mCBP, describing bipolar 4 Environment ACS Paragon Plus

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character of o-CzCN and m-CzCN. Moreover, the hole current density of m-CzCN was even higher than that of mCBP because of extended conformation of m-CzCN by 3- position modified carbazole backbone structure. High triplet energy, deep LUMO, molecular stability by C-C backbone structure and bipolar charge carrying property of o-CzCN and m-CzCN allowed the use of the two compounds as the host materials of blue TADF device and PhOLEDs. At first, blue TADF OLEDs were developed using the o-CzCN and m-CzCN host materials in comparison with a common mCBP host. Blue TADF emitting material was 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzCN) and it was doped at a doping concentration of 15%. Basic current density (J) and luminance data of the o-CzCN and m-CzCN devices collected by voltage (V) sweep are displayed in Figure 5 (a). Both J and L of the o-CzCN and m-CzCN device were greatly increased in comparison to those of mCBP dominantly by high hole and electron current density as demonstrated in the singlet carrier device. Deep LUMO level and corresponding reduced LUMO barrier for electron injection (Figure 6) and good hole transport character were key factors for the high J and L. The high J and L in the m-CzCN device compared to o-CzCN device are due to high hole and electron density in the m-CzCN device. In the ortholinkage based o-CzCN, hole and electron transport is rather hampered by distorted backbone structure due to difficult orbital overlap between molecules. The external quantum efficiency (EQE) of the blue TADF devices are shown in Figure 5 (b). The EQE was also jumped from 9.3% of mCBP device to 16.4% and 15.0% in the o-CzCN and m-CzCN devices, respectively. Reduced electron trapping by the 5CzCN dopant can explain the improved EQE of the oCzCN and m-CzCN devices. In the case of the mCBP device, strong electron trapping by 5CzCN disrupts hole and electron balance due to low electron density, but it is alleviated in the o-CzCN and m-CzCN devices owing to less LUMO gap between hosts and 5CzCN. Therefore, carrier balance was improved and EQE was enhanced above 20%.



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Electroluminescence (EL) spectra at 1000 cd/m2 were similar in the mCBP, o-CzCN and mCzCN devices (Figure 5 (c)). Operational stability of the mCBP, o-CzCN and m-CzCN devices was evaluated at the same initial luminance of 1,000 cd/m2. The stability data of the three devices in Figure 5 (d) demonstrated that the m-CzCN device is much more stable than mCBP and o-CzCN devices. The lifetime of the TADF device was almost doubled by the m-CzCN host material. The lifetime extension by the m-CzCN host can be explained by two factors. One is chemical bond stabilization effect by C-C backbone structure between carbazole and phenyl units. The other is facilitated electron injection and accompanying broad recombination zone in the 5CzCN doped emitting layer. Good electron injection in the m-CzCN host material increases electron density and less electron trapping broadens recombination zone, which are key contributors to the extended lifetime of the m-CzCN device. In the case of o-CzCN, relatively poor electron transport and weakened C-N bond by geometrical distortion induced by ortholinkage limited the stability. Encouraged by the simultaneous efficiency and lifetime boosting effect of m-CzCN, o-CzCN and m-CzCN were also reviewed as host materials of stable blue PhOLEDs. Emitting material of the blue PhOLEDs was tris(2-(1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-1H-imidazol-2yl)phenyl)iridium (Ir(dbi)3) which was widely used as blue triplet emitter for lifetime study. The (Ir(dbi)3) emitter was doped at a doping concentration of 5%. J and L of the blue PhOLEDs in Figure 7 (a) was also increased in the o-CzCN and m-CzCN blue PhOLEDs due to high carrier density and accompanying high exciton density. Exactly the same trend obtained in the TADF devices was observed in the blue PhOLEDs. The EQE of the device was also significantly enhanced as shown in Figure 7 (b). The EQE of the mCBP device was 11.0%, but the EQEs of o-CzCN and m-CzCN devices were 17.1% and 16.0%, respectively. This result also coincided with the EQE result of the blue TADF devices in Figure 5 (b). In the case of the EL spectra in Figure 7 (c), the three PhOLEDs exhibited almost the same EL 6 Environment ACS Paragon Plus

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spectra irrespective of the host materials. (Ir(dbi)3) emission was mainly observed with some weak emission from a hole transport material by hole trapping effect of (Ir(dbi)3).26 Lifetime data of the blue PhOLEDs are presented in Figure 7 (d). In contrast to the lifetime result of TADF devices, the lifetime of the o-CzCN and m-CzCN devices was shorter than that of the mCBP device. This result can be interpreted by the narrow emission zone of the oCzCN and m-CzCN devices compensating the better molecular stability of the host material. The Ir(dbi)3 dopant has been known as a strong hole trapping dopant confining emission zone of the device near hole transport layer.26 As explained in the single carrier device data, the oCzCN and m-CzCN hosts facilitate electron injection and transport, making the emission zone even narrower than that of the mCBP device. Therefore, the stability of the device would be degraded by triplet-triplet annihilation and triplet-polaron annihilation mechanisms in spite of robust molecular structure. As an objective to resolve the narrow emission zone issue of the o-CzCN and m-CzCN devices, the stable m-CzCN host was mixed with a strong hole transport type 9(dibenzo[b,d]thiophen-2-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole (DBTBCz) host. The o-CzCN mixed host device was not fabricated due to relatively poor stability of o-CzCN in comparison with m-CzCN. Lifetime data of the m-CzCN:DBTBCz mixed host device are presented in Figure 8. In the mixed host of m-CzCN:DBTBCz, the m-CzCN is an electron transport type host material due to relatively deep LUMO level. Therefore, the lifetime test result of the mixed host device can project the stability of m-CzCN under electron injection. The lifetime of the m-CzCN:DBTBCz mixed host device was greatly extended relative to that of the mCBP device, demonstrating good electron stability of the m-CzCN host. As already described, C-C molecular backbone structure stabilizing the molecules under radical, positive polaron and negative polaron contributed to the extended operational stability. The lifetime of the mixed host device was more than doubled compared to that of the mCBP device. Main reasons for the improved lifetime seem to be wide emission zone, carrier path separation, and 7 Environment ACS Paragon Plus


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electron stability of m-CzCN. Wide emission zone is induced by bipolar charge transport in the mixed host emitting layer, which extends the lifetime by reduced triplet-triplet annihilation. Additionally, the holes and electrons are separated in the emitting layer by the hole type and electron type hosts, which decreases degradation of each host material. Holes are transported by the DBTBCz host material and electrons are mostly carried by m-CzCN. Electron damage of the DBTBCz host is reduced because electron injection into DBTBCz is limited. Hole damage of the m-CzCN host is also relieved in the mixed host. Therefore, extended lifetime was realized in the m-CzCN mixed host.

Conclusions In conclusion, a molecular design having a CN modified carbazole instead of carbazole and carbazole-phenyl linkage at 3- position of carbazole instead of 9- position of carbazole in the backbone structure considerably improved the EQE and lifetime of the blue PhOLEDs and TADF devices at the same time. Chemical bond stabilization effect by C-C backbone structure was a key factor in both blue PhOLEDs and TADF OLEDs, and additional recombination zone adjusting function of the host was responsible for the superb lifetime of the blue TADF devices. Among the two host materials with different linkages, o-CzCN and m-CzCN, m-CzCN worked better than o-CzCN and common mCBP as the host material. Therefore, the molecular design concept of the host material applied in this work would be useful for both high efficiency and improved lifetime.

Experimental

General information 9H-carbazole-3-carbonitrile, (9-phenyl-9H-carbazol-3-yl)boronic acid, tetrakis(triphenylphosphine)palladium(0) (P&H tech), potassium carbonate, cesium carbonate, N,N-


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dimethylformamide (Duksan Sci. Co.), 1-bromo-2-fluorobenzene, and 1-bromo-3-fluorobenzene (Alfa aesar Co.) were used without further purification. Tetrahydrofuran (THF, Samchun pure chemical Co. Ltd) was distilled over sodium and calcium hydride. The 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Avance-500(Bruker, 500MHz) spectrometer. The ultraviolet-visible (UV-vis) spectra were obtained using UV-vis spectrophotometer (JASCO, V-730), and the photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (PerkinElmer, LS-55). UV-vis and PL measurements were carried out using a diluted THF solution, and triplet energy was measured under liquid nitrogen using a frozen THF solution. The energy levels of highest occupied molecular orbital and lowest unoccupied molecular orbital were estimated using a cyclic voltammetry (Ivium Tech., Iviumstat). The mass spectra were recorded using a Advion, ExpresionL CMS spectrometer in APCI mode.

Synthesis 9-(2-Bromophenyl)-9H-carbazole-3-carbonitrile 9H-carbazole-3-carbonitrile (1.0 g, 0.52 mmol) was dissolved in N,N-dimethylformamide (DMF, 15 mL) and poured into a pressure tube. 1-Bromo-2-fluorobenzene(1.2 g, 0.68 mmol) and cesium carbonate (2.5 g, 0.78 mmol) were added into the pressure tube, and stirred under 170 °C using oil bath for 12 h. The pressure tube was cooled to room temperature and the product in the pressure tube was extracted using methylene chloride (MC) / water. The organic layer was collected, dehydrated using magnesium sulfate (MgSO4), filtered and evaporated to remove solvent. A white powder was obtained after further purification using column chromatography. Yield 50%, 1H NMR (500 MHz, DMSO-d6): δ 8.86 (s, 1H), 8.38 (d, 1H, J=8.0HZ), 8.01 (d, 1H, J=8.0HZ), 7.78 (d, 1H, J=8.5HZ), 7.70 (d. 2H, J=8.0HZ), 7.62 (t, 1H, J=8.5HZ), 7.51 (t, 1H, J=7.5HZ), 7.39 (t, 1H, J=7.5HZ), 7.14 (d, 1H, J=8.5 HZ), 7.06 (d, 1H, 8.0HZ) . MS (APCI) m/z 323.1[(M+H)+].

9-(3-Bromophenyl)-9H-carbazole-3-carbonitrile



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9-(3-Bromophenyl)-9H-carbazole-3-carbonitrile was synthesized using same synthetic method of 9-(3bromophenyl)-9H-carbazole-2-carbonitrile. 1-Bromo-3-fluorobenzene(1.2 g, 0.68 mmol) was used instead of 1-bromo-2-fluorobenzene. Yield 50%, 1H NMR (300 MHz, DMSO-d6): δ 8.87 (s, 1H), 8.38 (d, 1H, J=7.8 HZ), 7.93 (s, 1H), 7.837.40 (m, 8H). MS (APCI) m/z 323.4 [(M+H)+].

9-(2-(9-Phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (o-CzCN) 9-(2-Bromophenyl)-9H-carbazole-3-carbonitrile (1.0 g, 0.31 mmol), (9-phenyl-9H-carbazol-3yl)boronic acid (0.98 g, 0.34 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.10 g, 0.1mmol) were dissolved in tetrahydrofuran, and K2CO3 (1.29 g, 0.93 mmol) aqueous solution was poured into the reaction mixture. The mixture was refluxed for 12 h and cooled to room temperature. The reaction mixture was extracted using MC/water and the organic layer was dehydrated using MgSO4. The mixture was purified using column chromatography, and obtained white powder was further purified using sublimation. Yield 65%. 1H NMR (500 MHz, DMSO-d6): δ 8.67 (s, 1H), 8.23 (d, 1H, J=7.5HZ), 7.97 (s, 1H), 7.84 (d, 1H, J=8.0HZ), 7.79-7.75 (m, 2H) , 7.68-7.52 (m, 5H), 7.45-7.32 (m, 5H), 7.28-7.17 (m, 5H), 6.99 (d, 1H, J=8.5HZ), 6.92 (d, 1H, J=8.5HZ). 13C NMR (125MHz, DMSO-d6): δ 142.5, 141.7, 141.0, 140.2, 139.1, 136.3, 132.9, 132.1, 130.0, 129.8, 129.6, 129.1, 128.9, 127.5, 127.4, 126.4, 126.3, 125.6, 125.5, 122.6, 122.5, 122.3, 121.4, 121.1, 120.1, 120.0, 119.9, 119.4, 110.7, 110.2, 109.7, 109.1, 101.3. MS (APCI) m/z 509.1 [(M+H)+].

9-(3-(9-Phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (m-CzCN) 9-(3-(9-Phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile was synthesized using same synthetic method of 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile. 9-(3Bromophenyl)-9H-carbazole-3-carbonitrile (1.0 g, 0.31 mmol) was used instead of 9-(2bromophenyl)-9H-carbazole-3-carbonitrile. Yield 75%. 1H NMR (500 MHz, DMSO-d6): δ 8.87 (s, 1H), 8.71 (s, 1H), 8.41 (d, 1H, J=7.5HZ), 8.33 (d, 1H, J=8.0HZ), 8.02-7.98 (m, 2H), 7.82-7.77 (m, 3H), 7.69 (t, 2H, J=7.75HZ), 7.62 (d, 2H, J=7.5HZ),


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7.58-7.53 (m, 4H), 7.49 (d, 1H, J=8.5HZ), 7.45-7.37 (m, 4H), 7.29 (t, 1H, J=7.5HZ). 13C NMR (125MHz, DMSO-d6): δ 143.0, 142.0, 141.0, 140.6, 139.9, 136.7, 136.4, 131.0, 130.8, 130.1, 129.4, 127.7, 127.6, 126.6, 126.5, 126.5, 125.7, 125.2, 124.9, 124.8, 123.4, 123.0, 122.8, 121.8, 121.2, 121.1, 120.8, 120.2, 120.1, 119.0, . MS (APCI) m/z 509.3 [(M+H)+].

Device fabrication and measurements Blue PhOLEDs and TADF devices were fabricated based on the same device platform except the host and emitter materials, and the same device was used for J-V-L and lifetime measurements. The device platform was indium tin oxide (ITO, 120 nm)/DNTPD (60 nm)/BPBPA (20 nm)/PCzAc (10 nm)/emitting layer (25 nm)/DBFTrz (5 nm)/NAPIm (30 nm)/LiF (1.5 nm)/Al (200 nm), where DNTPD is N,N'-diphenyl-N,N'-bis-[4-(phenyl-m-tolylamino)-phenyl]-biphenyl-4,4'-diamine, BPBPA is N,N,N'N'-tetra[(1,10-biphenyl)-4-yl]-(1,10biphenyl)-4,4'-diamine, PCzAc is 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10dihydroacridine, DBFTrz is 2,8-bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]thiophene, and NAPIm is 9,10-di(naphthalene-2-yl)anthracen-2-yl-(4,1-phenylene)(1-phenyl-1Hbenzo[ d]imidazole. The emitting layer was 5% Ir(dbi)3 doped mCBP, o-CzCN, and m-CzCN in the blue PhOLEDs, and the dopant was 15% doped 5CzCN in the TADF devices. Hole only devices and electron only devices were fabricated using device structure of ITO (120 nm)/DNTPD (60 nm)/BPBPA (20 nm)/PCzAc (10 nm)/o-CzCN or m-CzCN (30 nm)/DNTPD (10 nm)/Al (200 nm), and ITO (120 nm)/BCP (10 nm)/o-CzCN or m-CzCN (30 nm)/DBFTrz (5 nm)/NAPIm (30 nm)/LiF (1.5 nm)/Al (200 nm), respectively. Common vacuum thermal evaporation process was used for the whole device fabrication process and all devices were encapsulated inside a glove box for testing. All device performances were gather in ambient condition using the encapsulated devices. J-V-L performances were obtained using electrical and photometric measurement system equipped with Keithley 2400 and CS 1000 (Konica Minolta Inc.) spectroradiometer. Lifetime



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test was carried out in dark condition using Polaronix (McScience Co.) lifetime measurement system equipped with electrical source and photodiode as a detecting unit. Constant current driving of the devices at the same initial luminance was the driving method of the devices.

Acknowledgements This work was supported by Basic Science Research Program (2016R1A2B3008845) and Nano Materials Research Program (2016M3A7B4909243) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning.

References

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(6) Bulovic, V.; Deshpande, R.; Thompson, ME.; Forrest, SR. Tuning the Color Emission of Thin Film Molecular Organic Light Emitting Devices by the Solid State Solvation Effect. Chem. Phy. Lett. 1999, 308, 317-322 (7) Linton, KE.; Fisher, AL.; Pearson, C.; Fox, MA.; Palsson, M-O.; Bryce, MR.; Petty, MC. Colour Tuning of Blue Electroluminescence Using Bipolar Carbazole–Oadiazole Molecules in Single-Active-Layer Organic Light Emitting Devices (OLEDs). J. Mater. Chem. 2012, 22, 11816-11825. (8) Chen, CT. Evolution of Red Organic Light-Emitting Diodes: Materials and Devices. Chem. Mater. 2004, 16, 4389-4400. (9) Xiao, LX.; Chen, ZJ.; Qu, B.; Luo, JX.; Kong, S.; Gong, QH.; Kido, JJ. Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2012, 23, 926-952. (10) Tao, YT.; Yang, CL.; Qin, JG. Organic Host Materials for Phosphorescent Organic Light-Emitting Diodes. Chem. Soc. Rev. 2011, 40, 2943-2970 (11) Yook, KS.; Lee, JY. Organic Materials for Deep Blue Phosphorescent Organic LightEmitting Diodes. Adv. Mater. 2012, 24, 3169-3190. (12) Jeon, SO.; Jang, SE.; Son, HS.; Lee, JY. External Quantum Efficiency Above 20% in Deep Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2011, 23, 1436-1441. (13) Gong, SL.; He, X.; Chen, YH.; Jiang, ZQ.; Zhong, C.; Ma, DG.; Qin, JG.; Yang, CL. Simple CBP Isomers with High Triplet Energies for Highly Efficient Blue Electrophosphorescence. J. Mater. Chem. 2012, 22, 2894-2899. (14) Zhang, YF.; Lee, J.; Forrest, SR. Tenfold Increase in the Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes. Nat. Commun. 2014, 5, 5008. (15) Shin, H.; Lee, J-H.; Moon, C-K.; Huh, J-S.; Sim, B.; Kim, J-J. Sky-Blue Phosphorescent OLEDs with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting Layer. Adv. Mater. 2016, 28, 4920-4925. 13 Environment ACS Paragon Plus


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Hybrid Molecules: Highly Efficient Host Materials for Blue Phosphorescent OLEDs. Chem. Commun. 2015, 51, 1650-1653. (25) Zhao, YL.; Wu, C.; Qiu, PL.; Li, XP.; Wang, Q.; Chen, JS.; Ma, DG. New Benzimidazole-Based Bipolar Hosts: Highly Efficient Phosphorescent and Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes Employing the Same Device Structure. ACS Appl. Mater. Interfaces 2016, 8, 2635-2643 (26) Jeon, SK.; Lee, JY. Four Times Lifetime Improvement of Blue Phosphorescent Organic Light-Emitting Diodes by Managing Recombination Zone. Org. Electron. 2015, 27, 202-206



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List of figures Scheme 1. Synthetic scheme of o-CzCN and m-CzCN Figure 1. UV-vis, solution PL, and low temperature PL spectra of o-CzCN and m-CzCN. Figure 2. Calculated HOMO and LUMO distribution of o-CzCN and m-CzCN using B3LYP and 6-31G* basis set of Gaussian09 program Figure 3. Electrochemical scan data of o-CzCN and m-CzCN. Figure 4. Hole current density data of the hole only devices of o-CzCN and m-CzCN (a), and electron current density data of the electron only devices of o-CzCN and m-CzCN (b). Figure 5. Current density-voltage-luminance plots of 5CzCN doped o-CzCN and m-CzCN devices (a) and quantum efficiency-luminance plots of 5CzCN doped oCzCN and m-CzCN devices (b). EL spectra of 5CzCN doped oCzCN and m-CzCN device at 1,000 cd/m2 (c) and lifetime data of 5CzCN doped oCzCN and m-CzCN devices at an initial luminance of 1,000 cd/m2 (d) Figure 6. Device structure of blue TADF (a) and phosphorescent (b) devices. Figure 7. Current density-voltage-luminance plots of Ir(dbi)3 doped o-CzCN and m-CzCN devices (a) and quantum efficiency-luminance plots of Ir(dbi)3 doped o-CzCN and m-CzCN devices (b). EL spectra of Ir(dbi)3 doped o-CzCN and m-CzCN device (c) and lifetime data of Ir(dbi)3 doped o-CzCN and m-CzCN devices at an initial luminance of 1,000 cd/m2 (d) Figure 8. Lifetime data of Ir(dbi)3 doped m-CzCN:DBTBCz mixed host devices at an initial luminance of 1,000 cd/m2.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (arb. unit)


UV-Vis o-CzCN Solution PL o-CzCN LTPL o-CzCN

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UV-Vis m-CzCN Solution PL m-CzCN LTPL m-CzCN

1

0.5

0 200

300

400

500

Wavelength (nm) Figure 1


600

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HOMO

o-CzCN

m-CzCN

Figure 2


LUMO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

Current (arb. unit)


o-CzCN m-CzCN -3

-2

-1

0

1

2

Voltage (V) Figure 3


3

Page 21 of 26

(a)

(b) 600

900

Current density (mA/cm2)

1000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o-CzCN

800 m-CzCN

700

mCBP

600 500 400 300 200

o-CzCN

500

m-CzCN

400

mCBP

300 200 100

100 0

0 0

5

10

15

0

Voltage (V)

Figure 4


5

10

Voltage (V)

15


100000

(b)

20

mCBP ref.

35

o-CzCN 30

m-CzCN

1000

25 100

20 10

15 1

10 5

0.1

0

0.01

0

2

4

6

8

mCBP ref.

18

10000

Quantum efficiency (%)


40

Luminance (cd/m2)

(a)

o-CzCN

16

m-CzCN

14 12 10 8 6 4 2 0 1

10

10

Voltage (V)

(c)

100

1000

100

oPCzCN

10000

Luminance (cd/m2)

(d) mCBP ref.


mCBP ref. o-CzCN

95

mPCzCN

m-CzCN

Luminance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90 85 80 75

380

480

580

680

Wavelength (nm)

780

70 0

2

Figure 5


4

6

Time (h)

8

10

12

Page 23 of 26

(a) -2.10

Al(200 nm)

-2.40 -2.36

DNTPD(60 nm)

-5.71 -6.10 -6.07

PEDOT:PSS

-6.00

-2.90

ZADN

-3.14 -3.18

DBFTrz

mCBP

PCZAC

BPBPA -5.50

m-CzCN

-5.10 -5.20

-3.44

5CzCN

BPBPA(20 nm)

o-CzCN

PCZAc(10 nm)

ITO -4.70

DNTPD

DBFTrz(7.5 nm)

PEDOT:PSS

ZADN(27.5 nm)

HOST:5CzCN (30 nm, 15%)

-2.40 -2.57 -2.57

LiF(1.5 nm)

-6.10

-6.30 -6.71

ITO

(b) -2.40 -2.36

DNTPD(60 nm)

-5.50

-5.30

-3.14 -3.18

ZADN

mCBP

m-CzCN

-5.10 -5.20

-2.52

DBFTrz

BPBPA(20 nm)

-4.70

o-CzCN

PCZAc(10 nm)

ITO

PCZAC

DBFTrz(7.5 nm)

BPBPA

ZADN(27.5 nm)

DNTPD

LiF(1.5 nm)

Host:Ir(dbi)3 (30 nm, 10%)

-2.40 -2.57 -2.57

Ir(dbi)3

-2.10

Al(200 nm)

PEDOT:PSS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5.71 -6.10 -6.07 -6.00

PEDOT:PSS

-6.10 -6.71

ITO

Figure 6


-2.90


(a)

40

100000

(b)

20 18

o-CzCN

10000

m-CzCN

30

1000

25 20

100

15

10

10 1

5 0

Quantum efficiency (%)

35

Luminance (cd/m2)


mCBP ref.

16 14 12 10

0.1 0

2

4

6

8

(c)

8 6

mCBP ref.

4

o-CzCN

2

m-CzCN

0

10

1

Voltage (V)

10

100

1000

100

mCBP ref.

mCBP ref.

380

oPCzCN

Luminance (%)

o-CzCN m-CzCN

480

580

680

10000

Luminance (cd/m2)

(d)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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780

mPCzCN

95

90

85

80 0

Wavelength (nm)

Figure 7


5

Time (h)

10

15

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100

mCBP m-CzCN mixed host

Luminance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


95

90

85 0

2

4

6

Time (h)

8

10

12

Figure 8



Table of contents

100

mCBP o-CzCN

95

m-CzCN

Luminance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

90 85 80 75 70 0

2

4

6

Time (h)


8

10

12

CN-Modified Host Materials for Improved Efficiency and Lifetime in

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