一种新型交联电子传输材料在OLED中的应用

Application of A Novel Cross-linkable Electron Transport Material in OLED 一种新型交联电子传输材料在OLED中的应用 1 2 first-author 高朝(1992-), 男, 湖北黄冈人, 硕士研究生, 2013年于湖北工程学院获得学士学位, 主要从事OLED材料的合成以及器件等方面的研究。E-mail:zgao2018@sinano.ac.cn 高朝 (1992-), 男, 湖北黄冈人, 硕士研究生, 2013年于湖北工程学院获得学士学位, 主要从事OLED材料的合成以及器件等方面的研究。E-mail: zgao2018@sinano.ac.cn zgao2018@sinano.ac.cn 1 1 corresp SU Wen-ming.E-mail:wmsu2008@sinano.ac.cn SU Wen-ming.E-mail:wmsu2008@sinano.ac.cn 苏文明(1978-), 男, 湖南邵东人, 博士, 研究员, 2007年于中国科学院长春光学精密机械与物理研究所获得博士学位, 主要从事印刷发光与显示等方面的研究E-mail:wmsu2008@sinano.ac.cn 苏文明 (1978-), 男, 湖南邵东人, 博士, 研究员, 2007年于中国科学院长春光学精密机械与物理研究所获得博士学位, 主要从事印刷发光与显示等方面的研究E-mail: wmsu2008@sinano.ac.cn wmsu2008@sinano.ac.cn 1 2 Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China 中国科学院苏州纳米技术与纳米仿生研究所 印刷电子技术研究中心, 江苏 苏州 215123 中国科学院苏州纳米技术与纳米仿生研究所 印刷电子技术研究中心, 江苏 苏州 215123 School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China 上海科技大学 物质科学与技术学院, 上海 201210 上海科技大学 物质科学与技术学院, 上海 201210 A new cross-linkable electron transport layer(ETL) material, 2, 4, 6-tris(4'-vinyl-[1, 1'-biphenyl]-3-yl)-1, 3, 5-triazine(TV-T2T) was designed and synthesized. The film formation of the cross-linkable material TV-T2T has the excellent solvent resistance. The lowest unoccupied molecular orbital(LUMO) level of TV-T2T is -3.5 eV, which could deliver efficient charge injection from the zinc oxide(ZnO) to the emission layer. In addition, the surface roughness of 2.27 nm in the film of triple-layer ZnO/TV-T2T/2, 6-Dczppy:Ir(mppy) 3 , is lower than that of the film without TV-T2T(2.39 nm), resulting in a reduced leakage current. The solution-processed inverted OLED with ETL material TV-T2T, achieved an EQE of 5.1% compared to the 3.0% EQE of the device without TV-T2T layer, demonstrating an improvement of 1.7-fold for the inverted OLEDs. 设计并合成了一种新型的可交联电子传输材料TV-T2T。该材料经过热交联之后具有优异的抗溶剂特性，并且TV-T2T的LUMO能级为-3.5 eV，这将更有利于电子从ZnO层注入到发光层中。另外，溶液法制备的三层薄膜ZnO/TV-T2T/2，6-Dczppy:Ir（mppy） 3 ，其粗糙度低至2.27 nm，优于未加入TV-T2T电子传输层的双层薄膜（2.37 nm），可以有效减少漏电流的产生。随后，将TV-T2T应用于三层溶液法制备的倒置有机发光二极管中，获得了5.1% EQE的器件性能，是不加TV-T2T的器件性能（EQE为3.0%）的1.7倍。 cross-linkable material electron transport layer electron injection property solution processing inverted organic light emitting diodes 交联材料 电子传输层 电子注入能力 溶液法 倒置OLED Introduction 1 In recent years, organic light emitting diodes (OLEDs) have attracted a great deal of attentions in academia and industry since it was invented by C W Tang [ 1 1 ] more than 30 years ago. Vacuum evaporation deposition [ 2 2 - 7 7 2-7 ] of small molecular materials has been dominant in the past 30 years for OLED manufacturing. However, due to the disadvantage of high cost and low material usage in the vacuum evaporation process, solution processed materials which mainly relying on spin coating [ 8 8 ] and inkjet printing [ 9 9 - 12 12 9-12 ] deposition processes, have been actively pursued. Besides, the interlayer erosion has been a major obstacle in solution-processed OLEDs. There are two main approaches to solve the problem, using orthogonal solvents [ 13 13 - 17 17 13-17 ] or cross-linkable materials [ 18 18 - 21 21 18-21 ] . The use of cross-linkable materials has advantages of high air stability and could favor more compact film formation [ 22 22 ] , and a number of cross-linkable materials have been developed [ 23 23 - 26 26 23-26 ] . When OLEDs are used for display applications, passive-matrix OLEDs(PMOLEDs) [ 27 27 ] and active-matrix OLEDs(AMOLEDs) are the two main types [ 28 28 - 30 30 28-30 ] . For AMOLEDs, the light emission of each pixel is controlled by a number of thin film transistors(TFTs), which could potentially block the light emission on display panel. Owing to this, OLED with inverted structure takes an advantage of that the TFTs are made beneath the OLED pixels. However, the inverted OLED has the problem of highly efficiently injecting electrons from metal electrode to light emission layer. So, the electron material is very important to the inverted OLED. Many excellent electron materials have been reported [ 31 31 - 34 34 31-34 ] . Among them, triazine based compounds have been widely used as the electron transport layers [ 35 35 - 42 42 35-42 ] . Particularly, 2, 4, 6-tri([1, 1′-biphenyl]-3-yl)-1, 3, 5-triazine (T2T) has been chosen as the ETL for its good electron transporting ability [ 43 43 - 44 44 43-44 ] . Nevertheless, T2T as a small molecular material, is insoluble and only suitable for vacuum evaporation deposition, which is not compatible with solution processing. In this work, a modified version of T2T, the 2, 4, 6-tris(4′-vinyl-[1, 1′-biphenyl]-3-yl)-1, 3, 5-triazine(TV-T2T), has been designed and synthesized. TV-T2T has good solubility and cross-linkable unit while maintains the advantages of T2T as well. The lowest unoccupied molecular orbital (LUMO) of TV-T2T is -3.5 eV, lying between the LUMO levels of ZnO(-4.0 eV) and 2, 6-bis(3-(carbazol-9-yl)phenyl)pyridine(2, 6-Dczppy)(-2.6 eV):tris(2-(4-tolyl)phenylpyridine)iridium(Ir(mppy) 3 )(-2.4 eV), indicating that the TV-T2T would facilitate efficient charge transport. The surface roughness of ZnO/2, 6-Dczppy:Ir(mppy) 3 and ZnO/TV-T2T/2, 6-Dczppy:Ir(mppy) 3 have been measured to be 2.39 nm and 2.27 nm respectively. The thermodynamic property, photo-physical property, charge mobility and resistance to solvent erosion for TV-T2T were fully investigated. Tri-layer solution processing OLEDs were fabricated with the structure of ZnO/TV-T2T/2, 6-Dczppy:Ir(mppy) 3 .A maximum 5.1% EQE of OLEDs with the cross-linkable material TV-T2T has been achieved, comparing to the 3.0% EQE of the device without TV-T2T layer, delivering 1.7-fold of improvement. Experiments 1 General Information 2 The elemental analysis (EA) was performed using vario Micro cube. The 1 H and 13 C NMR in the CDCl 3 spectra were obtained with a Bruker NMR spectrometer. Thermogravimetric analysis(TGA) was measured with the NETZSCH TG 209 F1(Germany) on the condition of the nitrogen atmosphere by heating sample from 25 ℃ to 300 ℃ at the heating rate of 10 ℃/min. Differential scanning calorimetry(DSC) was used to investigate the TV-T2T of the thermodynamic stability with NETZSCH DSC 200F3 MAIA (Germany) at a heating rate of 10 ℃/min from room temperature to 300 ℃ under the nitrogen atmosphere. The ultraviolet-visible (UV) absorption and the photoluminescence(PL) spectra were performed on the Perkin-Elmer Lambda 750 spectrophotometer and a Hitachi F-4600 fluorescence spectrophotometer, respectively. Atom Force Microscopy(AFM) images were obtained with a veeco Dimension 3100 at the ambient temperature in the tapping mode. The current-voltage( I - V ) and luminance-voltage( L - V ) relations were characterized with a computer-controlled Keithley 2400 Sourcemeter. The UPS was measured with ThermoFisher. Fabrications and Measurements of Inverted OLEDs 2 Prior to device fabrications, ITO with glass substrate was etched into a specific pattern, and the active device area was 2 mm×2 mm. Next, the substrate was washed with detergent, deionized water, wiped with ethanol, and sonicated in ethanol solution for 30 min sequentially. At the beginning, the substrate was treated by the O 2 -plasma for 5 min under the condition of 120 W. ZnO NPs were spin-coated on a pre-cleaned ITO glass at 2 300 r/min for 40 s and then annealed at 120 ℃ for 10 min in the glove box. Next, the cross-linkable TV-T2T was spin-coated, and the corresponding film was annealed at 150 ℃ for 10 min and then was annealed at 230 ℃ for 30 min. The solution of light-emitting material(2, 6-Dczppy:Ir(mppy) 3 (1:0.1 wt. ratio)) in chlorobenzene was spin-coated onto the ETL and dried at 80 ℃ for 30 min to remove residual solvent. Finally, 1, 1-bis[(di-4-tolylamino) phenyl] cyclohexane (TAPC), hexaazatriphenylenehexacabonitrile(HAT-CN), MoO 3 and Al were sequentially vacuum deposited under the condition of 5×10 -4 Pa. Materials 2 The ZnO NPs [ 45 45 ] were synthesized through a solution-precipitation process using Zn acetate and tetramethylammonium hydroxide (TMAH) precursors. The reaction process proceeds as follows: a solution of 0.5 mol/L TMAH in ethanol and a solution of 0.1 mol/L zincacetate in dimethyl sulphoxide(DMSO) were mixed and stirred for 1 h in an ambient atmosphere. The prepared product was collected by centrifugation and then washed. The obtained transparent precipitate was dispersed in butanol at a concentration of 20 mg/mL Synthesis 2 Synthesis of The 2, 4, 6-tris(3-bromophenyl)-1, 3, 5-triazine(Br-T2T) 3 3-bromobenzonitrile (11.6 g, 63.7 mmol) was added to a triangular conical flask(250 mL). Then, trifluoromethanesulfonic acid (20.3 mL, 6.7 mmol) was injected into the conical flask with a syringe slowly in an ice bath. The resulting reaction mixture was stirred at 0 ℃ for about 30 min, then slowly warmed to room temperature and stirred for about 12 h. The mixture was finally purified to obtain the Br-T2T as a white solid (10.3 g, 59.3 mmol) (yield: 88.8%): 1 H NMR (400 MHz, CDCl 3 ): δ 8.80(t, 1H), 8.64(d, 1H), 7.76(d, 1H), 7.46 (t, 1H). 13 C (101 MHz, CDCl 3 ): δ 170.64, 137.65, 135.77, 131.84, 130.28, 127.64, 123.03, 77.36. Anal. Calcd for C 21 H 12 N 3 Br 3 (%): C 46.19, H 2.22, N 7.7; found: C 46.22, H 2.25, N 7.65. Synthesis of The Product, 4, 6-tris(4′-vinyl-[1, 1′-biphenyl]-3-yl)-1, 3, 5-triazine (TV-T2T) 3 Br-T2T(2 g, 3.7 mmol), (4-vinylphenyl) boronic acid (1.91 g, 12.88 mmol), Pd(PPh 3 ) 4 (424 mg, 0.368 mmol) and tert-butylphosphonium tetrafluoroborate(213.6 mg, 0.736 mmol) were added into the 100 mL flask. Then, 1, 4-dioxane was added in the flask and stirred to dissolve the Br-T2T. Afterwards, the flask was added with Na 2 CO 3 (7.8 g, 73.6 mmol) through a syringe and kept vacuum. Finally, the mixture was heated to 95 ℃ and reacted for 12 h, and purified to get the TV-T2T as a white solid (1.6 g, 2.6 mmol)(yield: 80.0%): 1 H NMR (400 MHz, CDCl 3 ) δ 9.00(s, 1H), 8.75(d, 1H), 7.84(d, 1H), 7.72(d, 1H), 7.65(t, 1H), 7.57(d, 1H), 6.81(m, 1H), 5.85(d, 1H), 5.33(d, 1H). 13 C NMR(101 MHz, CDCl 3 ) δ 171.68, 141.20, 140.13, 136.97, 136.74, 136.41, 131.10, 129.20, 127.99, 127.48, 127.42, 126.82, 114.17, 77.36, 77.04, 76.73. Anal. Calcd for C 45 H 33 N 3 (%): C 87.77, H 5.41, N 6.82; found: C 87.8, H 5.5, N 6.7. Results and Discussion 1 The route for TV-T2T synthesis is illustrated in Fig. 1 Fig. 1 , which was only a two-step reaction process. First, the compound of 3-bromobenzonitrile was reacted under the condition of trifluoromethanesulfonic acid at 0 ℃, and the intermediate 2, 4, 6-tris(3-bromophenyl)-1, 3, 5-triazine(Br-T2T) was obtained with a yield of 88.8%. Then, the Br-T2T was reacted with(4-vinylphenyl) boronic acid through Suzuki-Miyaura crossing-coupling reaction to reach the target product TV-T2T with a yield of 80.0%. The chemical structures of the Br-T2T and TV-T2T were confirmed by 1 H NMR, 13 C NMR, and EA. The photoluminescence(PL) spectra of the TV-T2T films before and after the cross-linking are shown in Fig. 2 Fig. 2 . The UV-Vis absorption peak positions of the films before and after the cross-linking stayed the same, and the PL peak of the TV-T2T is 446 nm before the cross-linking and 444 nm after the cross-linking, together implying that the cross-linking of TV-T2T would not influence on optical properties for their intrinsic unconjugated linking. To investigate the properties of the thermal stability, TGA and DSC were carried out for TV-T2T, and the TGA and DSC curves are shown in Fig. 3 Fig. 3 . They revealed that the polymerization peak was at about 218 ℃, as seen in Fig. 3(a) Fig. 3(a) , and the decomposition temperature ( T d ) was 430 ℃(5% weight loss), as seen in Fig. 3(b) Fig. 3(b) . After cross-linking, there was no obvious peak, as shown in Fig. 3(b) Fig. 3(b) , which showed that the TV-T2T was thermally stable. The highest occupied molecular orbit (HOMO) level of the TV-T2T was measured by the Ultraviolet Photoemission Spectroscopy(UPS), which was about -6.5 eV, as shown in Fig. 4 Fig. 4 . The LUMO of the TV-T2T was approximately -3.5 eV which is lower than that of the conduction band(CB) ZnO (-4.0 eV) and lies between the ZnO layer and the emission layer. Therefore, the TV-T2T as ETL could facilitate the electrons to inject from ZnO to TV-T2T and to the emission layer. All the above characterization data of TV-T2T are summarized in Tab. 1 Tab. 1 . a Obtained from DSC measurement, b Obtained from TGA measurement, c Measured in thin films by spin-coating from a tetrachloroethane solution, d Calculated from 21.22- E k from UPS, e Calculated from the edge of the UV-Vis absorption( E g =1240/ λ ). Compound T CL /℃ a T d /℃ b λ abs /nm c λ PL /nm c HOMO/eV d LUMO/eV E g /eV e TV-T2T 218 430 283 444 6.5 3.5 3.0 Cross-linked TV-T2T film was measured using UV-Vis spectroscopy on quartz substrates, with toluene, chlorobenzene, tetrahydrofuran and dichloroethane as the testing solvents. The results are shown in Fig. 5(a) Fig. 5(a) .The UV-Vis absorptions showed no change before and after rinsing of the TV-T2T films with the solvents, showing that the cross-linked TV-T2T has good resistance to the solvents. In addition, solvent resistance of TV-T2T with various crosslinking conditions were investigated( Fig. 5(b) Fig. 5(b) ). It is found that the best crosslinking condition was 220 ℃ for 30 min. To investigate the film morphology with the addition of the TV-T2T, four different film compositions were measured by AFM. The films were: (a)ZnO(30 nm), (b)ZnO(30 nm)/TV-T2T(10 nm), (c) ZnO(30 nm)/TV-T2T(10 nm)/2, 6-Dczppy:Ir(mppy) 3 (40 nm), and (d)ZnO(30 nm)/2, 6-Dczppy:Ir(mppy) 3 (40 nm). The morphology measurements are shown in Fig. 6 Fig. 6 .The surface roughness of the ZnO/TV-T2T/Ir(mppy) 3 (2.27 nm) was lower than that of the ZnO/2, 6-Dczppy:Ir(mppy) 3 (2.39 nm), which would reduce the leakage currency. Therefore, it may account for the better efficiency of the device with the TV-T2T in the following results. To investigate the property of charge injection from the ZnO to TV-T2T, three devices were fabricated with the TV-T2T and other two commonly used materials, 1, 3, 5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene(TPBi), and 1, 3, 5-tri[(3-pyridyl)-phen-3-yl]benzene(TmpyPb), as the ETL, respectively. The structure of the devices consisted of ITO/ZnO(20 nm)/ETL(40 nm)/8-hydroxyquinolinolato-lithium(Liq)(2 nm)/Al(100 nm), for TmPyPb, TPBi, TV-T2T as device 1, 2, 3 respectively. The measured current densities are shown in Fig. 7 Fig. 7 . The device fabricated with the TV-T2T as the ETL showed much better performance than the other so that the TV-T2T can effectively improve charge injection efficiency. Finally, OLEDs were fabricated with the device structure of ITO/ZnO(30 nm)/TV-T2T( x nm)/2, 6-Dczppy:Ir(mppy) 3 (40 nm)/TAPC(30 nm)/HAT-CN(10 nm)/MoO 3 (10 nm)/Al(120 nm), as shown in Fig. 8(a) Fig. 8(a) . The electron transport layer of ZnO and TV-T2T and the emission layer of 2, 6-Dczppy:Ir(mppy) 3 were fabricated by spin coating. The schematic energy-level diagrams of the solution-processed devices are shown in Fig. 8(b) Fig. 8(b) . For optimizations of film thickness of TV-T2T, four different devices were fabricated, namely device A: without two ETL materials based devices, indicating TV-T2T; device B: TV-T2T(5 nm); device C: TV-T2T(10 nm); device D: TV-T2T(15 nm). The device performances are shown in Fig. 8(c) Fig. 8(c) - (f) (f) . All the data are summarized in Tab. 2 Tab. 2 . The device C with a thickness of 10 nm achieved an EQE of 5.1%, a current efficiency(CE) of 16.9 cd/A, a power efficiency(PE) of 8.23 lm/W. In contrast, the device A without the TV-T2T only achieved a maximum EQE of 3.0%, a CE of 10.1 cd/A, a PE of 4.92 lm/W. In terms of EQE, the device with TV-T2T achieved 1.7-fold of improvement compared the device without it. a Turn on voltage at 1 cd·m -2 ; V on : V; thickness: nm; CE: cd·A -1 ; PE: lm·W -1 ; EQE: %; Commission Internationale de L'Eclairage (CIE) at 1 000 cd·m -2 . 2 Device 2 TV-T2T thickness 2 V on a 3 100 cd/m 2 3 1 000 cd/m 2 2 Maximum CE 2 Maximum PE 2 Maximum EQE 2 CIE ( x , y ) CE PE EQE CE PE EQE A without 2.5 2.7 1.8 0.8 9.7 4.9 2.8 10.2 4.9 3.0 (0.3, 0.6) B 5 2.5 2.2 1.6 0.5 7.5 4.3 2.2 12.2 5.3 3.6 (0.3, 0.6) C 10 2.5 4.0 2.9 1.0 15.3 8.2 4.6 16.9 8.2 5.1 (0.3, 0.6) D 15 2.5 2.1 1.6 0.6 10.0 5.5 3.0 13.0 6.2 3.9 (0.3, 0.6) Conclusion 1 A new ETL material, namely TV-T2T, has been designed and synthesized. TV-T2T maintains the best properties of T2T, and in the meantime, possesses good solubility for solution processing and cross-linking units. 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