Application of A Novel Cross-linkable Electron Transport Material in OLED

GAO Zhao XIE Li-ming SU Wen-ming* CUI Zheng

(1. Printable Electronics Research Center,Suzhou Institute of Nano-Tech and Nano-Bionics,Chinese Academy of Sciences,Suzhou 215123,China；2. School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China)

Abstract： 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.

Key words： cross-linkable material； electron transport layer； electron injection property； solution processing； inverted organic light emitting diodes

1 Introduction

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]more than 30 years ago. Vacuum evaporation deposition[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]and inkjet printing[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[13-17]or cross-linkable materials[18-21]. The use of cross-linkable materials has advantages of high air stability and could favor more compact film formation[22], and a number of crosslinkable materials have been developed[23-26].

When OLEDs are used for display applications,passive-matrix OLEDs(PMOLEDs)[27]and activematrix OLEDs ( AMOLEDs) are the two main types[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[31-34].Among them, triazine based compounds have been widely used as the electron transport layers[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[43-44]. 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)3and ZnO/TVT2T/2,6-Dczppy∶Ir(mppy)3have 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 TVT2T has been achieved, comparing to the 3. 0%EQE of the device without TV-T2T layer, delivering 1.7-fold of improvement.

2 Experiments

2.1 General Information

The elemental analysis (EA) was performed using vario Micro cube. The1H and13C NMR in the CDCl3spectra 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.

2.2 Fabrications and Measurements of Inverted OLEDs

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 O2-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 (HATCN),MoO3and Al were sequentially vacuum deposited under the condition of 5×10-4Pa.

2.3 Materials

The ZnO NPs[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

2.4 Synthesis

2.4.1 Synthesis of The 2,4,6-tris(3-bromophenyl)-1,3,5-triazine(Br-T2T)

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%)：1H NMR (400 MHz, CDCl3)：δ8.80(t,1H),8.64(d,1H),7.76(d,1H),7.46 (t,1H).13C(101 MHz, CDCl3)：δ170.64, 137.65, 135.77,131.84, 130. 28, 127. 64, 123. 03, 77. 36. Anal.Calcd for C21H12N3Br3(%)： C 46.19, H 2.22, N 7.7； found： C 46.22, H 2.25, N 7.65.

2.4.2 Synthesis of The Product 2,4,6-tris(4′-vinyl-[1,1′-biphenyl]-3-yl)-1,3,5-triazine(TV-T2T)

Br-T2T(2 g,3.7 mmol), (4-vinylphenyl) boronic acid (1. 91 g, 12. 88 mmol), Pd(PPh3)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 Na2CO3(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%)：1H NMR (400 MHz, CDCl3)δ9. 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).13C NMR(101 MHz,CDCl3)δ171.68,141.20,140.13,136.97,136.74,136.41,131.10,129.20,127.99,127.48, 127.42,126. 82, 114. 17, 77. 36, 77.04, 76. 73. Anal.Calcd for C45H33N3(%)： C 87. 77, H 5. 41, N 6.82； found： C 87.8, H 5.5, N 6.7.

3 Results and Discussion

The route for TV-T2T synthesis is illustrated in 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-Miyau racrossing-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 by1H NMR,13C NMR,and EA.

Fig.1 Synthetic route for TV-T2T

The photoluminescence(PL) spectra of the TVT2T films before and after the cross-linking are shown in 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.

Fig.2 UV-Vis absorption and PL spectra of the TV-T2T before and after cross-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.They revealed that the polymerization peak was at about 218 ℃,as seen in Fig.3(a),and the decomposition temperature (Td) was 430 ℃(5% weight loss), as seen in Fig. 3(b). After cross-linking,there was no obvious peak, as shown in Fig.3(b),which showed that the TV-T2T was thermally stable.

Fig.3 (a)DSC curve of the material TV-T2T with a heating of 10 ℃/min under nitrogen. (b)TGA curve of the material TV-T2T with a heating rate of 10 ℃/min under nitrogen.

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. 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.

Fig.4 UPS spectra of TV-T2T

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). The UV-Vis absorptions showed no change before and after rinsing of the TV-T2T films with the solvents, showing that the cross-linked TVT2T has good resistance to the solvents. In addition,solvent resistance of TV-T2T with various crosslinkinga 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-Ekfrom UPS,e Calculated from the edge of the UV-Vis absorption(Eg=1240/λ).conditions were investigated(Fig.5(b)). It is found that the best crosslinking condition was 220 ℃ for 30 min.

Tab.1 Thermal and photophysical properties of TV-T2T

Fig.5 (a)UV-Vis absorption spectra of the curved TV-T2T films before and after rinsing with toluene,chlorobenzene, tetrahydrofuran and dichloroethane. (b)Solvent resistance of TV-T2T rinsed with toluene at various crosslinking conditions.

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.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.

Fig.6 (a)AFM topographic images of the ZnO(30 nm).(b)AFM topographic images of the ZnO(30 nm)/TV-T2T(10 nm). (c)AFM topographic images of the ZnO(30 nm)/TV-T2T(10 nm)/2,6-Dczppy ∶Ir(mppy)3(40 nm). (d)AFM topographic images of the ZnO(30 nm)/2,6-Dczppy∶Ir(mppy)3(40 nm).

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. 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 .

Fig.7 Current density-voltage of EODs based on the TmPy-Pb, TPBi and cross-linked TV-T2T.

Fig.8 (a)OLED device structure diagram. (b)Schematic energy-level diagrams. (c)Current density/luminance vs. voltage(J-V-L). (d)Current efficiency/power efficiency vs. luminance(CE-L-PE). (e)Luminance-external quantum efficiency(L-EQE). (f)Wavelength-EL intensity.

Finally, OLEDs were fabricated with the device structure of ITO/ZnO(30 nm)/TV-T2T(xnm)/2,6-Dczppy∶Ir(mppy)3(40 nm)/TAPC(30 nm)/HAT-CN(10 nm)/MoO3(10 nm)/Al(120 nm), as shown in Fig.8(a). The electron transport layer of ZnO and TV-T2T and the emission layer of 2,6-Dczppy∶Ir(mppy)3were fabricated by spin coating. The schematic energy-level diagrams of the solution-processed devices are shown in 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 TVT2T； device B： TV-T2T(5 nm)； device C： TVT2T(10 nm)； device D： TV-T2T(15 nm). The device performances are shown in Fig.8(c) -(f).All the data are summarized in 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 comparedthe device without it.

Tab.2 EL data of the solution processed OLED

4 Conclusion

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. The LUMO level of TV-T2T( -3.5 eV) lies between the LUMO levels of ZnO( -4.0 eV) and 2,6-Dczppy( -2.6 eV)∶Ir(mppy)3( - 2. 4 eV), which can facilitate efficient charge injection from the ZnO to the 2,6-Dczppy∶Ir(mppy)3. Comparing the device without TV-T2T,the surface roughness of TV-T2T based device showed a lower value of 2. 27 nm. Using solution processed ZnO/TV-T2T/2,6-Dczppy∶Ir(mppy)3trilayer architecture, the fabricated OLEDs achieved a maximum EQE of 5. 1%. Comparing to a 3. 0%EQE of the device without TV-T2T, the tri-layered device delivered 1.7-fold of improvement.