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Pyridine-Based Electron Transport Materials with High Solubility, Excellent Film-Forming Ability and Wettability for Inkjet-Printed OLEDs Changting Wei, Jinyong Zhuang, Dongyu Zhang, Wenrui Guo, Dongfang Yang, Zhong-Zhi Xie, Jian-Xin Tang, Wenming Su, Haibo Zeng, and Zheng Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12190 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Applied Materials & Interfaces

Pyridine-Based

Electron

Transport

Materials

with

High

Solubility, Excellent Film-Forming Ability and Wettability for Inkjet-Printed OLEDs Changting Wei†,‡, Jinyong Zhuang*,†, Dongyu Zhang†, Wenrui Guo†, Dongfang Yang§, Zhongzhi Xie&, Jianxin Tang&, Wenming Su*,†, Haibo Zeng*‡ and Zheng Cui†

†

Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese

Academy of Sciences, 398 Ruoshui Road, Suzhou Industrial Park, Suzhou, Jiangsu, 215123, People’s Republic of China. Email: [email protected]; [email protected]. ‡

MIIT Key Laboratory of Advanced Display Materials and Devices, Institute of Optoelectronics &

Nanomaterials, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China. Email: [email protected]. §

Market & Product Planning Department, VOSBU, No.9 Dize Road, BDA, Beijing, 100176,

People’s Republic of China. &

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of

Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren-ai Road, Suzhou Industrial Park, Suzhou, 215123, People’s Republic of China.

ABSTRACT Film morphology has predominant influence on the performance of multi-layered organic light-emitting diodes (OLEDs), while there is little reported literature from the angle of molecular level to investigate the impact on film-forming ability and device performance. In this work, four isomeric cross-linkable electron transport materials constructed with pyridine, 1,2,4-triazole and vinylbenzyl ether groups were developed for inkjet-printed OLEDs. Their lowest unoccupied molecular orbital (LUMO: ∼3.20 eV) and highest occupied molecular orbital (HOMO: ∼6.50 eV) levels are similar, which are mainly determined by the 1,2,4-triazole groups. The triplet energies of these compounds can be tuned from 2.51 to 2.82 eV by different coupling 1

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modes with the core of pyridine, where the 2,6-pyridine based compound has the highest value of 2.82 eV. Film formation and solubility of the compounds were investigated. It was found that the 2,6-pyridine based compound outperformed the 2,4-pyridine, 2,5-pyridine and 3,5-pyridine based compounds. The spin-coated blue OLEDs based on the four compounds have achieved over 14.0% external quantum efficiencies (EQE) at the luminance of 100 cd m−2. And a maximum EQE of 12.1% was obtained for the inkjet-printed device with 2,6-pyridine based compound.

Keywords: inkjet printing, electron transport material, cross-linkable, blue phosphorescence, organic light-emitting diodes

1. INTRODUCTION Solution processing, especially inkjet printing is favorable for making organic light-emitting diodes (OLEDs), by virtue of its remarkable superiority in low-cost, mask-free, high material utilization (over 90%), and ease for making large-area device.1-9 It is known that OLEDs consist of multilayers including emitting layer, hole- and electron-transport layer. Therefore, it is a big challenge to obtain perfect interface morphology since multilayer stacking by solution processing tends to intermix of adjacent layers. Employing orthogonal solvent system is one way to solve the problem, which takes advantage of the solubility difference of the functional materials in different solvents.10-17 However, most organic materials can be dissolved in the upper solvents, which makes it hard to find suitable solvents for specific compounds in the solution-processed devices. Recently, cross-linkable materials have been actively used in the multilayer solution-processed light-emitting diodes and solar cells, showing their great potential.18-33 To pursue high performance OLEDs, it is crucial to develop highly efficient electron transport materials (ETMs). The ETMs should be provided with high charge transport ability, low LUMO (lowest unoccupied molecular orbital) level to lower the electron injection barrier from the cathode, deep HOMO (highest occupied molecular 2


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orbital) level to block the hole from the emitting layer (EML); suitable triplet energy level to confine the excitons in the EML.34-38 Pyridine group is a strong electron withdrawing group, which is widely used for the construction of ETMs.39-47 Meanwhile, 1,2,4-triazole based derivatives have excellent electron transport and hole-blocking abilities because of their electron deficient triazole moiety.48-53 In our previous study, we reported a cross-linkable ETM based on 1,2,4-triazole and applied the compound in inkjet-printed blue OLEDs with excellent device performance.30 In this work, the pyridine and 1,2,4-triazole groups were employed to construct

the

core

structure

of

four

isomeric

ETMs,

which

are

2,4-bis(4-(4-(4-(tert-butyl)phenyl)-5-(4-(((4-vinylbenzyl)oxy)methyl)phenyl)-4H-1,2, 4-triazol-3-yl)phenyl)pyridine

(DV-24PyTAZ),


(DV-25PyTAZ),


(DV-26PyTAZ),

3,5-bis(4-(4-(4-(tert-butyl)phenyl)-5-(4-(((4-vinylbenzyl)oxy)methyl)phenyl)-4H-1,2, 4-triazol-3-yl)phenyl)pyridine (DV-35PyTAZ), respectively. The vinylbenzyl ether group works as the cross-linking unit for these compounds, which make the ETMs can be thermally cross-linked without any initiator. The photophysical and thermal properties, film morphology and solvent resistance of the four compounds were systematically investigated, as well as the relationship between molecular structure and device performance. All the compounds show low LUMO levels of around -3.2 eV and deep HOMO levels of about -6.5 eV, which indicates their effective electron injection ability and hole-blocking ability. All the spin-coated blue OLEDs based on the four compounds achieved over 14.0% external quantum efficiencies (EQE) at the luminance of 100 cd m-2. DV-26PyTAZ outperformed the other three compounds in film forming ability and solubility, and a maximum EQE of 12.1% was obtained for the inkjet-printed blue OLED. These results imply that highly efficient pyridine-based electron transport materials can be obtained by molecular modulation, which is very promising for the inkjet-printed OLEDs. 3



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2. RESULTS AND DISCUSSION 2.1 Molecular structures and theoretical calculations The molecular structures of the isomeric ETMs DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ are shown in Scheme 1. The compounds are constructed with one pyriding and two 1,2,4-triazole groups, and the peripheral vinylbenzyl ether group is the cross-linkable unit. The synthetic procedures are depicted in Scheme S1 and the preparation of the target molecules is described in Experimental Details (Supporting Information).

Scheme 1. Molecular structures of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ and DV-35PyTAZ.

Density functional theory (DFT) calculations were conducted to study the molecular orbital spatial distributions of the ETMs. As shown in Figure 1, these materials have similar HOMO and LUMO distributions, except for the LUMO of DV-24PyTAZ.

The

LUMO

energy

levels

are

mainly

located

on

the

electron-deficient triazole and pyridine units. As for the HOMO, DV-25PyTAZ, DV-26PyTAZ and DV-35PyTAZ show symmetrical distribution on both the two triazole moieties. On the other hand, DV-24PyTAZ exhibits localized cloud distribution over the single triazole moiety, which could be attributed to the different coupling modes between pyridine and triazole.

4


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Figure 1. Calculated frontier orbitals and energy levels of the compounds.

2.2 Photophysical and thermal properties The UV-Vis absorption and photoluminescence (PL) spectra of the four compounds both in solution and film state are illustrated in Figure 2. As shown in Figure 2a, the absorption spectra of the isomeric compounds have similar absorption peak at about 256 nm in solution, which could be ascribed to their π-π* transitions.30, 41 Additionally, the longer wavelength at the absorption peaks of 285-323 nm corresponds to the n-π* transitions of 1,2,4-triazole groups, which are due to the different coupling modes between pyridine and triazole group.30, 44, 52, 54 It is also worthy to note that these compounds in thin films revealed a significant bathochromic shift in comparison with those in solution, which implies an intensive intramolecular interaction in solid state. According to the absorption edges of the thin films, the optical band gaps (Eg) before and after cross-linking were calculated and summarized in Table 1. The band gaps for each compound before and after cross-linking are almost the same. This phenomenon could be attributed to the cross-linkable unit, which is not conjugated with the core 5



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structure of the ETMs.

Figure 2. a) UV-Vis absorption (solid) and PL (open) spectra of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ and DV-35PyTAZ in CH2Cl2 (c= 1.0 × 10-5 mol·L-1), b) Normalized UV-Vis absorption (solid) and PL (open) spectra of these four in thin solid film.

Triplet energy level (ET) is important for phosphorescent OLEDs, because suitable ET value can ensure effective excitons confinement in the emitting layer. To obtain their triplet energies, phosphorescence spectra of the compounds were measured in methyl tetrahydrofuran solution at 77 K (Figure S1). It has been reported that the triplet energies of these isomers can be tuned by simply varying the coupling mode between pyridine and 1,2,4-triazole.55, 56 Based on the onset of the spectra, the ET values of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ and DV-35PyTAZ were calculated.40 The ET values of DV-25PyTAZ and DV-35PyTAZ are very similar, which are 2.52 and 2.51 eV, respectively. As for DV-24PyTAZ and DV-26PyTAZ, the ET values are 2.61 and 2.82 eV, respectively. Thus, the ET value can be easily tuned through different coupling modes with pyridine group, and the substitution of 5-position in pyridine unit should be avoided in order to acquire high triplet energy. The detailed photophysical data are summarized in Table 1. The thermal properties of the ETMs were performed by thermogravimetric analysis (TGA, Figure S2) and differential scanning calorimetry (DSC, Figure S3). All the compounds presented good thermal stability with the decomposition temperatures (Td) ranging from 305 to 354 °C corresponding to 5% weight loss. As shown in the first-heating scan, the glass-transition temperatures (Tg) of 114 and 118 °C were observed for DV-26PyTAZ and DV-35PyTAZ, respectively. As for DV-24PyTAZ, the Tg increases to 144 °C. Interestingly, only DV-25PyTAZ displayed a crystallization exotherm (Tc) at 171 °C from the first-heating scan, and yet no glass 6


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transition state was observed. In addition, all the styrene functionalized materials exhibited the exothermic peak above 200 °C, which is in accordance with the thermal polymerization of the vinylbenzyl ether groups.21, 23 There is a melting state before polymerization for DV-25PyTAZ and DV-26PyTAZ, which would lead to a high degree of cross-linking.24 Besides, no apparent signals were detected ranging from 25 to 300 °C in the second scan for these compounds, which indicates the excellent thermal stability of these ETMs after cross-linking. The main thermal data are outlined in Table S1. Table 1. Photophysical properties of the four compounds.

UV-Vis spectra Compounds DV-24PyTAZ DV-25PyTAZ DV-26PyTAZ DV-35PyTAZ

UPS (Before/After)

Phosphorescence

[eV]

Eg [eV]

HOMO[d] [eV]

LUMO[e] [eV]

ET[f] [eV]

3.36 3.22 3.35 3.28

3.38 3.15 3.35 3.27

-6.48/-6.57 n.a./-6.58 -6.63/-6.55 n.a./-6.49

-3.12/-3.19 n.a./-3.43 -3.28/-3.20 n.a./-3.22

2.61 2.52 2.82 2.51

Egop[a]

Egopt[b]

[eV] 3.63 3.41 3.54 3.43

opt[c]

[a] Calculated from the absorption edge of UV–Vis absorption in solution. [b] Determined from the film before cross-linking. [c] Determined from the film after cross-linking. [d] Calculated from 21.22 - Ek from the UPS data.57 [e] Calculated from the HOMO determined from UPS data and Egopt for the film before and after cross-linking. [f]] ET was calculated from the onset of the phosphorescence in dimethyl tetrahydrofuran solution at 77K.

It was reported that the density of solution-processed film is inferior to the corresponding vacuum-deposited one due to the different molecular orientation of film formation processes.58 Here, the densities of the before and after cross-linking ETM films before and after cross-linking

were investigated (Figure 3). The film

thickness after cross-linking is smaller than the ones before cross-linking for all the compounds. The average shrinkages for DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ and DV-35PyTAZ after cross-linking are 6.7%, 6.0%, 4.9% and 2.7%, respectively, which means that all the films become more compact after cross-linking. And this phenomenon could be attributed to the rearrangement of the molecular orientation during thermal curing.

7



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Figure 3. Film thicknesses of a) DV-24PymTAZ, b) DV-25PymTAZ, c) DV-26PymTAZ, and d) DV-35PymTAZ the films before and after cross-linking.

The energy levels of four compounds were measured by ultraviolet photoelectron spectroscopy (UPS, Figure S4). The HOMO levels of DV-24PyTAZ and DV-26PyTAZ films before cross-linking were calculated to be -6.48 and -6.63 eV, respectively. As for the cross-linked films, the HOMO levels are -6.57 eV and -6.55 eV.57 The LUMO levels of the two compounds before and after the cross-linking process were -3.12/-3.19 eV and -3.28/-3.20 eV, which were determined from the HOMO energy level and the Egopt value in the film state. The HOMO energy levels of DV-25PyTAZ and DV-35PyTAZ after the cross-linking were -6.55 and -6.49 eV. The corresponding LUMO energy levels were -3.40 and -3.22 eV. However, the HOMO energy levels of DV-25PyTAZ and DV-35PyTAZ before cross-linking could not be obtained under the same examination conditions, which may be due to their poor conductivity. Above all, the HOMO levels of the four compounds are all around -6.5 eV and the cross-linking hardly influence on the HOMO levels, as the HOMO levels before and after cross-linking are almost the same. The LUMO levels calculated from 8


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the HOMO energy level and the Egopt value are about -3.2 eV. Such low LUMO energy levels improve the electron injection, and the deep HOMO energy levels indicate their hole blocking ability in OLEDs.52, 54 2.3 Solvent resistance, solubility, film-forming ability and wettability It is well known that solvent resistance of interlayers is vital to fabricating well-defined multilayer structures during the succession of solution processes.21, 23, 60 The solvent resistance studies of the four compounds were performed by UV-Vis absorption spectroscopy for the films before and after rinsing with commonly used organic solvents such as chlorobenzene, toluene, xylene, tetrahydrofuran, and chloroform. The UV-Vis absorption spectra of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ and DV-35PyTAZ films on quartz are shown in Supporting Information (Figure S5-S8). As can be seen in Figure 4, more than 97% solvent resistance can be obtained by comparing the UV-Vis absorbance before and after washing.28, 61 These results demonstrate that all the compounds have excellent solvent resistance after cross-linking.

Figure 4. Solvent resistance results of the four ETMs after rinsing with chlorobenzene, toluene, xylene, tetrahydrofuran and chloroform, respectively.

To better understand the film properties of the four ETMs, the surface morphologies of films deposited by spin-coating were investigated by atomic force microscopy (AFM). Figure 5 presents the AFM topographic images of the spin-coated films before cross-linking, after cross-linking and after rinsing with chlorobenzene. All the films were smooth and uniform and the root-means-square (RMS) roughness were small, within the range of 0.42 to 1.48 nm. After rinsing with the chlorobenzene, 9



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the RMS roughness were 0.91, 1.40, 0.77, and 1.42 nm for DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ and DV-35PyTAZ, respectively. It is noted that DV-26PyTAZ has the smallest RMS values among the four compounds, which should be beneficial to lower the current leakage.

Figure 5. AFM topographic images (2 µm × 2 µm) of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ thin films.

The material solubility is directly connected to the solution processing ability, especially for inkjet printing, because the solvents used in inkjet printing are usually aromatics with high boiled points. Besides, the reported solvents in inkjet-printed OLEDs such as indan, tetralin, cyclohexylbenzene, α-chloronaphthalene, et.al, normally have poorer solubility than the solvents used in spin-coating.8, 30, 33, 62, 63 Thus, it is necessary to investigate the solubility of the four isomeric compounds. As shown in Figure 6, DV-24PyTAZ, DV-25PyTAZ, and DV-35PyTAZ have the solubility of 35∼40 mg/mL, 5∼10 mg/mL, 15∼20 mg/mL, respectively. As for DV-26PyTAZ, the solubility is over 40 mg/mL at the same condition, which is the highest among these compounds (Figure 6). Both DV-24PyTAZ and DV-25PyTAZ have the solubility over 35 mg/mL, which resulted in the better film morphologies of the two compounds.

10


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Figure 6. Solubility test of the four compounds (1,2-dichloroethane, 28 °C)

The wettability of the four ETMs was investigated by measuring their corresponding contact angles, as the film surface energy plays a crucial role for the deposition of upper layer. By automated image analysis, the contact angles were determined to be 48.2°, 47.1°, 39.5°, and 50.3°, respectively, which were measured with diiodomethane. As for deionized water (Figure 7), the contact angles were measured to be 85.5°, 90.2°, 88.1° and 94.5° for DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ, respectively. The surface energies of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ were calculated to be 35.6, 35.8, 39.9, and 34.2 mN/m, according to Owens-Wendt-Rabel-Kaelble (OWRK) method.64 The detailed data are summarized in Table S2. DV-26PyTAZ has the highest surface energy among these compounds, which suggests better wettability for the deposition of next layer by solution-processing.

11



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Figure 7. Images of a drop of diiodomethane and deionized water on the four cross-linked ETM films during the contact angle tests.

To carefully evaluate the result of the as-formed surface energy on upper layer film forming ability, the emitting layer (EML) was cast on the four ETMs. The emitting layer consists of 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) as the host and

fac-tris[1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-2-phenyl-1H-imidazole]

iridium(III) (Ir(dbi)3, 10wt%) as the dopant. Figure 8 shows the optical microscopy images of the spin-coated EML. The film morphology of EML on DV-24PyTAZ is rough and full of grains. For EML on DV-25PyTAZ and DV-35PyTAZ, the films are covered with spots and cracks. On the other hand, the film morphology of EML on DV-26PyTAZ is very smooth and uniform, indicating the superiority of DV-26PyTAZ for solution deposition of EML. For the better as-formed film morphology of EML on DV-26PyTAZ, which could be attributed to its smaller RMS values and better wettability that tolerate the film-forming of upper EML layer to a greater extent.

Figure 8. Optical microscopy images of the spin-coated EML on the four cross-linked ETM films. 12


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2.4 Device performance. All the compounds were applied in blue OLEDs to systematically evaluate these molecules as electron transport materials. The device structure consists of ITO/ZnO (35

nm)/ETMs/26DCzPPy:

Ir(dbi)3

(30

wt%)/1,1-bis[(di-4-tolylami-no)phenyl]cyclohexane nm)/1,4,5,8,9,11-hexaazatriphenylene

hexacarbonitrile

nm,

10

(TAPC) (30 (HAT-CN) (10 nm)/MoO3

(10 nm)/Al. The DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ were all spin-coated. The molecular structures, device configuration and energy level diagram are demonstrated in Figure 9.

Figure 9. a) Molecular structures , b) schematic configuration, and c) energy-level diagram of the devices.

The thicknesses of all the ETMs were fully optimized in the devices, which were 15 nm, 15 nm, 15 nm, and 10 nm, respectively. The emitting layers were deposited by vacuum-deposition to exclude the effect of the different surface energy of each compound. For comparison, the electroluminescence (EL) properties of the four optimized devices are gathered in Figure 10 and the detailed performance parameters are outlined in Table 2. Figure S9-S12 illustrate the current density-voltage-luminance (J-V-L), current efficiency-power efficiency-luminance (CE-L-PE), EQE-luminance 13



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(EQE-L) characteristics, and EL spectra and detailed data are summarized in Table 2. All the devices based on the four ETMs showed over 14.0% EQEs at the luminance of 100 cd m−2. Considering the results, the difference of excellent film morphology or the high triplet energy result into the enhancement of the efficiency of these devices. For current efficiency and EQE values for DV-25PyTAZ are higher than the DV-26PyTAZ, which may be owing to the better electron injection ability of DV-26PyTAZ.

Figure 10. a) J-V-L, b) CE-L-PE, c) EQE-L characteristics, d) EL spectra at the current density of 20 mA cm-2 of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ based devices.

Tables 2. EL data of DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ based devices. CE Von

Device

PE

EQE

Voltage

CE

PE -2

Maximum

EQE

@100 cd m / @1000 cd m

-2

DV-24PyTAZ (15 nm)

6.6

31.7

14.7

14.7

8.5/9.9

31.5/21.4

11.6/6.8

14.5/9.9

DV-25PyTAZ (15 nm)

6.5

36.1

16.4

16.6

8.2/9.5

31.2/23.8

12.1/8.0

14.5/11.0

DV-26PyTAZ (15 nm)

5.8

32.9

18.1

15.4

7.6/9.1

30.7/20.6

12.9/7.0

14.2/9.4

DV-35PyTAZ (10 nm)

7.6

32.6

11.1

14.6

9.5/10.8

31.6/21.2

10.7/6.2

14.4/9.6

-1

-1

CE: cd A , PE: lm W , EQE: %, Von and Voltage: V.

14


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In addition, inkjet printing was also performed using the electron transport materials. The ink consisted of indan, tetralin, and butyl phenyl ether as the solvents. First, the solubility of these ETMs was investigated to ensure the film forming property. As expected, DV-26PyTAZ has better solubility than the other compounds, which is over 10 mg/mL in the mixed solvents (Figure S13). As a result, DV-26PyTAZ was applied in the inkjet-printed OLEDs and the related J-V-L,

CE-L-PE), EQE-L characteristics, and EL spectra are shown in Figure 11. The controlled device without ETM was employed for comparison, which exhibited maximum EQE of 9.1% (19.8 cd A-1 & 6.9 lm W-1). The device based on the inkjet-printed DV-26PyTAZ in air achieved a high EQE of 12.1% (31.5 cd A-1& 12.1 lm W-1), which is in close proximity to the spin-coated one fabricated in glovebox. The slight decrease in efficiency for the ink-jet printed device may be due to the higher roughness (1.70 nm ， Figure S17b) of the film in comparison with the spin-coated one (0.70 nm). Encouraged by these results, trilayer solution-processed devices were also fabricated. The device exhibited an inspiring result with a maximum EQE of 11.1% (23.3 cd A-1& 8.4 lm W-1). The main EL parameters are outlined in Table 3. And Figure S14 presents the average peak EQEs of the DV-26PyTAZ-based devices. Besides, the electrical stability of DV-26PyTAZ based devices was studied (Figure S15). The inkjet-printed device showed similar lifetime with the spin-coated devices in glove box, which indicates the air stability of DV-26PyTAZ.

15



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Figure 11. a) J-V-L, b) CE-L-PE, c) EQE-L characteristics, d) EL spectra at the current density at 20 mA cm-2 of the devices. Table 3. EL data of the DV-26PyTAZ based devices. Device

Von

CE

PE

EQE

Voltage

CE

PE -2

Maximum

EQE

@100 cd m / @1000 cd m

-2

Without ETM

7.1

19.8

6.9

9.1

9.0/10.1

19.6/17.2

6.8/5.3

9.0/7.9

Spin-coated ETM

5.8

32.9

18.1

15.4

7.6/9.1

30.7/20.6

12.9/7.0

14.2/9.4

Inkjet-printed ETM

6.2

31.5

13.9

12.1

8.2/10.6

31.2/28.1

11.9/8.3

12.0/8.6

Trilayer solution-processed

5.6

23.3

8.4

11.1

9.7/13.8

23.2/17.5

7.5/4.0

11.1/8.1

Von: 1 cd m-2, CE: cd A-1, PE: lm W-1, EQE: %, Von and Voltage: V.

3. CONCLUSIONS Herein, we have designed and synthesized four isomeric cross-linkable electron transport materials, DV-24PyTAZ, DV-25PyTAZ, DV-26PyTAZ, and DV-35PyTAZ. The photophysical, thermal properties, energy levels, film morphology, and solubility of the four compounds were investigated. The low LUMO (∼3.20 eV) and deep HOMO (∼6.50 eV) levels ensured their efficient electron injection and hole blocking abilities. The triplet energies of these compounds can be tuned from 2.51 to 2.82 eV by different coupling modes to the core of pyridine, in which DV-26PyTAZ has the 16


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highest value of 2.82 eV. Film-forming ability and solubility of the compounds were investigated, with the 2,6-pyridine based compound outperformed the 2,4-pyridine, 2,5-pyridine and 3,5-pyridine based compounds. As for inkjet printing, DV-26PyTAZ outperformed the other three compounds in solubility and film forming ability. Blue OLEDs achieved a maximum EQE of 12.1% based on the inkjet-printed DV-26PyTAZ. Further, a blue device was fabricated by trilayer solution-processing with DV-26PyTAZ, which exhibited a maximum EQE of 11.1%. Our work indicates that by modulating the molecular structures, high efficiency ETMs with high solubility excellent film-forming ability can be obtained for inkjet-printed OLEDs.

4. EXPERIMENTAL SECTION General information. Unless otherwise stated, all commercially available reagents and chemicals were used as received without further purification. 1H NMR and

13

C

NMR spectra were recorded on a 400 MHz Varian NMR spectrometer at ambient temperature. High-performance liquid chromatograph mass spectra (HPLC-MS) were performed on Agilent Technologies 1260 Infinity (America). UV-Vis spectra of the isomeric ETMs in CH2Cl2 solution, uncross-linked and

cross-linked films were

performed on a Perkin Elmer Lambda 750 spectrophotometer. PL spectra and phosphorescent spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer, respectively. Thermogravimetric analysis (TGA) was undertaken with a NETZSCH TG 209 F1 (Germany) under a nitrogen atmosphere and

the

samples were heated from 25 to 700 °C at an isothermal heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) was conducted by a Netzsch DSC F3 Maia (Germany) at an isothermal heating rate of 10 °C min−1 from room temperature to 300 °C under a nitrogen atmosphere. The thicknesses of solution-processed films were measured using an AlphaStep profilometer (Veeo, Dektak150). HOMO levels were determined from UPS analysis with an unfiltered He Ⅰ (21.22 eV) gas discharge lamp and a hemispherical analyzer (Kratos Analytical Shimadzu Group Company). Atomic force microscopy (AFM) measurements were recorded by using a Dimension 3100 Scanning Probe Microscope at ambient temperature in tapping mode. The contact 17



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Page 18 of 26

angles were tested by using a contact angle meter model SL150 (USA KINO Industry).

Materials. Zinc oxide (ZnO) nanoparticles (NPs) were synthesized using an adapted procedure according to a previous report with slight modification.65 The ZnO NPs were prepared by dispersing them into 2-ethoxyethanol and the concentration was 20 mg/mL. imidazole]-iridium

Tris[1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-2-phenyl-1H(III)

(Ir(dbi)3)

was

purchased

from

SunaTech

Inc.

2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26Dczppy), 1,1-bis[(di-4-tolylamino) phenyl]cyclohexane (TAPC) were purchased from Nichem Fine Technology Co. Dipyrazino [2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11- hexacarbonitrile (HAT-CN) were purchased from Shanghai Han Feng Chemical Co.,Ltd. MoO3 was purchased from Alfa Aesar.

OLED Device Fabrication and Measurements. 30 × 30 mm2 indium tin oxide (ITO) coated glass substrates with resistance of 10 Ω per square were patterned by the conventional photolithography method and a wet etching process. The ITO substrates were cleaned by sanitation in several solvents orderly, then dried in an oven at 110 °C and treated by UV ozone for 30 min.30 The actual procedures involved first spin-coating of ZnO NPs at 2000 rpm for 60 s on a pre-cleaned ITO anode and then drying at 120 °C for 10 min to form a 35 nm thickness film in the glovebox. The ETLs were dissolved in 1,2-dichloroethane and spin-coated at 2,000 rpm for 60 s onto the ZnO layer. After the coating of ETL, the substrate was baked at 84 °C for 20 min to remove residual solvent, and then it was cured for cross-linking. By adjusting the concentration of the solution, the films thicknesses can be controlled at 10 nm, 15 nm, 20 nm for these four ETMs, respectively. Next, as for vacuum-deposited EML, 10 wt% Ir(dbi)3 was co-deposited with 26Dczppy to form a 30 nm EML. As for spin-coating the EML, 10 wt% Ir(dbi)3 and 26Dczppy were dissolved in chlorobenzene and stirred for 30 min. The resulting solution was spin-coated at 2500 rpm for 60 s and then dried at 80 °C for 30 min under nitrogen to form 40 nm thickness film.32 Finally, a 18


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hole-transporting layer of TAPC (30 nm), hole-injection layers HAT-CN (10 nm), MoO3 (10 nm) and Al, were deposited layer by layer in the vacuum of 4.5×10-4 Pa by vacuum deposition.30 All electrical testing and optical measurements were carried out performed under ambient conditions without further encapsulation. The device performance (EL spectra, J-V curves, L-V, and EQE values) was measured with a Spectra Scan PR655 and a computer controlled Keithley 2400 Sourcemeter. All the devices have an emitting area of 2 × 2 mm2, and only measuring the luminance in the forward direction. The lifetime of the devices was measured with a lifetime tester (Polaronix M6000) in a constant current mode.

Inkjet Printing of DV-26PyTAZ. DV-26PyTAZ was printed with a inkjet printer (Dimatix 2831, USA) in air. Figure S15 shows the film morphology of the inkjet-printed DV-26PyTAZ before and after formula optimization. After optimizing, the ink for printing is a 5 mg/mL solution in indan, tetralin, and butyl phenyl ether, as the drop mode displayed in Figure S16a. Based on the above-mentioned results, after the ink-jet printing of DV-26PyTAZ after cross-linking on the ZnO layer, the roughness was only 1.70 nm (Figure S16b). Accordingly, a decent device performance can be expected for the inkjet-printed DV-26PyTAZ.

Synthesis and characterization. All reactants and solvents, were purchased from Sigma-Aldrich, J&K Co, Energy Chemicals, Alfa Aesar, Aladdin, Sinopharm Chemical, Nanjing Chemlin Chemical Industrial Co., Ltd and used without further purification. Additionally, anhydrous THF were dried over CaCl2 and Na/Ph2CO, distilled and stored at N2 atmosphere. All reactions involving air-sensitive reagents were carried out under the nitrogen atmosphere. The reactions were carried out in a standard Schlenk line under N2 atmosphere. Four isomeric electron transport materials were synthesized following the same Suzuki coupling reaction, reduction reaction, and etherification reaction from boron ester-substituted 1,2,4-triazole (1) and four dibromopyridine derivatives. All compounds were purified with silica columns using

19



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dichloromethane, ethyl acetate, hexane, and methanol as eluents. And all of the compounds were structurally confirmed by 1H NMR, 13C NMR, and HPLC-MS.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Supplemental figures and

tables, experimental details,

synthesis,

characterization, phosphorescent spectra, TGA, UPS data, solvent resistance, contact angle, optical microscopy images, optimization of the devices, and lifetime test.

AUTHOR INFORMATION Corresponding Authors Email: [email protected], [email protected], [email protected].

Notes The authors declare no competing financial interest.

Acknowledgements This

work

was

supported

National Program on Key Research Project (2017YFB0404400),

by

the Strategic

Priority Research Program of the Chinese Academy of Sciences (grant number XDA0 9020201),

National Key R&D Program of China (2016YFB0401500 and

2016YFB0401600), Science Foundation of Two sides of Strait (Key Program, No. U1605244), Program on Key Research Project of Jiangsu Province of China (No. BE 2016173), National Natural Science Foundation of China (NSFC) (21402233), the Natural Science Foundation of Jiangsu Province (BK20140387), Project on the Integration of Industry, Education and Research of Jiangsu Province (BY2014066), and Prospective Application Research Program of Suzhou City (SYG201631). 20


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The authors also thank Youth Innovation Promotion Association CAS (No. 2013206) for financial support.

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