A new thermally crosslinkable hole injection material for OLEDs

Organic Electronics 13 (2012) 2508–2515

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A new thermally crosslinkable hole injection material for OLEDs Wen-Yi Hung a,⇑, Chi-Yen Lin b, Tsang-Lung Cheng a, Shih-Wei Yang a, Atul Chaskar b, Gang-Lun Fan b, Ken-Tsung Wong b,⇑, Teng-Chih Chao c, Mei-Rurng Tseng c a

Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan Department of Chemistry, National Taiwan University, Taipei 106, Taiwan c Material and Chemical Research Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu 310, Taiwan b

a r t i c l e

i n f o

Article history: Received 18 May 2012 Received in revised form 15 June 2012 Accepted 15 June 2012 Available online 13 July 2012 Keywords: OLEDs Hole injection layer Thermal crosslink Hole mobility

a b s t r a c t A new thermal cross-linkable hole injection monomer VB-DATA derived from famous mMTDATA as core peripherally functionalized with styryl (vinylbenzene) moiety as polymerizable group has been synthesized and characterized. The propensity of VB-DATA thin films formation is sensitive to the nature of solvent, in which the dichloroethane solution gave smooth polymeric thin films with surface roughness of RMS 0.84 nm by spin-casting followed by thermal treatment at 190 °C. The introduction of oxygen-linked vinylbenzene group shifted HOMO energy level of VB-DATA to 5.1 eV along with good nondispersive hole transport property (lh  10–6 cm2 V–1 s–1) makes it suitable for serving as HIL on top of ITO electrode. The replacement of PEDOT:PSS by thermally cross-linked VB-DATA films showed comparable OLEDs performance, giving more flexibility on material selection for future OLEDs applications, especially solution-processed ones. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The explosive growth in the application of organic lightemitting diodes (OLEDs) as flat-panel-display is a strong evidence of success in organic electronics. In order to be more competitive with contemporary display technologies, tremendous efforts have been made to improve device performance and reduce the manufacturing cost. So far, it is inevitable to adopt a multilayer device architecture. In this regard, all functional layers are equally important and play a specific role in overall device performance, which is mainly govern by the charge injection and transport from electrodes as they are responsible for the formation of excitons. Among various active layers, the hole injection layer (HIL) in OLEDs is an interfacial connection layer between the anode (usually ITO) and the hole-transporting layer (HTL). HIL is utilized to facilitate efficient cascade ⇑ Corresponding authors. Tel.: +886 (02)24622192x6718; fax: +886 (02)24634360 (W.-Y. Hung). E-mail addresses: [email protected] (W.-Y. Hung), [email protected] (K.-T. Wong). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.023

hole injection from the ITO anode to HTL, rendering a lower turn-on voltage. HIL can also improve the film forming property of the subsequent organic layer by planarizing the ITO surface [1,2], giving longer device lifetime. In order to provide such functions, hole injection materials (HIMs) usually have higher ionization potentials and good adhesion to the anode surface. A conductive polymer blend, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is widely used as the hole injection material owing to its good electrical, optical and mechanical properties [3,4]. Subsequently, a hole transport layer (HTL) is deposited as an interlayer between the HIL and emitting layer (EML), leading to effective prevention of exciton quenching at the adjacent PEDOT:PSS interface [5] and occasionally function as the electron-blocking layer [6,7]. In spite of the known advantages, PEDOT:PSS is associated with some potential risks. The strong acidic colloidal solution of PEDOT:PSS may cause degradation of adjacent layers and etching of ITO [8]. Furthermore, PEDOT:PSS would degrade with high curing temperature, for example, Friedel et al. recently showed that the device performance dramatically reduced as the annealing temperature of

W.-Y. Hung et al. / Organic Electronics 13 (2012) 2508–2515

PEDOT:PSS is higher than 260 °C, which associated with the chemical decomposition to give sulfonic acid [9]. Thus, in an endeavor to realize efficient OLED with high power efficiency at low driving voltage, it is desirable to develop new HILs with better stability. In this regard, crosslinkable HIMs with the resistance of organic solvent erosion could be an alternative to PEDOT:PSS, rendering high performance OLEDs by solution processable fabrication feasible. In addition, crosslinkable HIMs approach can offer a number of advantages, such as convenience of fabrication, flexibility in choosing hole transport materials, and avoid luminescence quenching. In 2000, Jen first incorporated trifluorovinyl ethers (TFVE) as polymerization group in traditional HTMs [10,11]. It is well-documented that aromatic TFVE can undergo thermal cyclodimerization to form high molecular weight polymers with aromatic perfluorocyclobutane (PFCB) ether linkages. In 2006, Kim introduced an arylamine-based hole injection material (TPA-TFVE) containing TFVE as thermal polymerization group [12]. The thermally (at 230 °C) crosslinked TPA-TFVE ontop of ITO showed a better surface smoothness than PEDOT:PSS. In addition, the OLED device adopted TPA-TFVE as HIL gave the maximum luminance and luminance efficiency which were 2 times higher than that of using the counterpart PEDOT:PSS. The better device characteristics were ascribed to the superior electron-blocking property of the TPA-TFVE polymer than that of PEDOT:PSS. In 2008, Jen demonstrated an innovative double-crosslinked-HTLs approach using triaryldiamine (TPD) equipped with trifluorovinyl ether group (PS-TPD-TFV) and tricarbazole (TCTA) containing with vinylbenzyl group (TCTA-BVB) as cross-linker to facilitate cascade hole injections in a blue-emitting phosphorescent OLED devices [13]. Due to their ability to avoid interfacial mixing and erosion of bottom layer, solutionprocessed device with the configuration of ITO/crosslinked PS-TPD-TFV/crosslinked TCTA-BVB/PVK-FIr6 (10 wt.%)/ TPBI/CsF/Al, where PS-TPD-TFV and TCTA-BVB were used as the HIL and HTL, respectively. The maximum external quantum efficiency reached to 3.17%, corresponding to a current efficiency of 6.6 cd A1. Due to the reduced holeinjection barrier and better electron/exciton confinement, the external quantum efficiency and brightness were significantly improved as compared with those of the device using only PEDOT:PSS. This double-crosslinked HTLs approach is especially useful for the blue emitting device where the HOMO levels of emitting material or the host are usually lower than 6.0 eV. Previously, we reported a thermally polymerizable 9,9-diarylfluorene-based arylamine monomer (VB-FNPD) equipped with styrene for polymerization [14]. This is the first report about the intrinsic charge carrier mobility of styrene-based thermally polymerizable HTL materials. The non-dispersive and high hole drift mobility (lh  104 cm2V1 s1) allowed us to achieve OLED with a maximum external quantum efficiency of 1.4%. In this contribution, we report the synthesis and application of a new thermally crosslinkable HIM (VB-DATA, Scheme 1). This new crosslinkable HIM is derived from famous HIM 4,40 ,400 -tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) used in vaccum process. DATA exhibits reversible anodic oxidation at low oxidation potential, implying a high HOMO energy level

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capable of hole injecting. In addition, we introduced styryl moiety as peripheral pendent groups with regards to its mild thermal polymerization in the absence of an initiator, giving the resulting polymer with high solvent resistance which can facilitate the deposition of subsequent layers through solution-processing. This star-shaped VB-DATA molecule exhibits low curing polymerization temperature without any initiator, giving the thin HIL films with high morphological stability, high HOMO energy level (5.1 eV) and hole transport mobility (lh  10–6 cm2 V1 s). The devices utilized PEDOT:PSS and thermally crosslinked VB-DATA as HIMs show comparable performance.

2. Experimental 2.1. Synthesis 1-Bromo-3-(4-vinylbenzyloxy)benzene (1): 18-Crown6 (4.63 g, 17.5 mmol), potassium carbonate (24.19 g, 175 mmol), and 4-vinylbenzyl chloride (13.35 g, 87.5 mmol) were added to a solution of 3-bromophenol (15.59 mg, 90.1 mmol) in acetone (875 ml). After heating under reflux for 24 h, the suspension was cooled to room temperature, filtered off and the residue was washed extensively with ethyl acetate. The combined organic solution was dried over MgSO4 and concentrated. The resulting residue was washed with methanol to afford pure 1-bromo-3-(4-vinylbenzyloxy)benzene (1) (24.1 g, 95%) as a white solid; IR (KBr) m 3088 (w), 3059 (w), 2929 (w), 2877 (w), 1919 (w), 1693 (w), 1587 (m), 1569 (s), 1513 (m), 1474 (m), 1406 (m), 1376 (m), 1303 (m), 1241 (s), 1064 (m), 1241 (s), 1064 (m), 1008 (m), 990 (s), 890 (m), 838 (s), 829 (s), 771 (s), 678 (s) cm1. 1H NMR (acetoned6, 400 MHz): d 7.50 (d, J = 8.0 Hz, 2H), 7.45 (d, J = 8.0 Hz, 2H), 7.26–7.20 (m, 2H), 7.11 (d, J = 8.4 Hz, 1H), 7.02 (d, J = 8.4 Hz, 1H), 6.77 (dd, J = 17.6, 10.8 Hz, 1H), 5.83 (d, J = 17.6 Hz, 1H), 5.24 (d, J = 10.8 Hz, 1H), 5.15 (s, 2H). 13C NMR (acetone-d6, 100 MHz): d 160.5, 138.1, 137.3, 137.2, 131.7, 128.6, 127.0, 124.5, 123.0, 118.7, 114.9, 114.3, 70.5. MS (m/z, FAB+) 289.1 (10), 281.0 (10), 267.1 (10), 221.2 (10), 207.1 (20), 193.1 (10), 180.2 (10), 168.2 (60), 147.1 (30), 136.1 (40), 117.1 (90), 115.1 (20), 91.0 (30), 73.0 (100). HRMS (M+, FAB+) Calcd. C15H13BrO 288.0150, found 288.0147. N-Phenyl-3-(4-vinylbenzyloxy)aniline (2): 1-Bromo-3(4-vinylbenzyloxy)benzene (1) (2.89 g, 10 mmol), aniline (1.0 ml, 11 mmol), Pd2(dba)3 (270 mg, 0.3 mmol), sodium tert-butoxide (3.84 g, 40 mmol) were dissolved in 70 ml toluene and tert-butyl phospine (6 ml, 0.3 mmol, 0.05 M in toluene) was added. The reaction was heated at 75 °C under Argon atmosphere for 15 h and quenched with water. The reaction mixture was filtered through Celite and then it was washed with CH2Cl2. The filtrate was extracted with CH2Cl2 and partitioned using brine. The combined organic phase was dried over MgSO4 and concentrated. The residue was purified by column chromatography (SiO2:EA/hexane, 1:7) to afford N-phenyl-3-(4vinylbenzyloxy)aniline (2) (1.77 g, 59%) as a yellow solid; IR (KBr) m 3373 (m), 3085 (w), 3039 (w), 3002 (w), 2926 (w), 2872 (w), 1907 (w), 1818 (w), 1587 (s), 1513 (m),

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OH

O

Cl 18-crown-6, K2CO3 Acetone, reflux 95%

Br

1

Br

Pd2(dba)3, Pt Bu3, NaOt Bu, toluene, 75 oC

O

NH2

+

59%

O

NH

1

Br

2

O Br

Br

O

N +

R

Pd(OAc)2, Pt Bu3, NaOt Bu, toluene, 75 oC

NH

N

N N

66%

O

2 Br O

R:

N

VB-DATA Scheme 1. Synthetic route of monomer VB-DATA.

1498 (s), 1453 (m), 1403 (m), 1380 (m),1340 (m),1307 (m), 1259 (s), 1239 (m), 1185 (m), 1158 (m), 1117 (w), 1082 (w), 1024 (m), 1007 (m), 988 (m), 972 (m), 903 (m), 866 (m), 828 (s), 777 (m), 768 (m), 738 (s), 687 (s) cm1. 1H NMR (DMSO-d6, 400 MHz): d 8.14 (s, 1H), 7.48 (d, J = 8.0 Hz, 2H), 7.39 (d, J = 8.0 Hz, 2H), 7.19 (t, J = 8.0 Hz, 2H), 7.10 (t, J = 8.0 Hz, 1H), 7.01 (d, J = 8.0 Hz, 2H), 6.83– 6.70 (m, 2H), 6.64–6.61 (m, 2H), 6.46 (d, J = 8.0 Hz, 1H), 5.83 (d, J = 17.6 Hz, 1H), 5.26 (d, J = 10.8 Hz, 1H), 5.05 (s, 2H). 13C NMR (DMSO-d6, 100 MHz): d 158.9, 144.5, 142.8, 136.7, 136.3, 136.1, 129.7, 128.9, 127.6, 126.0, 119.7, 116.9, 114.2, 109.2, 105.9, 102.7, 68.6. MS (m/z, FAB+) 301.2 (15), 281.1 (15), 267.1 (7), 265.1 (5), 221.2 (20), 207.1 (20), 191.1 (10), 147.1 (30), 136.1 (20), 117.1 (30), 95.2 (100), 73.0 (80). HRMS (M+, FAB+) Calcd. C21H19NO 301.1467, found 301.1465. VB-DATA: N-Phenyl-3-(4-vinylbenzyloxy)aniline (2) (312 mg, 1.03 mmol), tris(4-bromophenyl)amine (161 mg, 0.33 mmol), palladium acetate (20 mg, 0.09 mmol), sodium tert-butoxide (288 mg, 3 mmol) were dissolved in 18 ml toluene and tert-butyl phospine (2 ml, 0.1 mmol, 0.05 M in toluene) was added. The reaction was heated at 75 °C under Argon for 24 h, quenched with water and filtered through Celite which subsequently washed with CH2Cl2. The filtrate was extracted with CH2Cl2 and partitioned using brine. The combined organic layer was dried over MgSO4 and concentrated. The residue was purified by column chromatography (SiO2:CH2Cl2/hexane, 1:1.2) to afford VB-DATA (249 mg, 66%) as a light yellow solid; IR (KBr) m 3085 (w), 3033 (w), 2863 (w), 1588 (m), 1500 (s), 1406 (w), 1377 (w), 1306 (m), 1264 (m), 1215 (m),

1146 (m), 1030 (w), 1015 (w), 990 (w), 909 (w), 825 (w), 753 (w), 695 (w), 618 (w) cm1. 1H NMR (DMSO-d6, 400 MHz): d 7.40 (d, J = 8.0 Hz, 6H), 7.31–7.24 (m, 12H), 7.15 (t, J = 8.4 Hz, 3H), 7.02–6.91 (m, 21H), 6.72–6.62 (m, 6H), 6.52–6.51 (m, 6H), 5.78 (d, J = 17.6 Hz, 3H), 5.22 (J = 10.8 Hz, 3H), 4.98 (s, 6H). 13C NMR (C6D6, 100 MHz): d 160.6, 150.0, 148.6, 143.9, 143.3, 137.7, 137.4, 137.3, 130.7, 130.0, 128.3, 127.0, 126.5, 125.6, 124.9, 123.3, 117.1, 114.2, 111.1, 109.9, 70.1. MS (m/z, FAB+) 1142.5 (50), 1026.5 (10), 766.4 (5), 616.3 (10), 391.4 (10), 307.2 (20), 289.2 (10), 154.1 (100), 136.1 (70), 57 (30). HRMS (M+, FAB+) Calcd. C81H66N4O3 1142.5135, found 1142.5133. 2.2. Measurement The oxidation potentials were determined by cyclic voltammetry (CV) in CH2Cl2 solution (1.0 mM) containing 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte at a scan rate of 100 mV s–1. A glassy carbon electrode or a thermal polymerized-VB-DATA coated ITO plate (2.25 cm2) which was washed with dichloromethane (CH2Cl2) prior as the working electrode and platinum wire were used as the working and counter electrodes, respectively. The ferrocene/ferrocenium redox couples occur at values of Eo0 of +0.46 V in CH2Cl2/TBAPF6. All potentials were recorded versus Ag/ AgCl (saturated) as a reference electrode. Absorption spectra were recorded with a U2800A spectrophotometer (Hitachi). AFM images were collected under ambient conditions on multiple sets of films using the

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1st scan 2nd scan

Heat Flow(a.u., Exo up)

o

75 C o

110 C

50

100

150

200

250

300

o

Temperature ( C)

2.3. Device fabrication

Fig. 1. DSC analysis of the thermally crosslinkable monomers VB-DATA before and after thermal polymerization.

VB-DATA before polymerization VB-DATA after polymerization m-MTDATA

0.15

0.8 Absorbance

Absorption(a.u.)

1.0

0.6 0.4

0.10 0.05 0.00

0.2 0.0 300

400

crosslink before rinsing crosslink after rinsing

300

400 500 600 Wavelength (nm)

500

600

Wavelength (nm) Fig. 2. UV–vis absorption spectra of thin films of m-MTDATA and VBDATA. Inset is the absorption spectra of crosslinked VB-DATA before washing and after washing by acetone.

Innova scanning probe microscope (Veeco Inc.) and Nanoscope Controller IIIa (Veeco Inc.). Silicon probes with spring constants of 5 N m1 and resonant frequencies of 75 kHz (Budget Sensors) were used for tapping mode AFM measurements. TOF samples were prepared by dissolving appropriate weight ratios (up to 25 wt.%) of the monomer VB-DATA in THF and then dip-coating the solutions onto an ITO substrate within a glove box. The film thicknesses were controlled by varying the solution concentrations and the dip-coating conditions; the films were subsequently dried through baking at 100 °C for 30 min to remove the residual solvent and then cured at 185 °C for 30 min to form the polymers. A Dektak surface profilometer was used to measure the thicknesses of the VB-DATA (ca. 2.5 lm) polymer film. The device samples were then completed through thermal deposition of the back electrode [Ag (100 nm)] through a shadow mask. For the TOF measurements, the samples were mounted in a cryostat under vacuum (ca. 103 torr). A sheet of charge carriers in the organic layer was generated by radiating a short excitation pulse (k = 337 nm, 10 Hz, 800 ps pulses) through the transparent

The OLEDs were fabricated on ITO sheets having a resistance of 15 X/h. The substrates were washed sequentially with isopropyl alcohol, acetone, and methanol in an ultrasonic bath, followed by UV-ozone treatment prior to use. One hole-injection layer (HIL) of poly(3,4-ethylenedioxythiophene):poly(4-stylenesulfonate) (PEDOT:PSS) was first spin-coated onto the ITO substrate to a thickness of 30 nm and then dried at 130 °C for 30 min to remove residual solvent. Another HIL used the monomer VB-DATA (0.5 wt.% in 1,2-dichloroethane) was spin-coated at 2000 rpm for 60 s onto the ITO substrate then thermally cured (70 °C for 30 min, and 185 °C for 40 min) in a N2-filled glovebox yielding a layer of 30 nm. After coating HIL, the monomer VB-FNPD (1.2 wt.% in THF) was dip-coated onto the substrate to form a 50-nm-thick HTL. The monomer VB-FNPD was baked at 100 °C for 30 min to remove residual solvent and then it was heated at 170 °C for 30 min to expedite polymerization. Finally, a 50-nm-thick EML of Alq3 was vacuum-deposited on top of the polymerized VB-FNPD at a deposition rate of 2 Å s1. Cathodes consisting of a 0.5-nm-thick layer of LiF followed by a 100-nmthick layer of Al were patterned using a shadow mask without breaking the vacuum. OLED device characterization was performed under glovebox using a computer-controlled Keithley 6430 source meter and a Keithley 6487 picoammeter equipped with a calibrated silicon photodetector. EL spectra were measured using a photodiode array

0.26 0.62

0.43

Current (a.u.)

o

175 C

ITO electrode. Under an applied DC voltage, these charge carriers sweep over the organic sample toward the counter electrode (Ag) and discharge to result in a transient photocurrent. When the carriers reached the counter electrode, the current dropped to zero; the time at which this event occurred corresponded to the transit time of the carriers. The photocurrent signal was detected using a digital storage oscilloscope. Selected carriers (holes or electrons) drifted across the sample upon switching the polarity of the applied bias. The carrier mobility (l) was calculated from the transit time (tT), the sample thickness (d), and the applied voltage (V) using the expression l = d2/tTV.

0.31

0.79 0.65

0.43 0.38

0.67 0.77

0.40

0.0

0.2

m-MTDATA VB-DATA (solu.) VB-DATA (Film) on ITO

0.4

0.69

0.6

0.8

1.0

1.2

Potential (V vs Ag/AgCl) Fig. 3. Cyclic voltammograms of m-MTDATA and VB-DATA in solution.

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Fig. 4. Film-forming propensities of VB-DATA monitored by optical microscopy. Films cast from (a, b) chlorobenzene and (c, d) 1,2-DCE solutions on ITO substrate before (a, c) and after (b, d) thermal curing.

Fig. 5. AFM topographic images of crosslinked VB-DATA films cast from (a) THF and (b) 1,2-dichloroethane (1,2-DCE) on ITO.

detector (Ocean Optics USB2000) over the spectral range from 200 to 850 nm at a resolution of 2 nm.

3. Results and discussion 3.1. Synthesis Scheme 1 displays the synthetic route of monomer VBDATA. The synthesis of VB-DATA started from the etherification of 3-bromophenol with 4-vinylbenzyl chloride with K2CO3 to furnish 1-bromo-3-(4-vinylbenzyloxy)benzene (1) in 95% yield. A two-step amination was subsequently adopted to yield VB-DATA. 1-Bromo-3-(4-vinylbenzyloxy)benzene (1) was first mono-aminated in the presence of a catalytic amount of Pd2(dba)3 to afford N-phenyl-3-(4vinylbenzyloxy)aniline (2) in 59% yield, which was subsequently subjected to a Pd-catalyzed amination with tris(4-bromophenyl)amine to give VB-DATA in 66% yield.

3.2. Physical properties The thermal property of the thermally crosslinkable VBDATA was determined by using differential scanning calorimeter (DSC). From the first scan DSC data (Fig. 1), VBDATA displayed the endothermic peaks at 75 and 110 °C. We assigned the first endothermic transition at 75 °C to be the glass transition temperature (Tg) of VB-DATA, which is similar to that of its parent compound m-MTDATA [15], whereas the second sharp endothermic peak at 110 °C to be the melting point (Tm) of VB-DATA. These assignments were confirmed by the observation of polarized optical microscope, in which there is no indication of liquid crystal property. Obviously, VB-DATA exhibits a broad exothermic peak centered at ca. 175 °C, which was assigned to the thermal polymerization temperature. The thermal crosslinking of VB-DATA led to a robust polymeric material which showed no significant phase change upon heating on the second run DSC analysis.

W.-Y. Hung et al. / Organic Electronics 13 (2012) 2508–2515

Fig. 6. (a) Nondispersive transient photocurrents of the polymerized VBDATA (thickness: 2.5 lm; E = 2.4  105 cm V1). (b) Hole mobility plotted as a function of the square root of the electric field.

Fig. 2 displays UV–vis absorption of the VB-DATA thin films before and after thermal polymerization. The absorption spectra of VB-DATA thin films were slightly blueshifted as compared to that of m-MTDATA. This result clearly revealed that the introduction of oxygen-containing substituent at the meta position of the peripheral phenylene ring of the parent core gave a larger bandgap due to the electron-withdrawing nature of the meta-substitution. Solvent resistance of the crosslinked VB-DATA film was investigated by monitoring UV–vis spectra. As shown in the inset of Fig. 2, the absorption intensity of the crosslinked VB-DATA remains intact before and after rinsing with acetone, indicating the sufficient solvent resistance has been generated. Electrochemical property of VB-DATA monomer was investigated using cyclic voltammetry (CV) (Fig. 3). VBDATA exhibited two reversible oxidation potentials, which were assigned to the oxidation of central triarylamine and peripheral amino center, respectively. As compared to those of m-MTDATA, the oxidation of VB-DATA were slightly shifted to higher potentials. This result is consistent with the observation of UV–vis electronic absorption spectra, where the meta-substitution gives wider bandgap by lowering down the HOMO level. The cyclic voltammetry of polymerized VB-DATA thin film coated on an ITO electrode (2.25 cm2) indicated the electrochemically reversible

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and stable character of VB-DATA after thermal polymerization. For a more practical evaluation of the frontier energy level, we used a Riken AC-2 photoemission spectrometer to determine the HOMO energy level of thermal-cured VB-DATA thin films. The ionization potential (5.1 eV) of thermal-cured VB-DATA thin films is close to the work function of ITO (4.9 eV), implies that it could serve as efficient hole injection material. Film smoothness is critical in the OLEDs applications. It is always desirable to obtain thin films with good homogeneity. Since the defects such as aggregation, pinhole and microcracks are always account for reducing lifetime and device performance. More importantly, hole injection materials (HIMs) should have good adhesion to the ITO substrate and provide a smooth surface for the subsequent depositing layers. However, for most of the small molecules used for solution process, there is no polymer-like chain entanglement. They are susceptible to crystallization and form rough films as the solvent dries out. Furthermore, the hydrophilic ITO substrate usually has poor surface energy matching with the hydrophobic aromatic materials [16], impeding the formation of smooth thin films. In order to overcome these problems and to obtain excellent crosslinked films via a wet process, the choice of solvent is therefore important. In the present study, the film-forming propensity of VB-DATA from different solvent systems were investigated and monitored by optical microscopy and atomic force microscopy (AFM). As shown in Fig. 4a and c, smooth films can be freshly deposited atop ITO substrate from both chlorobenzene and 1,2-dichloroethane (1,2-DCE) solutions. After thermal polymerization (100 °C for 30 min then 195 °C for 40 min) under nitrogen, uniform surface with no evidence of aggregations or pinholes was still observed in the deposited film (Fig. 4d) from 1,2-dichloroethane solution. In contrast, using chlorobenzene the film morphology changed dramatically in the deposited film, where dewetting was observed for the film after thermal polymerization (Fig. 4b). The volume shrinkage of materials indicated the poor adhesion to the ITO substrate when VB-DATA was deposited from chlorobenzene solution. The surface roughness of VB-DATA films after thermal curing was investigated using tapping mode AFM. Similar to chlorobenzene-deposited film, dewetting was observed again after the film was casted from THF solution and then thermally polymerized (Fig. 5a). In sharp contrast, thin film casted from 1,2-dichloroethane is very smooth with a RMS roughness of ca. 0.84 nm (Fig. 5b). Furthermore, the atomic force microscopy (AFM) images of the upper HTL (VBFNPD) layer on top of ITO/VB-DATA also showed that smooth and uniform films with no observed cracks or pinholes, and the RMS surface roughnesses is 0.67 nm. This indicates that the thermally crosslinked VB-DATA film can effectively smooth the rough surface of ITO substrate which is usually with a RMS roughness of ca. 3 nm [17,18]. This result can lead the OLED device to have reduced device current leakage and improved lifetime [19– 21]. To characterize the hole transport property of thermally polymerized VB-DATA films and to quantify the mobility, we used time-of-flight (TOF) techniques to measure the charge carrier mobility [22]. Fig. 6(a) displays the

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4000

2

HIL/ HTL VB-DATA/ NPB m-MTDATA/ NPB VB-DATA/ VB-FNPD PEDOT:PSS/ VB-FNPD

3

10

3000

1

2000

-1

1000

10

10

2

Current Density (mA/cm )

Brightness (cd/m )

(a) 105

0

-3

10

0

3

6

9

12

15

1.8

3.0

1.5

2.5

1.2

2.0

0.9

1.5 EL (a.u.)

CIE(0.32, 0.56)

0.6

1.0

0.3

0.5 400

500

600

700

Power Efficiency (lm/W)

(b) Quantum Efficiency (%)

Voltage(V)

800

Wavelength (nm)

0.0 0 10

1

2

10

10

3

4

10

10

0.0

5

10

2

Brightness (cd/m ) Fig. 7. (a) J–V–L characteristics and (b) external quantum and power efficiencies plotted with respect to the luminance for Alq3 devices (inset: EL spectrum).

units within the VB-DATA polymeric films, hindering the hopping of the charge carriers among adjacent molecules.

Table 1 Device performance. HIL/HTL

Von Lmax [V] [cd m2]

Imax EQEmax PEmax [mA cm2] [%] [lm W1]

VB-DATA/ NPB m-MTDATA/ NPB VB-DATA/ VB-FNPD PEDOT:PSS/ VB-FNPD

2.5 76,900 (14.5 V) 2.5 81,100 (13.5 V) 2 65,200 (12.5 V) 2.5 52,600 (12 V)

2950

1.6%

2.5

3400

1.48%

2.1

3760

1.34%

2.3

2600

1.4%

2.6

TOF transients of the VB-DATA polymer film. Distinctive plateaus are visible in the typical TOF transient photocurrents, indicating the non-dispersive transport behavior of the VB-DATA polymer toward holes. Fig. 6(b) displays the hole mobility of the thermally cured VB-DATA polymer films plotted as functions of the square root of the electric field, which exhibited hole mobilities in the range from 2  106 to 8  106 cm2 V1 s1 at electric fields. We attribute this decrease in hole mobility to the polystyrene

3.3. Device performance To assess the feasibility of using VB-DATA polymeric thin films as hole injecting layers (HILs), bilayer fluorescent OLEDs were first fabricated with a device structure involving ITO/HIL (30 nm)/NPB (40 nm)/Alq3 (60 nm)/LiF (0.5 nm)/Al (100 nm), in which the vapor deposited NPB works as a hole-transporting layer and Alq3 works as both an electron-transporting layer and an emitting layer. For comparison, the corresponding vapor deposited small molecule m-MTDATA was also used as an HIL. We find that device used crosslinked VB-DATA as HIL showed an external quantum efficiencies (EQE) of 1.6% which is higher than the device using the vacuum deposited small molecule m-MTDATA with EQE of 1.48%. The results suggest that the cross-linkable VB-DATA copolymer can work very well as a hole-injecting material for OLEDs application. Therefore we tested the potential use of the crosslinked copolymer in more performing solution-processed hybrid OLEDs in the device structure: ITO/HIL (30 nm)/VB-FNPD

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(50 nm)/Alq3 (50 nm)/LiF (0.5 nm)/Al (100 nm). In this work, VB-FNPD was introduced to serve as the thermally crosslinkable hole transport material to fabricate the double-crosslinked-HTLs OLEDs device. For comparison, the commercial PEDOT:PSS was also used as an HIL in its corresponding device architecture. As indicated in Fig. 7 and Table 1, the OLEDs fabricated using VB-DATA as the hole injection layer and VB-FNPD as the hole transport layer exhibited comparable device performances (1.34%, 2.3 lm W-1) to the device composed of conventional PEDOT:PSS (1.4%, 2.6 lm W-1). In other words, the newly developed crosslinkable VB-DATA can serve as good alternatives for PEDOT:PSS and gave us more material choices for future OLEDs applications. Inspired by this fascinating result, all solution-processed multilayer OLEDs via crosslinkable approach is possible to achieve. 4. Conclusions We have successfully developed a new crosslinkable HIL monomer VB-DATA derived from a famous HIM mMTDATA as core equipped with styryl moiety as thermally polymerizable functional groups. VB-DATA can polymerize at 190 °C to give homogeneous thin films with remarkable thermal and morphological stabilities. The thermally polymerized VB-DATA thin films performed well-aligned energy level with adjacent layers, as a consequence of reducing barrier for hole-injection. The replacement of PEDOT:PSS by thermally crosslinked VB-DATA thin films showed comparable OLEDs performance, making VB-DATA thin films as a potential substitution for conventional acidic HIL, PEDOT:PSS. Acknowledgment We greatly appreciate the financial support from National Science Council of Taiwan (NSC 98-2119-M-002007-MY3 and 100-2112-M-019-002-MY3).

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