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Synthetic Metals 161 (2011) 2422– 2426

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

j o ur nal homep ag e: www.elsev ier .com/ locate /synmet

rganic field effect transistors fabricated using a composite ofoly(9-vinylcarbazole) and pentacene precursor

ong-Su Kima, Jae Ho Kwona, Song Yun Chob, Changjin Leeb, Kwang-Sup Leea,∗, Tae-Dong Kima,∗

Department of Advanced Materials, Hannam University, Daejeon 305-811, Republic of KoreaKorea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea

r t i c l e i n f o

rticle history:eceived 14 June 2011eceived in revised form 7 September 2011ccepted 9 September 2011vailable online 2 October 2011

a b s t r a c t

Organic field effect transistors (OFETs) were fabricated using a solution processable composite of a pen-tacene precursor and poly(9-vinylcarbazole) (PVK) used for the combinatorial properties of its smallmolecules and polymers. The pentacene precursor was synthesized by a Diels–Alder reaction from pen-tacene with N-sulfinylbutylcarbamate in the presence of a catalytic amount of a palladium compound.A composite system of the pentacene precursor with PVK was investigated for OFET device perfor-

eywords:rganic field-effect transistorsentacene precursoroly(9-vinylcarbazole)

mance. The photophysical and thermal properties of the blending system were characterized, and thefilm morphology was investigated by using atomic force microscopy (AFM). A maximum hole mobility of2.2 × 10−2 cm2 V−1 s−1 was achieved with a threshold voltage of −4 V and on/off current ratio of 1.8 × 106.

ole mobilityolution processing

. Introduction

There has been growing interest in organic field-effect transis-ors (OFETs) because of their potential applications in a variety ofarge-area electronics, such as smart cards, electronic identificationags, flat-panel displays, and electronic papers. Notably, solutionrocessable organic semiconducting materials are of great inter-st for large area coverage, structural flexibility, low-temperaturerocessing, and low-cost applications since they allow for the usef spin-coating, spray-coating, screen printing, or ink-jet printing1–3].

To date, acene- and heteroacene-based organic semiconduc-ors, such as pentacene and fused oligothiophenes, have beenntensively investigated for OFET applications [4–6]. Among them,entacene and its derivatives have field effect mobilities over

cm2 V−1 s−1 [7]. However the main drawback for the pentacenes poor solubility that can hinder solution processing for OFETevices. Several attempts have been made to synthesize solubleentacene derivatives or pentacene precursors. Anthony and co-orkers reported a class of soluble triisopropylsilylethynyl (TIPS)

ubstituted pentacene derivatives [2]. Compared to the pentacene,he bulky TIPS substituents make the resulting pentacene deriva-

ives very soluble in common organic solvents and afford improvednvironmental stability. Another approach for synthesizing solu-ion processable pentacene has been attempted using a retro

∗ Corresponding authors. Tel.: +82 42 629 8855; fax: +82 42 629 8854.E-mail addresses: kslee@hnu.kr (K.-S. Lee), tdkim@hnu.kr (T.-D. Kim).

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.09.019

© 2011 Elsevier B.V. All rights reserved.

Diels–Alder reaction from a soluble pentacene precursor to forma thin film, followed by conversion to the insoluble pentacene formby heating at an elevated temperature [8]. Afzali and co-workersreported a highest field effect mobility of 0.8 cm2 V−1 s−1 affordedfrom the pentacene precursors which was comprised of hetero-Diels–Alder adducts of pentacene reacted with N-sulfinylacetamidein a high yield [9]. However, although these approaches have pro-vided enough solubility of the pentacene for solution processingwith high field effect mobilities, the small conjugated moleculesoften generate poor film quality due to high crystallinity and makelarge area deposition difficult due to their anisotropic morphol-ogy.

Blending small conjugated molecules with active polymersappears to be the most promising way not only to enhance OFETperformance but also to demonstrate excellent device uniformityfor large area printing [10]. Very recently, Anthopoulos and co-workers reported that acene-based semiconductors blended withan amorphous polymer, poly(triarylamine), can give carrier mobil-ities of greater than 2 cm2 V−1 s−1 in devices employing a top-gate,bottom-contact architecture [11].

Here we have demonstrated high performance p-type OFETsusing a pentacene precursor blended with poly(9-vinylcarbazole)(PVK) to control film morphology with high mobility. PVK was cho-sen due to its good hole transporting properties, being widely usedin polymeric light emitting diodes (PLEDs). It is noted that pre-

vious studies on blends of polyfluorene with PVK were shown toimprove color stability and device efficiency [12]. In this study wefound that the pentacene precursor blending with PVK improveddevice reliability without compromising charge mobility.

D.-S. Kim et al. / Synthetic Metals 161 (2011) 2422– 2426 2423

O N

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pent acene

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Fig. 1. Synthesis of a pentacene precursor, pentac

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. Experimental details

.1. Materials

Pentacene, n-butylcarbamate, and poly(9-vinylcarbazole)Mn = 25,000–50,000), were purchased from Aldrich Chemicalo. and were used as received. Dichloromethane and tolueneere dried by distillation over calcium hydride. All reactionsere carried out under an inert nitrogen atmosphere. A pen-

acene precursor, pentacene-N-sulfinyl-n-butylcarbamate adductpentacene-pre-A) was synthesized according to the methodsescribed in the literature [13].

.2. N-Sulfinylbutylcarbamate

Thionyl chloride (3.0 g, 0.025 mol) was added to the solution ofmidazole (6.8 g, 0.10 mol) in anhydrous dichloromethane (100 mL)t 10 ◦C. After stirring the solution for 30 min at room tempera-ure, the remaining solids were filtered off, and the filtrates wereransferred to a round flask charged under N2. To the solution addi-ional thionyl chloride (3.0 g, 0.025 mol) was added and stirred for0 min to give a light yellow solution of N-chlorosulfinylimidazole.he resulting solution was slowly added into the solution of n-utylcarbamate (4.7 g, 0.04 mol) in anhydrous dichloromethane100 mL) and was stirred for 1 h at room temperature. The pre-ipitate was filtered off and filtrates were evaporated on a rotaryvaporator and the oily residue was distilled under high vacuumhile the receiver flask was kept at −78 ◦C. The product was col-

ected as a colorless liquid (b.p. = 40–43 ◦C at 0.15 mmHg). 1H NMRCDCl3, ppm): 0.95 (t, j = 8 Hz, 3H), 1.42 (m, 2H), 1.72 (m, 2H), 4.32t, j = 7 Hz, 2H). 13C NMR (CDCl3, ppm): 14, 19, 31, 69, 152. GC MS:/z = 163.

ene-pre-A, and a chemical structure of PVK.

2.3. Pentacene-pre-A

Cationic palladium catalyst [Pd (dppp)(PhCN)2](BF4)2 wasadded to the mixture of pentacene (0.10 g, 0.36 mmol) andN-sulfinylbutylcarbamate (0.12 g, 0.72 mmol) in anhydrous chlo-roform (15 mL) and the mixture was refluxed for 2 h under N2.The solvent was evaporated under reduced pressure and the solidresidue was purified by flash chromatography eluented with 1:1of hexane and dichloromethane. 1H NMR (CDCl3, ppm): 0.93 (t,j = 8 Hz, 3H), 1.35 (m, 2H), 1.62 (m, 2H), 4.21 (t, j = 8 Hz, 2H), 5.9 (s,1H), 6.7 (s, 1H), 7.5 (m, 4H), 7.85 (m, 4H), 8.02 (s, 2H), 8.07 (s, 2H).13C NMR (CDCl3, ppm): 14, 19, 31, 65, 68, 70, 123, 124, 127, 128,129, 130, 132, 133, 134, 138, 158. GC MS: m/z = 443.

2.4. Characterization

1H and 13C NMR spectra (300 MHz) were taken on a Varian300 spectrometer and UV/vis spectra were obtained on a Perkin-Elmer spectrophotometer. Differential scanning calorimetry (DSC)and thermogravimetric analysis (TGA) were performed on a TAinstruments Q50 at a ramping rate of 10 ◦C/min under a nitrogenatmosphere. Out-of-plane XRD (Cu K� radiation source, ShimadzuCorp.) was used to characterize the film structures of the pen-tacene precursor. Atomic force microscopy (AFM) was performedusing a Digital Instruments Nanoscope IV operated in tapping mode(∼350 kHz frequency, Si tip).

2.5. Device fabrication and measurement

Bottom-contact transistors were fabricated on a common gateof heavily doped p-type silicon wafers covered with a 3000 A-thick SiO2 (a capacitance per unit area of 10 nF/cm2). Bilayerbottom-contact electrodes consisting of Cr/Au (5 nm/80 nm) wereevaporated under high vacuum (10−6 mbar) through a shadowmask. The channel length (L) and width (W) of the transistorswere 50 �m and 3 mm, respectively. The substrates were thentreated with hexamethyldisilazane (HMDS) to produce hydropho-bic dielectric surfaces. Binary blends of pentacene-pre-A withPVK (4:1 and 3:1 weight ratios) were prepared by mixing with2.0 wt% solid contents in dried toluene. Spin-coating was carriedout at 1000 rpm for 30 s and was followed by drying at 160 ◦Cfor 30 min in nitrogen to remove N-sulfinylbutylcarbamate byretro-Diels–Alder reaction. All the OFET devices were encapsu-lated by a glass can and getters in an inert argon environmentinside a glove box system. Electrical measurements were per-formed at RT under an argon atmosphere using an HP4156C

semiconductor parameter analyzer. Parameters such as mobil-ity, Ion/Ioff ratio, and threshold voltage were calculated based onstandard semiconductor FET equations in the saturation-currentregime.

2424 D.-S. Kim et al. / Synthetic Metals 161 (2011) 2422– 2426

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Before annealing (Pentacene-pre-A)After annealing (Pentacene-pre-A with PVK (4:1))After annealing (Pentacene-pre-A)

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ig. 3. UV/vis spectra of a binary blend (4:1 weight ratio of pentacene-pre-A withVK) in a thin film before and after annealing at 160 ◦C.

. Results and discussion

The synthesis of pentacene-pre-A is shown in Fig. 1.-sulfinylbutylcarbamate was prepared by reaction of n-utylcarbamate and N-chlorosulfinylimidazole and the productas purified by vacuum distillation. The Diels–Alder reaction ofentacene with N-sulfinylbutylcarbamate was catalyzed with thealladium compound [Pd(dppp)(PhCN)2](BF4)2. The adduct ofentacene-pre-A is easily purified by flash column chromatogra-hy on silica gel with an eluent of dichloromethane and hexane1:1). The structure and purity of pentacene-pre-A have beenerified by 1H NMR, UV/vis and mass spectroscopy, and thermalnalysis.

The pentacene-pre-A is soluble in common organic solventsuch as acetone, ethyl acetate, tetrahydrofuran, toluene, and chlo-inated solvents. The thermal properties of pentacene-pre-A were

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ig. 5. ID–VD characteristics ((a) and (c)) and ID–VG (open circle) and I1/2D − VG (closed circ

f pentacene-pre-A with PVK), respectively.

Fig. 4. XRD profiles of pentacene precursor thin films before and after annealing at160 ◦C.

determined by differential scanning calorimetry (DSC) and ther-mogravimetric analysis (TGA) in a nitrogen atmosphere at a rateof heating of 10 ◦C/min. Upon heating, one endothermic peak at160 ◦C corresponding to the loss of N-sulfinylbutylcarbamate wasobserved for pentacene-pre-A by DSC. The initial decompositiontemperature (defined as 25% weight loss) at 160 ◦C, which wasmeasured by TGA, was also indicated by a clear retro-Diels–Alderreaction to form pentacene molecules (Fig. 2).

The absorption spectra of a thin film for the binary blend ofpentacene-pre-A with PVK (4:1 weight ratio) are shown in Fig. 3.Before heating, the film has absorptions at 329 and 345 nm anda shoulder at 370 nm corresponding to the intrinsic absorption

of PVK. After heating the thin film at 160 ◦C for 5 min in air, sev-eral peaks between 550 and 700 nm appear which are identical tothe absorption resonances of pentacene. There is slightly reduced

-100-80-60-40-2002040-11

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Mobility: 2.2 X 10Threshold Voltage: 4.0On/Off Ratio: 1.8 X 10V : -40 V

Gate Voltage (V)

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urrent (A1/2)

(d)

-10-20-30-40-50-60-70-80-90-10001020304050-13

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Mobility : 3.6 X 10Threshold voltage : -1.7On/Off Ratio : 8.4X10V : -40 V

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Drain C

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le) plots ((b) and (d)) of pentacene-pre-A only and a binary blend (4:1 weight ratio

D.-S. Kim et al. / Synthetic Metals 161 (2011) 2422– 2426 2425

Table 1Comparison of OTFT performance between pentacene-pre-A only and binary blends of pentacene-pre-A with PVK.

Materials Pentacene-pre-Aonly

Pentacene-pre-Awith PVK (4:1)

Pentacene-pre-Awith PVK (3:1)

Mobility ( cm2 V−1 s−1) 3.6 × 10−2 2.2 × 10−2 –Threshold voltage (V) −1.7 −4.0 –On/off ratio 8.4 × 106 1.8 × 106 –

F nneaa ) after

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ig. 6. AFM height images of pentacene-pre-A only system (a) before and (d) after annealing; 3:1 weight ratio systems of pentacene-pre-A with PVK (c) before and (f

bsorption intensity after annealing at 160 ◦C for 30 min. It wasound that the blending of pentacene-pre-A and PVK with 3:1eight ratio showed similar absorption behavior.

Fig. 4 shows the XRD profiles of spin-coated thin films forentacene-pre-A only and the binary blend of pentacene-pre-Aith PVK (4:1 weight ratio). Before annealing, the films were amor-hous without any diffraction peaks. However, after the annealingt 160 ◦C for 30 min, the (0 0 1) peak clearly appeared at 6.1◦ corre-ponding to the d0 0 1-spacing of 14.5 A for pentacene-pre-A. Thateans the pentacene precursor in the films converted back to pen-

acene molecules during the annealing process and aggregated intoicrocrystals at the same time. For the binary blend of pentacene-

re-A with PVK, the (0 0 1) peak is around 6.2◦ which corresponds

o d0 0 1-spacing of 14.2 A.

A typical plot of drain current ID versus drain voltage VDt various gate voltages VG obtained from an OTFT based onhe pentacene-pre-A only system and the binary blend of

ling; 4:1 weight ratio systems of pentacene-pre-A with PVK (b) before and (e) after annealing.

pentacene-pre-A with PVK (4:1) is shown in Fig. 5(a) and (c).The device annealed at 160 ◦C for 30 min showed good p-channelcharacteristics. Also, the transfer characteristics at VD = −40 V areshown in Fig. 5(b) and (d). The on/off current ratios (Ion/Ioff) for thepentacene-pre-A only and the blend are 8.4 × 106 and 1.8 × 106,respectively. The field-effect mobility (�) and the threshold voltage(VT) were estimated from the square root of the drain current–gatevoltage (I1/2

D − VG) plots of Fig. 5(b) and (d), according to the stan-dard equation in the saturation regime, ID = (W/2L)�Ci(VG − VT),where ID is the drain current, W and L are the conduction chan-nel width and length, respectively, Ci is the capacitance per unitarea of gate dielectric, and VG is the gate voltage. The pentacene-pre-A only system showed the mobility of 3.6 × 10−2 cm2 V−1 s−1.

In the case of the binary blend of pentacene-pre-A with PVK(4:1), � and VT were 2.2 × 10−2 cm2 V−1 s−1 and 4.0 V, respectively.However the devices from the blending of pentacene-pre-A andPVK with 3:1 weight ratio exhibited negligible mobility less than

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[10] A. Babel, J.D. Wind, S.A. Jenekhe, Adv. Funct. Mater. 14 (2004) 891.

426 D.-S. Kim et al. / Synthetic

0−6 cm2 V−1 s−1. This behavior is attributed to uniformity andomogeneity of pentacene-pre-A crystallization in the PVK poly-er.The film morphologies of pentacene-pre-A only and binary

lends of pentacene-pre-A with PVK were investigated using AFMn standard tapping mode. Fig. 5 shows AFM images (5 × 5 �m2)efore and after annealing. Fig. 6(a)–(c) is height images ofentacene-pre-A only, blends of pentacene-pre-A with PVK (4:1nd 3:1), respectively, before annealing. The films exhibit uniformnd smooth surfaces having rms roughnesses of 10–20 nm andaximum heights of 41 nm. Notably, the blended samples haveore uniform surfaces. After annealing the film of pentacene-

re-A only at 160 ◦C for 30 min, circular cluster features with anverage size of 150 nm, rms roughness of 210 nm, and a heightf 300 nm were seen distributed throughout the image (Fig. 6(d)),ndicating a crystallite of pentacene molecules formed by retro-iels–Alder reaction. As for blending pentacene-pre-A with PVK

4:1), there is a slight decrease in the cluster size and more uni-orm cluster shape (Fig. 6(e)). Furthermore, rms roughness of thelend system was ∼120 nm which is almost half the value of theentacene-pre-A only system. Interestingly, the pentacene clus-ers form discontinuous islands in the blend of pentacene-pre-And PVK with 3:1 weight ratio (Fig. 6(f)), which prevent pentaceneolecules forming highly ordered crystals. This result indicates

hat the presence of PVK in the pentacene-pre-A can help improv-ng film-forming property for OTFT devices, while excessive PVKn the pentacene-pre-A can disturb molecular ordering resulted inoor OTFT performance.

Table 1 shows comparison of OTFT performance betweenentacene-pre-A only and binary blends of pentacene-pre-A withVK (4:1 and 3:1). OTFT devices from the pentacene-pre-A onlyystem have better performance showing a maximum mobility of.6 × 10−2 cm2 V−1 s−1, threshold voltage of −1.7 V, and Ion/Ioff of.4 × 106 compared with the blending system. However the resultsere varied depending on film quality and uniformity. After thelms were annealed at 160 ◦C for 30 min, only a few devices wereeasurable for OTFT performances due to the damage to the films

aused by the high crystallization of the pentacene molecules. Inhe case of the binary blend of pentacene-pre-A and PVK with:1 weight ratio, highly reliable and constant mobilities ranged in0−2 cm2 V−1 s−1 were found in the OTFT devices.

[

[[

ls 161 (2011) 2422– 2426

4. Conclusion

We have demonstrated p-channel OTFTs based on a binaryblend system using a pentacene precursor, pentacene-pre-A, withPVK. The devices were fabricated using a spin coating method andexhibited good OTFT performance with a highest electron mobil-ity of 2.2 × 10−2 cm2 V−1 s−1. Ion/Ioff ratio and threshold voltagewere 1.8 × 106 and 4.0 V, respectively. Since the binary blend ofa small molecule and polymer has merits, such as facile processing,film uniformity, and device reliability, it will be further utilized fordeveloping organic semiconductors in flexible electronic applica-tions.

Acknowledgements

This work was supported by Korea Research Council IndustrialScience & Technology (SK-0903-01). The part of the work for K.-S. LEE was supported by Mid-career Researcher Program throughNRF grant funded by the MEST (No. 2010-0000499) and the 2011Research Program by Hannam University.

References

[1] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W.Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P. Herwig, D.M. deLeeuw, Nature 401 (1999) 685.

[2] J.E. Anthony, J.S. Brooks, D.L. Eaton, S.R. Parkin, J. Am. Chem. Soc. 123 (2001)9482.

[3] E.Y. Park, J.S. Park, T.-D. Kim, K.-S. Lee, H.S. Lim, J.S. Lim, C. Lee, Org. Electron.10 (2009) 1028.

[4] K. Xiao, Y. Liu, T. Qi, W. Zhang, F. Wang, J. Gao, W. Qiu, Y. Ma, G. Cui, S. Chen, X.Zhan, G. Yu, J. Qin, W. Hu, D. Zhu, J. Am. Chem. Soc. 127 (2005) 13281.

[5] T. Okamoto, Z. Bao, J. Am. Chem. Soc. 129 (2007) 10309.[6] K.P. Weidkamp, A. Afzali, R.M. Tromp, R.J. Hamers, J. Am. Chem. Soc. 126 (2004)

12740.[7] L. Yen-Yi, D.I. Gundlach, S.F. Nelson, T.N. Jackson, IEEE Trans. Electron. Dev. 44

(1997) 1325.[8] P.T. Herwig, K. Mullen, Adv. Mater. 11 (1999) 480.[9] A. Afzali, C.D. Dimitrakopoulos, T.L. Breen, J. Am. Chem. Soc. 124 (2002)

8812.

11] R. Hamilton, J. Smith, S. Ogier, M. Heeney, J.E. Anthony, I. McCulloch, J. Veres,D.D.C. Bradley, T.D. Anthopoulos, Adv. Mater. 21 (2009) 1166.

12] G. Bernardo, A. Charas, J. Morgado, J. Phys. Chem. Solids 71 (2010) 340.13] A. Afzali, C.R. Kagan, G.P. Traub, Syn. Metal 155 (2005) 490.