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Synergistic effect of polymer and oligomer blends for solution-processable organic thin-film transistors
Organic Electronics 9 (2008) 952–958
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Synergistic effect of polymer and oligomer blends for solution-processable organic thin-film transistors Eunhee Lim a,d, Byung-Jun Jung a, Masayuki Chikamatsu b, Reiko Azumi b, Kiyoshi Yase b, Lee-Mi Do c, Hong-Ku Shim a,* a
Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea b Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Electronics and Telecommunications Research Institute (ETRI), 161 Kajeong-dong, Yuseong-gu, Daejeon 305-350, Republic of Korea d Korea Institute of Industrial Technology (KITECH), 35-3 Hongchon-ri, Ibjangmyun, Chonan-si, Chungnam 330-825, Republic of Korea
a r t i c l e
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Article history: Received 10 September 2007 Accepted 28 June 2008 Available online 8 July 2008
PACS: 85.30-z 68.55.J31.15 82.35.Cd
a b s t r a c t The thin-film morphologies and thin-film transistor (TFT) characteristics of a series of binary blends of poly(9,90 -dioctylfluorene-alt-bithiophene) (F8T2) and a,x-dihexylquarterthiophene (DH4T) are reported. The blends of F8T2 and DH4T exhibit good solubility and produce TFT devices with better performances than F8T2 and DH4T devices. The 50% DH4T blend device was found to have a hole mobility of 0.011 cm 2 V1 s1, which is four times higher than the mobility of the F8T2 device, with a high-on/off ratio of about 105 and a low-off current of 17 pA. The polymer and oligomer domains are phase-separated with large domain size and arranged in characteristic molecular alignments. It was found that carrier transport in the blend systems is mainly controlled by the polymer component, and that the nature of the blended oligomer affects the OTFT performance of the blends. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Thin-film transistor Charge carrier mobility Blend Polyfluorene Thiophene
1. Introduction Interest in organic thin-film transistors (TFTs) has increased because of their various advantages over inorganic amorphous silicon TFTs, including facile processability, chemical tunability, compatibility with plastic substrates, and low-cost processing. To minimize manufacturing costs, the TFT fabrication process should ideally include solution-based methods such as spin-coating, drop-casting, ink-jet printing and screen-printing. The use of soluble * Corresponding author. Tel.: +82 42 350 2827; fax: +82 42 350 2810. E-mail address: [email protected] (H.-K. Shim). 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.06.018
polymeric semiconductors has made possible the development of active-matrix multipixel displays using solutionbased technology. Most p-channel solution-processable organic semiconductors fabricated to date have been based on thiophenebased polymers such as poly(3-hexylthiophene) (P3HT) [1]. However, because of their relatively low-band gaps and high highest occupied molecular orbital (HOMO) levels, thiophene-based polymers tend to be easily oxidized, which results in their degradation, and their low-on/off ratio and high-off current in OTFTs restricts their use in display applications. In attempts to solve these problems, the design and synthesis of organic semiconductors with a
E. Lim et al. / Organic Electronics 9 (2008) 952–958
high-charge carrier mobility and good stability has become a focus of research into organic electronic materials [2–4]. Fluorene-based copolymers have recently emerged as promising materials for polymer TFTs [5–7]. Fluorenebased polymers such as poly(9,90 -dioctylfluorene-alt-bithiophene) (F8T2) have better stability than thiophene-based polymers because of their rigid structures and lower HOMO levels, as well as good film-forming and hole-transporting properties. On the other hand, organic small molecules have also been introduced as active layers in solution-processed OTFTs, through the introduction of various alkyl side chains or by using precursor systems containing thermally removable solubilizing groups. Some of these compounds have exhibited promising behavior as solution-deposited semiconductors with hole mobilities on the order of 0.1 cm2 V1 s1, however, the number of organic small molecules with good solubility and TFT performance is limited so far [8–10]. The use of blends of p-conjugated polymers has proven effective in improving the electronic and optoelectronic properties of OLEDs and photovoltaic devices [11,12]. Recently, blends and other multicomponent systems are also introduced in solution-processable OTFT applications for tuning and improving the properties of TFTs. For example, solution-processed blends of an n-type polymer, poly(benzobisimidazobenzophenanthroline) (BBL) and a p-type small molecule, copper phthalocyanine resulted in ambipolar thin-film transistors that transport both holes and electrons [13]. The incorporation of polyethylene into regioregular P3HT yielded mechanically robust, high-performance TFTs, owing to a highly favourable, crystallization-induced phase segregation of the two components [14]. Moreover, solution-processed rubrene based transistor exhibited saturated mobilities of up to 0.7 cm2 V1 s1 and on/off ratios of over 106 by incorporating a glass-inducing diluent, diphenylanthracene [15]. Therefore, blending two or more electrically active materials can offer multiple advantages into active semiconductor for realizing solution-processable, highperformance OTFTs. Here, we report the synergistic effect of the blend systems consisting of fluorene–thiophene based copolymer F8T2 and thiophene-based oligomer DH4T in OTFTs. Despite of its high-on/off ratio and low-off current, on the order of pA, F8T2 is known to be unsuitable for real applications because of the relatively low-charge carrier mobility on the order of 103 cm2 V1 s1, without the use of additional alignment techniques. Thus our approach is to incorporate the polymer/oligomer blend system into the fluorene-based polymer semiconductor with the aim of improving the hole mobility of this class of polymers while preserving its high-on/off ratio. The morphologies and charge carrier mobilities of a series of binary blends of polymeric and oligomeric semiconductors are investigated. Moreover, comparison of these characteristics with those of the polyfluorene homopolymer and terfluorene oligomer devices enables us to further investigate the charge carrier transport in the blend devices. These results demonstrate that the use of polymer/oligomer blend systems is an effective way to increase the performance of solution-processable OTFTs.
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2. Experimental section 2.1. Materials The fluorene–thiophene based copolymer, F8T2, and the thiophene-based oligomer, DH4T, were prepared with methods described in a previous report, through the Suzuki coupling reaction [16]. Polyfluorene homopolymer, PF8, and the fluorene-based oligomer, 3FL, were also prepared for comparison. The number-average molecular weight (Mn) of F8T2 was found to be 38,000. F8T2 was found to exhibit a crystallization peak at 159 °C and a melting endothermal peak at 268 °C. 2.2. Physical measurements The number-average molecular weight was determined by gel permeation chromatography (GPC) on a Waters GPC-150C instrument calibrated with polystyrene standards, with THF used as the eluent. Differential scanning calorimetry (DSC) was performed under a nitrogen atmosphere at a heating rate of 10 °C min1. UV–vis spectra were obtained using an MPS2000 (SHIMADZU) UV/vis spectrometer. The morphological characterizations with XRD and AFM were performed on the same films as used in the OTFT measurements. The XRD measurements were carried out with a Rigaku Denki RU-300 using Cu Ka radiation (40 kV, 200 mA) in the h–2h scan mode with 0.01° steps in 2h and 0.6 s per step. Non-contact mode (Dynamic Force Mode) AFM images were recorded using a Seiko Instruments SPA-300/SPI3800 probe system, equipped with a Si cantilever (Seiko Instruments SII-DF20, force constant 15 N m1, resonance frequency 130 kHz, and tip curvature radius = 10 nm). 2.3. Fabrication of the TFT devices TFTs were fabricated using the bottom contact geometry. Gold was used for the source and drain contacts and silicon oxide (SiO2) with a thickness of 300 nm was used as the dielectric. The SiO2 surface was cleaned with UV– ozone treatment (UV irradiation for 20 min in an oxygen atmosphere) and pretreated with hexamethyldisilazane (HMDS) to produce apolar and smooth surfaces onto which the polymer could be spin-coated. The polymer and/or oligomer solutions were dissolved in 0.5 wt.% o-dichlorobenzene, and filtered through a 0.45 lm pore size polytetrafluoroethylene (PTFE) membrane syringe filter before use. The polymer solutions were applied dropwise onto the substrates and spin-coated at 1500 rpm. The films were dried on a hot plate at 150 °C for 20 min in a glovebox. The electrical characteristics of the OTFT devices were measured in a glovebox using an Agilent 4155C Semiconductor Parameter Analyzer. 3. Results and discussion 3.1. Optical properties of the F8T2/DH4T blends The chemical structures of all the semiconducting materials used in this study are shown in Fig. 1. The UV–
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Fig. 1. Chemical structures of the semiconductors.
Fig. 2. UV–vis absorption spectra of the F8T2, DH4T, and F8T2/DH4T blend films. Fig. 3. XRD patterns of the F8T2, DH4T, and F8T2/DH4T blend films.
vis absorption spectra of the F8T2/DH4T blend films are shown in Fig. 2 with those of the F8T2 and DH4T films. The absorption peaks of F8T2 are at 210, 460 and 485 nm and those of the DH4T film are at 220, 268, and 355 nm. The absorption peaks of the blend films can be understood as the superpositions of those of the F8T2 and DH4T films. The intensities of the absorption peaks vary with the blend ratios. As more DH4T is incorporated, the intensities of the absorption peaks originating from F8T2 and DH4T decrease and increase, respectively. 3.2. Morphological characteristics of the F8T2/DH4T blend films Crystallization of the polymer and oligomer was confirmed with film X-ray diffraction (XRD), which enabled the identification of the nature of the mesophase. As shown in Fig. 3, the DH4T film produces multiple (h 0 0) reflections with a (1 0 0) reflection at 2h = 3.1°, indicating a d spacing of 28.5 Å, which is comparable to the molecular length of DH4T calculated using MOPAC-PM3 (31.0 Å). In other words, the molecules in the spin-coated DH4T film are oriented normal to the surface, which is a similar result to that reported previously for a vacuum-deposited DH4T film with a d spacing of 27.8 Å [17]. Interestingly, blend
films containing 50% or more DH4T also exhibit the peak at 2h = 3.1° as shoulder. This means that the vertical alignment of the D4HT molecules is somewhat maintained in 50% and 75% DH4T blend films. As more DH4T was incorporated, the intensity of the reflection at 3.1° was found to increase. In contrast, there is no peak for the 25% DH4T blend film in the low-angle region of the XRD pattern, which indicates that the minor DH4T phase in the 25% DH4T blend film is not aligned as in the 50% and 75% DH4T films, and so might act as an impurity disturbing the alignment of F8T2. This result is consistent with the relatively low-thin-film mobility of the 25% DH4T film, even though its domain size is comparable to that of the 50% DH4T film. The morphologies of the F8T2/DH4T blend films were also investigated using atomic force microscopy (AFM). As shown in Fig. 4, the morphologies of the blend films are clearly different to those of the polymer and oligomer films. Whereas the F8T2 and DH4T films have a smooth and homogeneous surface morphology, two distinct phases can be seen in the blend films. During the evaporation process, the oligomer and polymer components separate due to differences in solubilities as the solution is concentrated [18]. The bright features in Fig. 4b–d
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Fig. 4. AFM images of the (a) F8T2 film, (b) 25% DH4T, (c) 50% DH4T and (d) 75% DH4T blend films and (e) DH4T film (10 lm 10 lm). The scale bar represents 2 lm.
decrease with increasing DH4T content in the blends, indicating that the brighter regions correspond to the F8T2 polymer phase. The surface morphologies of the 75% and 50% F8T2 films are similar with those previously reported for P3HT and dibenzotetrathiafulvalene (DBTTF) blends, in which the film surfaces became rough and branching domains such as dendrite and seaweed appeared by blending [19]. The morphologies of polymer/oligomer blends are comparable to those of two polymer blends. Binary blends of P3HT with poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPH) and with polystyrene formed spherical domains or a bicontinuous network on
the length scale of 100–600 nm depending on the blend composition [20]. 3.3. Transistor properties of the F8T2/DH4T blends The TFT devices of DH4T/F8T2 blend films were fabricated and compared with those of DH4T and F8T2 films. The output characteristics of the devices at different gate voltages are shown in Fig. 5. The devices of the blend films showed typical p-type behaviors with clear saturation currents. A contact resistance, likely due to the relatively low solubility of DH4T, was however observed at low source-
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Fig. 5. Output characteristics of the (a) F8T2 film and (b) 50% DH4T blend film (channel width W = 100 lm and length L = 10 lm).
Fig. 6. Plots of the transfer curves for the F8T2 and F8T2/DH4T films at constant VD = 80 V; (a) ID versus VG (inset: semilogarithmic plot) and (b) (ID)1/2 versus VG (channel width W = 100 lm and length L = 10 lm).
drain voltages. Plots of the transfer curve [i.e., ID = f(VG)] at constant VD = 80 V are shown in Fig. 6. The field-effect mobilities were calculated in the saturation regime at VD = 80 V using the conventional TFT equation proposed by Horowitz for saturation regimes [21]. The TFT fabricated with F8T2 under the same conditions as used for the blend devices was found to have a hole mobility in the range 0.001–0.003 cm2 V1 s1 with an on/off ratio of the order of 105, which is in close agreement with the reported hole mobilities of F8T2 devices fabricated under similar conditions [5]. On the other hand, as expected, the field-effect mobility of the TFT device fabricated with a spin-coated DH4T film could not be measured. This result can be compared with that of our previous report for a vacuum-deposited DH4T film, which found a
hole-mobility of 0.02 cm2 V1 s1 using the same DH4T batch and bottom-contact device as used in the blend devices [17]. The absence of TFT characteristics for the spincoated DH4T film can be explained by the poor quality of the film, which results from DH4T’s low solubility. Blending the oligomer with a polymer can improve the low solubility of the oligomer as well as the relatively poor TFT performance of the polymer. The compositional dependence of the hole mobility of the F8T2/DH4T blend films is shown in Fig. 7. The 50% and 75% DH4T blend films produce better TFT performances than the F8T2 and DH4T films. The hole mobilities of the 50% and 75% DH4T blends were found to be 0.011 cm2 V1 s1, which is four times higher than that of the F8T2 polymer film. Three channel widths (W) and
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3.4. Comparative study
Fig. 7. Dependence of the hole mobility of F8T2/DH4T blend devices on the DH4T concentration.
lengths (L) [W(lm)/L(lm) = 100/10, 50/5 and 25/5] were used in one device: only slight variation of the hole mobilities with the W/L ratio was observed, resulting in the same dependence of the hole mobility on DH4T composition. As shown in Fig. 6, the incorporation of the oligomer into the polymer causes an increase in the on current without much increase in the off current. The 50% and 75% DH4T blend films exhibit on-currents of up to 1.5 lA, which are two and a half times higher than that of the F8T2 film. At the same time, the off-currents of the 25% and 50% DH4T blend films were still in the range 10–20 pA, which is apparently lower than that of the other thiophene-based polymers (e.g., >100 pA for stable polythiophene semiconductors incorporating thieno[2,3-b]thiophene [22]). As a result, the F8T2/DH4T blend devices have on/off ratios in the range 104–105, and in particular the on/off ratio of the 50% DH4T blend film device (0.9 105) was found to be close to that of the F8T2 film device. The optimal blend ratio for our polymer/oligomer blend system was found to be 50/50 with a hole mobility of 0.011 cm2 V1 s1, a highon/off ratio of about 105 and a low-off current of 17 pA. The TFT characteristics of the blend films are summarized in Table 1. The improvements in the TFT characteristics that result from the incorporation of 50% or 75% oligomer were also observed with other thiophene-based oligomers, T2TT and T2FL. (The chemical structures of T2TT and T2FL are shown in Ref. [17].)
At this point the question arises as to which mechanism predominantly controls the TFT characteristics of the polymer/oligomer blends. To assess the roles of the polymer and the oligomer in the blend films, polyfluorene homopolymer, poly(9,90 -dioctylfluorene) (PF8) and the fluorene-based oligomer, 9,9,90 ,90 ,900 ,900 -dihexyl-[2,20 ;70 , 300 ]terfluorene (3FL) were used in blends instead of F8T2 and DH4T, respectively. The TFT devices fabricated with only PF8 or 3FL were found to have no TFT characteristics, because of their relatively low-HOMO levels, which result in difficult hole injection from the Au electrode. This result is consistent with those of previous reports, which found a high barrier to charge injection from the Au electrode to PF8. Although PF8 is a liquid crystalline polymer that tends to be highly ordered in thin films, PF8 devices are known to have no TFT characteristics [23]. Films of PF8/DH4T and F8T2/3FL blends were fabricated under the same conditions as used for the F8T2/DH4T blends. While all the PF8/DH4T blend films were found to exhibit no TFT characteristics, the F8T2/3FL blend films were found to exhibit TFT characteristics with hole mobilities of 0.001–0.002 cm2 V1 s1. This result indicates that the polymer component controls the TFT characteristics
Fig. 8. Dependence of the hole mobility of F8T2/3FL blend devices on the 3FL concentration.
Table 1 TFT characteristics of the polymer/oligomer blend films (channel width W = 100 lm and length L = 10 lm)a Blend ratios (polymer:oligomer, wt.%)
F8T2:DH4T
l/cm2 V1 s1
F8T2:3FL PF8:DH4T PF8:3FL
Ion/Ioff Ioff/pA Ion/lA l/cm2 V1 s1 l/cm2 V1 s1 l/cm2 V1 s1
a b
100:0
75:25
50:50
25:75
0:100
0.0026 1.3 105 5.2 0.67 0.0026 –b –b
0.0011 0.3 105 20.3 0.62 0.0023 –b –b
0.011 0.9 105 17.4 1.6 0.0010 –b –b
0.011 0.3 105 49.1 1.5 0.0007 –b –b
–b –b –b –b –b –b –b
Determined from the transfer characteristics at VD = 80 V. The mobilities (l) were calculated in the saturation regime. No TFT characteristics was observed.
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of the polymer/oligomer blend system, which is consistent with a previous report, in which the transport in such a blend system was analyzed using percolation theory and found to be dominated by the performance of the polymer [24]. On the other hand, it is worth noting that the TFT performances of the F8T2/DH4T and F8T2/3FL blend films were found to be different. The dependence of the hole mobility of the F8T2/3FL blend devices on the 3FL concentration is shown in Fig. 8. Whereas the hole mobility of the 50:50 F8T2/DH4T blend film was found to be four times higher than that of the F8T2 film, that of the 50:50 F8T2/3FL blend film is half that of the F8T2 film. The difference can be explained by the nature of the incorporated oligomers. The reduced TFT performance of the F8T2/3FL blend can be attributed to the relatively low-HOMO levels of the 3FL oligomer, which result in difficult hole injection, and the position of the introduced alkyl groups at the 9-position of fluorene. It has been reported that the thin-film morphologies and TFT characteristics of oligothiophene films depend on the position and bulkiness of the introduced side groups. Due to the ease of solid-state core packing, a,x-dihexyl-substituted systems exhibit better TFT performance than b,b-dihexyl-functionalized systems [25,26]. The positions of the substituted alkyl chains in DH4T and 3FL correspond to those of a,x-dihexyl-substituted and b,b-dihexyl-functionalized thiophene derivatives, respectively. The comparative study of the TFT characteristics of PF8/ DH4T, F8T2/3FL and F8T2/DH4T blends shows that (1) the main pathway for hole transport in polymeric and oligomeric semiconductors blend systems is the polymer and (2) the nature of the blended oligomer affects the OTFT performance of the blends. 4. Conclusions We successfully produced synergistic effects in polymer and oligomer semiconductor blend systems without compromising their other OTFT characteristics. The good solubility of the polymer was found to complement the high performance of the small molecules. The F8T2/DH4T blends were found to exhibit good solubility and could be introduced onto the substrate by simple spin-coating. The 50:50 F8T2/DH4T blend film device was found to have a hole mobility of 0.011 cm2 V1 s1 and a high-on/off ratio of up to 105. The F8T2/DH4T blend films were found to have improved on-currents – about twice that of F8T2 – without much increase in the off-currents. The enhanced TFT performances of the blend films were explained in terms of morphological characteristics revealed by the XRD and AFM analyses. A comparative study with polyfluorene homopolymer and terfluorene showed that carrier transport in the blend systems is mainly controlled by
the polymer component and that the nature of the blended oligomer affects the OTFT performance of the blends. Acknowledgements This research was supported by a Grant (No. AOD-02-A) from the Information Display R&D Center, one of the 21st Century Frontier R& D Program funded by the Ministry of Commerce, Industry and Energy of the Korean Government. We gratefully acknowledge Professor I.-N. Kang (Catholic University), Dr. M. Misaki (AIST) and Dr. Y.-Y. Noh (ETRI) for the fruitful discussion. E. Lim was supported by the Winter Institute Program of KJF and JISTEC for visiting research at AIST. 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. de Leeuw, Nature 401 (1999) 685. [2] B.S. Ong, Y. Wu, P. Liu, S. Gardner, J. Am. Chem. Soc. 126 (2004) 3378. [3] I. Mcculloch, M. Heeney, C. Bailey, C.K. Genevicius, I. Macdonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang, M.L. Chabinyc, R.J. Kline, M.D. Mcgehee, M.F. Toney, Nat. Mater. 5 (2006) 328. [4] H. Pan, Y. Li, Y. Wu, P. Liu, B.S. Ong, S. Zhu, G. Xu, J. Am. Chem. Soc. 129 (2007) 4112. [5] H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekaran, W. Wu, E.P. Woo, M. Grell, D.D.C. Bradly, Appl. Phys. Lett. 77 (2000) 406. [6] E. Lim, B.-J. Jung, J. Lee, H.-K. Shim, J.-I. Lee, Y.S. Yang, L-.M. Do, Macromolecules 38 (2005) 4531. [7] Y.M. Kim, E. Lim, I.-N. Kang, B.-J. Jung, J. Lee, B.W. Koo, L.-M. Do, H.-K. Shim, Macromolecules 39 (2006) 4081. [8] K.C. Dickey, J.E. Anthony, Y.-L. Loo, Adv. Mater. 18 (2006) 1721. [9] A. Afzali, C.D. Dimitrakopoulos, T.L. Breen, J. Am. Chem. Soc. 124 (2002) 8812. [10] Y. Li, Y. Wu, P. Lin, Z. Prostran, S. Gardner, B.S. Ong, Chem. Mater. 19 (2007) 418. [11] M. Berggren, O. Inganäs, G. Gustafsson, J. Rasmusson, M.R. Andersson, T. Hjertberg, O. Wennerström, Nature 372 (1994) 444. [12] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [13] A. Babel, J.D. Wind, S.A. Jenekhe, Adv. Funct. Mater. 14 (2004) 891. [14] S. Goffri, C. Muller, N. Stingelin-Stutzmann, D.W. Breiby, C.P. Radano, J.W. Andreasen, R. Thompson, R.A.J. Janssen, M.M. Nielsen, P. Smith, H. Sirringhaus, Nat. Mater. 5 (2006) 950. [15] N. Stingelin-Stutzmann, E. Smits, H. Wondergem, C. Tanase, P. Blom, P. Smith, D. de Leeuw, Nat. Mater. 4 (2005) 601. [16] E. Lim, B.-J. Jung, H.-K. Shim, Macromolecules 36 (2003) 4288. [17] E. Lim, B.-J. Jung, H.-K. Shim, T. Taguchi, B. Noda, T. Kambayashi, T. Mori, K. Ishikawa, H. Takezoe, L.-M. Do, Org. Electron. 7 (2006) 121. [18] R.H. Pater, M.G. Hansen, U.S. Patent 5770676, 1998. [19] T. Kambayashi, H. Wada, M. Goto, T. Mori, B. Park, H. Takezoe, K. Ishidawa, Org. Electron. 7 (2006) 440. [20] A. Babel, S.A. Jenekhe, Macromolecules 37 (2004) 9835. [21] C.D. Dimitrakoupoulos, P.R.L. Melenfant, Adv. Mater. 14 (2002) 99. [22] M. Heeney, C. Bailey, K. Genevicius, M. Shkunov, D. Sparrowe, S. Tierny, I. McCulloch, J. Am. Chem. Soc. 127 (2005) 1078. [23] A. Babel, S.A. Jenekhe, Macromolecules 36 (2003) 7759. [24] D.M. Russell, C.J. Newsome, S.P. Li, T. Kugler, M. Ishida, T. Shimoda, Appl. Phys. Lett. 87 (2005) 222109. [25] H.E. Katz, Z. Bao, S.L. Gilat, Accounts Chem. Res. 34 (2001) 359. [26] A. Facchetti, M. Mushrush, M.-H. Yoon, G.R. Hutchison, M.A. Ratner, T.J. Marks, J. Am. Chem. Soc. 126 (2004) 13859.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Synergistic effect of polymer and oligomer blends for solution-processable organic thin-film transistors Eunhee Lim a,d, Byung-Jun Jung a, Masayuki Chikamatsu b, Reiko Azumi b, Kiyoshi Yase b, Lee-Mi Do c, Hong-Ku Shim a,* a
Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea b Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Electronics and Telecommunications Research Institute (ETRI), 161 Kajeong-dong, Yuseong-gu, Daejeon 305-350, Republic of Korea d Korea Institute of Industrial Technology (KITECH), 35-3 Hongchon-ri, Ibjangmyun, Chonan-si, Chungnam 330-825, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 September 2007 Accepted 28 June 2008 Available online 8 July 2008
PACS: 85.30-z 68.55.J31.15 82.35.Cd
a b s t r a c t The thin-film morphologies and thin-film transistor (TFT) characteristics of a series of binary blends of poly(9,90 -dioctylfluorene-alt-bithiophene) (F8T2) and a,x-dihexylquarterthiophene (DH4T) are reported. The blends of F8T2 and DH4T exhibit good solubility and produce TFT devices with better performances than F8T2 and DH4T devices. The 50% DH4T blend device was found to have a hole mobility of 0.011 cm 2 V1 s1, which is four times higher than the mobility of the F8T2 device, with a high-on/off ratio of about 105 and a low-off current of 17 pA. The polymer and oligomer domains are phase-separated with large domain size and arranged in characteristic molecular alignments. It was found that carrier transport in the blend systems is mainly controlled by the polymer component, and that the nature of the blended oligomer affects the OTFT performance of the blends. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Thin-film transistor Charge carrier mobility Blend Polyfluorene Thiophene
1. Introduction Interest in organic thin-film transistors (TFTs) has increased because of their various advantages over inorganic amorphous silicon TFTs, including facile processability, chemical tunability, compatibility with plastic substrates, and low-cost processing. To minimize manufacturing costs, the TFT fabrication process should ideally include solution-based methods such as spin-coating, drop-casting, ink-jet printing and screen-printing. The use of soluble * Corresponding author. Tel.: +82 42 350 2827; fax: +82 42 350 2810. E-mail address: [email protected] (H.-K. Shim). 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.06.018
polymeric semiconductors has made possible the development of active-matrix multipixel displays using solutionbased technology. Most p-channel solution-processable organic semiconductors fabricated to date have been based on thiophenebased polymers such as poly(3-hexylthiophene) (P3HT) [1]. However, because of their relatively low-band gaps and high highest occupied molecular orbital (HOMO) levels, thiophene-based polymers tend to be easily oxidized, which results in their degradation, and their low-on/off ratio and high-off current in OTFTs restricts their use in display applications. In attempts to solve these problems, the design and synthesis of organic semiconductors with a
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high-charge carrier mobility and good stability has become a focus of research into organic electronic materials [2–4]. Fluorene-based copolymers have recently emerged as promising materials for polymer TFTs [5–7]. Fluorenebased polymers such as poly(9,90 -dioctylfluorene-alt-bithiophene) (F8T2) have better stability than thiophene-based polymers because of their rigid structures and lower HOMO levels, as well as good film-forming and hole-transporting properties. On the other hand, organic small molecules have also been introduced as active layers in solution-processed OTFTs, through the introduction of various alkyl side chains or by using precursor systems containing thermally removable solubilizing groups. Some of these compounds have exhibited promising behavior as solution-deposited semiconductors with hole mobilities on the order of 0.1 cm2 V1 s1, however, the number of organic small molecules with good solubility and TFT performance is limited so far [8–10]. The use of blends of p-conjugated polymers has proven effective in improving the electronic and optoelectronic properties of OLEDs and photovoltaic devices [11,12]. Recently, blends and other multicomponent systems are also introduced in solution-processable OTFT applications for tuning and improving the properties of TFTs. For example, solution-processed blends of an n-type polymer, poly(benzobisimidazobenzophenanthroline) (BBL) and a p-type small molecule, copper phthalocyanine resulted in ambipolar thin-film transistors that transport both holes and electrons [13]. The incorporation of polyethylene into regioregular P3HT yielded mechanically robust, high-performance TFTs, owing to a highly favourable, crystallization-induced phase segregation of the two components [14]. Moreover, solution-processed rubrene based transistor exhibited saturated mobilities of up to 0.7 cm2 V1 s1 and on/off ratios of over 106 by incorporating a glass-inducing diluent, diphenylanthracene [15]. Therefore, blending two or more electrically active materials can offer multiple advantages into active semiconductor for realizing solution-processable, highperformance OTFTs. Here, we report the synergistic effect of the blend systems consisting of fluorene–thiophene based copolymer F8T2 and thiophene-based oligomer DH4T in OTFTs. Despite of its high-on/off ratio and low-off current, on the order of pA, F8T2 is known to be unsuitable for real applications because of the relatively low-charge carrier mobility on the order of 103 cm2 V1 s1, without the use of additional alignment techniques. Thus our approach is to incorporate the polymer/oligomer blend system into the fluorene-based polymer semiconductor with the aim of improving the hole mobility of this class of polymers while preserving its high-on/off ratio. The morphologies and charge carrier mobilities of a series of binary blends of polymeric and oligomeric semiconductors are investigated. Moreover, comparison of these characteristics with those of the polyfluorene homopolymer and terfluorene oligomer devices enables us to further investigate the charge carrier transport in the blend devices. These results demonstrate that the use of polymer/oligomer blend systems is an effective way to increase the performance of solution-processable OTFTs.
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2. Experimental section 2.1. Materials The fluorene–thiophene based copolymer, F8T2, and the thiophene-based oligomer, DH4T, were prepared with methods described in a previous report, through the Suzuki coupling reaction [16]. Polyfluorene homopolymer, PF8, and the fluorene-based oligomer, 3FL, were also prepared for comparison. The number-average molecular weight (Mn) of F8T2 was found to be 38,000. F8T2 was found to exhibit a crystallization peak at 159 °C and a melting endothermal peak at 268 °C. 2.2. Physical measurements The number-average molecular weight was determined by gel permeation chromatography (GPC) on a Waters GPC-150C instrument calibrated with polystyrene standards, with THF used as the eluent. Differential scanning calorimetry (DSC) was performed under a nitrogen atmosphere at a heating rate of 10 °C min1. UV–vis spectra were obtained using an MPS2000 (SHIMADZU) UV/vis spectrometer. The morphological characterizations with XRD and AFM were performed on the same films as used in the OTFT measurements. The XRD measurements were carried out with a Rigaku Denki RU-300 using Cu Ka radiation (40 kV, 200 mA) in the h–2h scan mode with 0.01° steps in 2h and 0.6 s per step. Non-contact mode (Dynamic Force Mode) AFM images were recorded using a Seiko Instruments SPA-300/SPI3800 probe system, equipped with a Si cantilever (Seiko Instruments SII-DF20, force constant 15 N m1, resonance frequency 130 kHz, and tip curvature radius = 10 nm). 2.3. Fabrication of the TFT devices TFTs were fabricated using the bottom contact geometry. Gold was used for the source and drain contacts and silicon oxide (SiO2) with a thickness of 300 nm was used as the dielectric. The SiO2 surface was cleaned with UV– ozone treatment (UV irradiation for 20 min in an oxygen atmosphere) and pretreated with hexamethyldisilazane (HMDS) to produce apolar and smooth surfaces onto which the polymer could be spin-coated. The polymer and/or oligomer solutions were dissolved in 0.5 wt.% o-dichlorobenzene, and filtered through a 0.45 lm pore size polytetrafluoroethylene (PTFE) membrane syringe filter before use. The polymer solutions were applied dropwise onto the substrates and spin-coated at 1500 rpm. The films were dried on a hot plate at 150 °C for 20 min in a glovebox. The electrical characteristics of the OTFT devices were measured in a glovebox using an Agilent 4155C Semiconductor Parameter Analyzer. 3. Results and discussion 3.1. Optical properties of the F8T2/DH4T blends The chemical structures of all the semiconducting materials used in this study are shown in Fig. 1. The UV–
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Fig. 1. Chemical structures of the semiconductors.
Fig. 2. UV–vis absorption spectra of the F8T2, DH4T, and F8T2/DH4T blend films. Fig. 3. XRD patterns of the F8T2, DH4T, and F8T2/DH4T blend films.
vis absorption spectra of the F8T2/DH4T blend films are shown in Fig. 2 with those of the F8T2 and DH4T films. The absorption peaks of F8T2 are at 210, 460 and 485 nm and those of the DH4T film are at 220, 268, and 355 nm. The absorption peaks of the blend films can be understood as the superpositions of those of the F8T2 and DH4T films. The intensities of the absorption peaks vary with the blend ratios. As more DH4T is incorporated, the intensities of the absorption peaks originating from F8T2 and DH4T decrease and increase, respectively. 3.2. Morphological characteristics of the F8T2/DH4T blend films Crystallization of the polymer and oligomer was confirmed with film X-ray diffraction (XRD), which enabled the identification of the nature of the mesophase. As shown in Fig. 3, the DH4T film produces multiple (h 0 0) reflections with a (1 0 0) reflection at 2h = 3.1°, indicating a d spacing of 28.5 Å, which is comparable to the molecular length of DH4T calculated using MOPAC-PM3 (31.0 Å). In other words, the molecules in the spin-coated DH4T film are oriented normal to the surface, which is a similar result to that reported previously for a vacuum-deposited DH4T film with a d spacing of 27.8 Å [17]. Interestingly, blend
films containing 50% or more DH4T also exhibit the peak at 2h = 3.1° as shoulder. This means that the vertical alignment of the D4HT molecules is somewhat maintained in 50% and 75% DH4T blend films. As more DH4T was incorporated, the intensity of the reflection at 3.1° was found to increase. In contrast, there is no peak for the 25% DH4T blend film in the low-angle region of the XRD pattern, which indicates that the minor DH4T phase in the 25% DH4T blend film is not aligned as in the 50% and 75% DH4T films, and so might act as an impurity disturbing the alignment of F8T2. This result is consistent with the relatively low-thin-film mobility of the 25% DH4T film, even though its domain size is comparable to that of the 50% DH4T film. The morphologies of the F8T2/DH4T blend films were also investigated using atomic force microscopy (AFM). As shown in Fig. 4, the morphologies of the blend films are clearly different to those of the polymer and oligomer films. Whereas the F8T2 and DH4T films have a smooth and homogeneous surface morphology, two distinct phases can be seen in the blend films. During the evaporation process, the oligomer and polymer components separate due to differences in solubilities as the solution is concentrated [18]. The bright features in Fig. 4b–d
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Fig. 4. AFM images of the (a) F8T2 film, (b) 25% DH4T, (c) 50% DH4T and (d) 75% DH4T blend films and (e) DH4T film (10 lm 10 lm). The scale bar represents 2 lm.
decrease with increasing DH4T content in the blends, indicating that the brighter regions correspond to the F8T2 polymer phase. The surface morphologies of the 75% and 50% F8T2 films are similar with those previously reported for P3HT and dibenzotetrathiafulvalene (DBTTF) blends, in which the film surfaces became rough and branching domains such as dendrite and seaweed appeared by blending [19]. The morphologies of polymer/oligomer blends are comparable to those of two polymer blends. Binary blends of P3HT with poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPH) and with polystyrene formed spherical domains or a bicontinuous network on
the length scale of 100–600 nm depending on the blend composition [20]. 3.3. Transistor properties of the F8T2/DH4T blends The TFT devices of DH4T/F8T2 blend films were fabricated and compared with those of DH4T and F8T2 films. The output characteristics of the devices at different gate voltages are shown in Fig. 5. The devices of the blend films showed typical p-type behaviors with clear saturation currents. A contact resistance, likely due to the relatively low solubility of DH4T, was however observed at low source-
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Fig. 5. Output characteristics of the (a) F8T2 film and (b) 50% DH4T blend film (channel width W = 100 lm and length L = 10 lm).
Fig. 6. Plots of the transfer curves for the F8T2 and F8T2/DH4T films at constant VD = 80 V; (a) ID versus VG (inset: semilogarithmic plot) and (b) (ID)1/2 versus VG (channel width W = 100 lm and length L = 10 lm).
drain voltages. Plots of the transfer curve [i.e., ID = f(VG)] at constant VD = 80 V are shown in Fig. 6. The field-effect mobilities were calculated in the saturation regime at VD = 80 V using the conventional TFT equation proposed by Horowitz for saturation regimes [21]. The TFT fabricated with F8T2 under the same conditions as used for the blend devices was found to have a hole mobility in the range 0.001–0.003 cm2 V1 s1 with an on/off ratio of the order of 105, which is in close agreement with the reported hole mobilities of F8T2 devices fabricated under similar conditions [5]. On the other hand, as expected, the field-effect mobility of the TFT device fabricated with a spin-coated DH4T film could not be measured. This result can be compared with that of our previous report for a vacuum-deposited DH4T film, which found a
hole-mobility of 0.02 cm2 V1 s1 using the same DH4T batch and bottom-contact device as used in the blend devices [17]. The absence of TFT characteristics for the spincoated DH4T film can be explained by the poor quality of the film, which results from DH4T’s low solubility. Blending the oligomer with a polymer can improve the low solubility of the oligomer as well as the relatively poor TFT performance of the polymer. The compositional dependence of the hole mobility of the F8T2/DH4T blend films is shown in Fig. 7. The 50% and 75% DH4T blend films produce better TFT performances than the F8T2 and DH4T films. The hole mobilities of the 50% and 75% DH4T blends were found to be 0.011 cm2 V1 s1, which is four times higher than that of the F8T2 polymer film. Three channel widths (W) and
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3.4. Comparative study
Fig. 7. Dependence of the hole mobility of F8T2/DH4T blend devices on the DH4T concentration.
lengths (L) [W(lm)/L(lm) = 100/10, 50/5 and 25/5] were used in one device: only slight variation of the hole mobilities with the W/L ratio was observed, resulting in the same dependence of the hole mobility on DH4T composition. As shown in Fig. 6, the incorporation of the oligomer into the polymer causes an increase in the on current without much increase in the off current. The 50% and 75% DH4T blend films exhibit on-currents of up to 1.5 lA, which are two and a half times higher than that of the F8T2 film. At the same time, the off-currents of the 25% and 50% DH4T blend films were still in the range 10–20 pA, which is apparently lower than that of the other thiophene-based polymers (e.g., >100 pA for stable polythiophene semiconductors incorporating thieno[2,3-b]thiophene [22]). As a result, the F8T2/DH4T blend devices have on/off ratios in the range 104–105, and in particular the on/off ratio of the 50% DH4T blend film device (0.9 105) was found to be close to that of the F8T2 film device. The optimal blend ratio for our polymer/oligomer blend system was found to be 50/50 with a hole mobility of 0.011 cm2 V1 s1, a highon/off ratio of about 105 and a low-off current of 17 pA. The TFT characteristics of the blend films are summarized in Table 1. The improvements in the TFT characteristics that result from the incorporation of 50% or 75% oligomer were also observed with other thiophene-based oligomers, T2TT and T2FL. (The chemical structures of T2TT and T2FL are shown in Ref. [17].)
At this point the question arises as to which mechanism predominantly controls the TFT characteristics of the polymer/oligomer blends. To assess the roles of the polymer and the oligomer in the blend films, polyfluorene homopolymer, poly(9,90 -dioctylfluorene) (PF8) and the fluorene-based oligomer, 9,9,90 ,90 ,900 ,900 -dihexyl-[2,20 ;70 , 300 ]terfluorene (3FL) were used in blends instead of F8T2 and DH4T, respectively. The TFT devices fabricated with only PF8 or 3FL were found to have no TFT characteristics, because of their relatively low-HOMO levels, which result in difficult hole injection from the Au electrode. This result is consistent with those of previous reports, which found a high barrier to charge injection from the Au electrode to PF8. Although PF8 is a liquid crystalline polymer that tends to be highly ordered in thin films, PF8 devices are known to have no TFT characteristics [23]. Films of PF8/DH4T and F8T2/3FL blends were fabricated under the same conditions as used for the F8T2/DH4T blends. While all the PF8/DH4T blend films were found to exhibit no TFT characteristics, the F8T2/3FL blend films were found to exhibit TFT characteristics with hole mobilities of 0.001–0.002 cm2 V1 s1. This result indicates that the polymer component controls the TFT characteristics
Fig. 8. Dependence of the hole mobility of F8T2/3FL blend devices on the 3FL concentration.
Table 1 TFT characteristics of the polymer/oligomer blend films (channel width W = 100 lm and length L = 10 lm)a Blend ratios (polymer:oligomer, wt.%)
F8T2:DH4T
l/cm2 V1 s1
F8T2:3FL PF8:DH4T PF8:3FL
Ion/Ioff Ioff/pA Ion/lA l/cm2 V1 s1 l/cm2 V1 s1 l/cm2 V1 s1
a b
100:0
75:25
50:50
25:75
0:100
0.0026 1.3 105 5.2 0.67 0.0026 –b –b
0.0011 0.3 105 20.3 0.62 0.0023 –b –b
0.011 0.9 105 17.4 1.6 0.0010 –b –b
0.011 0.3 105 49.1 1.5 0.0007 –b –b
–b –b –b –b –b –b –b
Determined from the transfer characteristics at VD = 80 V. The mobilities (l) were calculated in the saturation regime. No TFT characteristics was observed.
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of the polymer/oligomer blend system, which is consistent with a previous report, in which the transport in such a blend system was analyzed using percolation theory and found to be dominated by the performance of the polymer [24]. On the other hand, it is worth noting that the TFT performances of the F8T2/DH4T and F8T2/3FL blend films were found to be different. The dependence of the hole mobility of the F8T2/3FL blend devices on the 3FL concentration is shown in Fig. 8. Whereas the hole mobility of the 50:50 F8T2/DH4T blend film was found to be four times higher than that of the F8T2 film, that of the 50:50 F8T2/3FL blend film is half that of the F8T2 film. The difference can be explained by the nature of the incorporated oligomers. The reduced TFT performance of the F8T2/3FL blend can be attributed to the relatively low-HOMO levels of the 3FL oligomer, which result in difficult hole injection, and the position of the introduced alkyl groups at the 9-position of fluorene. It has been reported that the thin-film morphologies and TFT characteristics of oligothiophene films depend on the position and bulkiness of the introduced side groups. Due to the ease of solid-state core packing, a,x-dihexyl-substituted systems exhibit better TFT performance than b,b-dihexyl-functionalized systems [25,26]. The positions of the substituted alkyl chains in DH4T and 3FL correspond to those of a,x-dihexyl-substituted and b,b-dihexyl-functionalized thiophene derivatives, respectively. The comparative study of the TFT characteristics of PF8/ DH4T, F8T2/3FL and F8T2/DH4T blends shows that (1) the main pathway for hole transport in polymeric and oligomeric semiconductors blend systems is the polymer and (2) the nature of the blended oligomer affects the OTFT performance of the blends. 4. Conclusions We successfully produced synergistic effects in polymer and oligomer semiconductor blend systems without compromising their other OTFT characteristics. The good solubility of the polymer was found to complement the high performance of the small molecules. The F8T2/DH4T blends were found to exhibit good solubility and could be introduced onto the substrate by simple spin-coating. The 50:50 F8T2/DH4T blend film device was found to have a hole mobility of 0.011 cm2 V1 s1 and a high-on/off ratio of up to 105. The F8T2/DH4T blend films were found to have improved on-currents – about twice that of F8T2 – without much increase in the off-currents. The enhanced TFT performances of the blend films were explained in terms of morphological characteristics revealed by the XRD and AFM analyses. A comparative study with polyfluorene homopolymer and terfluorene showed that carrier transport in the blend systems is mainly controlled by
the polymer component and that the nature of the blended oligomer affects the OTFT performance of the blends. Acknowledgements This research was supported by a Grant (No. AOD-02-A) from the Information Display R&D Center, one of the 21st Century Frontier R& D Program funded by the Ministry of Commerce, Industry and Energy of the Korean Government. We gratefully acknowledge Professor I.-N. Kang (Catholic University), Dr. M. Misaki (AIST) and Dr. Y.-Y. Noh (ETRI) for the fruitful discussion. E. Lim was supported by the Winter Institute Program of KJF and JISTEC for visiting research at AIST. 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. de Leeuw, Nature 401 (1999) 685. [2] B.S. Ong, Y. Wu, P. Liu, S. Gardner, J. Am. Chem. Soc. 126 (2004) 3378. [3] I. Mcculloch, M. Heeney, C. Bailey, C.K. Genevicius, I. Macdonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang, M.L. Chabinyc, R.J. Kline, M.D. Mcgehee, M.F. Toney, Nat. Mater. 5 (2006) 328. [4] H. Pan, Y. Li, Y. Wu, P. Liu, B.S. Ong, S. Zhu, G. Xu, J. Am. Chem. Soc. 129 (2007) 4112. [5] H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekaran, W. Wu, E.P. Woo, M. Grell, D.D.C. Bradly, Appl. Phys. Lett. 77 (2000) 406. [6] E. Lim, B.-J. Jung, J. Lee, H.-K. Shim, J.-I. Lee, Y.S. Yang, L-.M. Do, Macromolecules 38 (2005) 4531. [7] Y.M. Kim, E. Lim, I.-N. Kang, B.-J. Jung, J. Lee, B.W. Koo, L.-M. Do, H.-K. Shim, Macromolecules 39 (2006) 4081. [8] K.C. Dickey, J.E. Anthony, Y.-L. Loo, Adv. Mater. 18 (2006) 1721. [9] A. Afzali, C.D. Dimitrakopoulos, T.L. Breen, J. Am. Chem. Soc. 124 (2002) 8812. [10] Y. Li, Y. Wu, P. Lin, Z. Prostran, S. Gardner, B.S. Ong, Chem. Mater. 19 (2007) 418. [11] M. Berggren, O. Inganäs, G. Gustafsson, J. Rasmusson, M.R. Andersson, T. Hjertberg, O. Wennerström, Nature 372 (1994) 444. [12] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [13] A. Babel, J.D. Wind, S.A. Jenekhe, Adv. Funct. Mater. 14 (2004) 891. [14] S. Goffri, C. Muller, N. Stingelin-Stutzmann, D.W. Breiby, C.P. Radano, J.W. Andreasen, R. Thompson, R.A.J. Janssen, M.M. Nielsen, P. Smith, H. Sirringhaus, Nat. Mater. 5 (2006) 950. [15] N. Stingelin-Stutzmann, E. Smits, H. Wondergem, C. Tanase, P. Blom, P. Smith, D. de Leeuw, Nat. Mater. 4 (2005) 601. [16] E. Lim, B.-J. Jung, H.-K. Shim, Macromolecules 36 (2003) 4288. [17] E. Lim, B.-J. Jung, H.-K. Shim, T. Taguchi, B. Noda, T. Kambayashi, T. Mori, K. Ishikawa, H. Takezoe, L.-M. Do, Org. Electron. 7 (2006) 121. [18] R.H. Pater, M.G. Hansen, U.S. Patent 5770676, 1998. [19] T. Kambayashi, H. Wada, M. Goto, T. Mori, B. Park, H. Takezoe, K. Ishidawa, Org. Electron. 7 (2006) 440. [20] A. Babel, S.A. Jenekhe, Macromolecules 37 (2004) 9835. [21] C.D. Dimitrakoupoulos, P.R.L. Melenfant, Adv. Mater. 14 (2002) 99. [22] M. Heeney, C. Bailey, K. Genevicius, M. Shkunov, D. Sparrowe, S. Tierny, I. McCulloch, J. Am. Chem. Soc. 127 (2005) 1078. [23] A. Babel, S.A. Jenekhe, Macromolecules 36 (2003) 7759. [24] D.M. Russell, C.J. Newsome, S.P. Li, T. Kugler, M. Ishida, T. Shimoda, Appl. Phys. Lett. 87 (2005) 222109. [25] H.E. Katz, Z. Bao, S.L. Gilat, Accounts Chem. Res. 34 (2001) 359. [26] A. Facchetti, M. Mushrush, M.-H. Yoon, G.R. Hutchison, M.A. Ratner, T.J. Marks, J. Am. Chem. Soc. 126 (2004) 13859.