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Hall mobility and characteristics of gas-phase polymerized poly(3-iodothiophene) thin films
Current Applied Physics 12 (2012) 1148e1152
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Hall mobility and characteristics of gas-phase polymerized poly(3-iodothiophene) thin films Gum-Joo Lee, Sang-Hyun Jo, Jong-Won Yang, Jin-Yeol Kim* School of Advanced Materials Engineering, Kookmin University, 861-1 Jeongneung, Seongbuk-gu, Seoul 136-702, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2011 Received in revised form 4 February 2012 Accepted 9 February 2012 Available online 28 February 2012
The semi-conductive poly(3-iodothiophene)(P3IT) films were fabricated by gas-phase polymerization through a chemical vapor deposition process. The P3IT nanoscale films have a high crystalline morphologies, and possessed a high Hall mobility up to 10 cm2/Vs. The degree of crystalline and the mobility values measured through Scanning Electron Microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy with structural analysis. These conductive thin films, possessing polycrystalline structures, have a very high mobility and are capable of being applied to organic electronic layers for electrical devices such as the thin film transistors and organic photovoltaic cells. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Poly(3-iodothiophene) Conductive polymers Gas-phase polymerization Hall mobility
1. Introduction
p-electron conjugated polymers have attracted considerable attention in the past few decades because of their electronics/ physical properties and potential application in electronic devices. Recently, these polymers have been developed into very useful organic materials that can be used as an alternative to silicon for the active layer of thin film transistors (TFTs) [1e3] and organic photovoltaic cells (OPVs) [4e6] and for large-area and low-cost applications, including organic electroluminescence displays (OELDs) [7,8]. The electronic devices, especially OPVs and organic semiconductors etc., in which a high light transmittance, electronic conductivity, and electron/hall mobility are required as a important factor, need ultrathin films and crystalline structures for more improved performance of the devices. In polymeric materials, most research into microstructure formation during solidification has focused on crystal growth, ranging from faced crystals to symmetric dendrites. These growth forms can be perturbed by heterogeneities, yielding a rich variety of polycrystalline growth patterns. Among conjugated polymers, poly(3-alkylthiophene)s (P3ATs) have been most extensively studied as a conducting polymer for the development of electronic devices such as organic TFTs and OPVs from both a fundamental and technological point of view.
* Corresponding author. Tel.: þ82 2 910 4663; fax: þ82 2 943 1016. E-mail address: [email protected] (J.-Y. Kim). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.02.037
Obtaining conjugated polymers with high conductivity usually implies that a high carrier mobility is achieved, which results from better regularity of the extended conjugated chain structure and/or the chain packing. Controlling crystalline structure, previous studies have reported that the regioregularity and morphology in semi-conducting polymers are important factors for obtaining a high carrier mobility [9e12]. The charge-carrier mobility in polymers has also been related to the molecular weight [13], casting solvent [14], and carrier concentration [15]. Previous reports showed that most conjugated polymers have a carrier mobility in the range of 0.01 and 0.1 cm2/Vs and typically do not exceed 0.1 cm2/Vs [16e18], which is well below the typical level for amorphous silicon (about 1 cm2/Vs). We have developed a vapor-phase-deposited polymerization (VPP) technique using a self-assembly method to create conducting polymer ultrathin films and patterning at the microscale. Here, the vapor-phase polymerization technique is one of the nanofabrication techniques based on a bottom-up processing method which can utilize well the organic arrangement of macromolecules to produce ordered aggregates at the scale of a nano-layer. This process is the self-assembly polymerization progressing in the gasphase. Conventionally, the conducting polymers having p-conjugated bond have been synthesized by either an oxidative chemical or electrochemical polymerization of a monomer in the liquid phase. In this paper, we present studies on the structure and properties of the conductive ultrathin films prepared by selfassembling poly(3-iodothiophene), head-to-tail structured P3IT (Fig. 1), which is grown on the substrate film in the gas-phase and
G.-J. Lee et al. / Current Applied Physics 12 (2012) 1148e1152
Fig. 1. Head-to-tail molecular structure of poly(3-iodothiophene).
chemically deposited. Distinctive features of these conductive ultrathin films of p-doped conducting polymer include high carrier mobility and their having polycrystalline microstructures. Especially, these synthesized P3IT films had a high carrier mobility up to 10 cm2/Vs and degree of crystalline. 2. Experimental The P3IT films were synthesized by gas-phase deposit polymerization using a self-assembly method. The electrically semiconducting P3IT thin films were directly grown on the polymeric film substrates by polymerizing the 3-iodothiophene monomer in vapor-phase. Solutions of ferric chloride hexahydrate (FeCl3$6H2O; 10e20wt%) in methyl alcohol/isopropyl alcohol/2-methoxyethanol (3:1:1) mixed solvent were prepared as an oxidant. First, FeCl3$6H2O solution was pretreated on clean bare polymeric substrate films (PET, PI, PC etc.) by meyer bar or spin coating. In the second step, after drying at 60 C, these pretreated substrates films with an oxidant were exposed to a 3-iodothiophene monomer vapor at 80 C condition for 10e30 min in a vapor deposition chamber at ambient condition. Finally, after polymerization, the P3IT films produced directly through a washing process, using methanol to remove the impurities in the third step. The P3IT films were assembled on the substrate using this method, and the thickness of the assembled polymer layer or the height of the grown pattern were freely controlled between 200 and 600 nm by varying the conditions of deposition temperature and time. Materials such as 3-iodothiophene, ferric chloride, methanol, isopropyl alcohol, 2-methoxyethanol were obtained from Aldrich and used as received. The thickness and the conductivity of the semi-conducting P3IT thin films grown on the plastic substrates were measured using the scanning electron microscopy (SEM) (JSM-633F, Jeol) and a standard 4-probe technique (Loresta-GP, Mitsubishi Chemical) from the film samples, respectively. The polymer morphologies were observed with an AFM (Nanoscope Ⅲa DI, USA). UV/vis (UV-3150, SHIMADZU) spectroscopy and X-ray diffraction were used for the structural analysis. 3. Results and discussion Conventionally, semi-conducting polymers with a p-conjugated bond have been synthesized by either oxidative chemical or electrochemical polymerization of the monomer in the liquid phase, and these polymers are coated or casted for film formation under wet conditions. On the other hand, polymer semiconductors are more amenable to solution-based processing techniques, and their crystallinity can be achieved by annealing and/or cooling, but the carrier mobility of these films is not very high. In this study, regionregular P3ATs, which have a high molecular weight and an intrinsic tendency to form well-ordered polymer structures, achieve favorable alignment in the film simply by vapor-phase polymerization using a self-assembling technique in a vapor deposition chamber. The vapor-phase polymerization technique is a bottom-up
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processing method, which exploits the organic arrangement of macromolecules to produce ordered nanoscale aggregates. This method can also be used to prepare thin films of self-assembled molecules. The self-assembly of molecules or small clusters, that is, the spontaneous association of atomic or molecular building blocks under conditions of equilibrium, is emerging as a successful chemical strategy to create well-defined structures of nanometer dimensions, with potential applications in many areas of nanotechnology. In this work, region-regular P3ITs, which have a high molecular weight and an intrinsic tendency to form well-ordered polymer structures, attained a favorable alignment in the film simply by vapor-phase polymerization using the self-assembling technique in dry condition. Compared with traditional polymerization and film-forming procedures, this process increases the crystallinity for carrier mobility, increases the Hall mobility by several orders of magnitude. These conductive films, possessing polycrystalline structures, have a high level of carrier mobility. Highly region-regular P3IT films with a carrier mobility of 10 cm2/ Vs were actually prepared by in-situ gas-phase polymerization. Figs. 2 and 3 presents SEM and AFM topographic images of the P3IT film surfaces formed at several stages on insulating substrates (Si/SiO2). In Fig. 2, image I show the initial stage of growth when the 3-iodothiophene monomer was exposed in the VPP chamber for 5 min at 80 C condition. As shown in this figure, the thin, fine crystalline structured films were generated in the form of the netlike structure, and this fine crystalline was increased greatly with an increasing deposition time as shown in image III. The grown pattern was clearly observed at 30 min deposit time. The reticulum was clearly resolved in the AFM topography (Fig. 3). Fig 3-I show a two-dimensional AFM topographic images at a 10 10 mm scale. The surface of the crystallized P3IT films differed from that of the amorphous phase. The amorphous zones appeared dark in these images. On the contrary, crystalline zones appeared in white images. These crystalline structures, which exhibited a netlike structure, grew to a height of almost 400 nm in the P3IT film. This growth pattern can be explained by assuming that the crystalline probably advanced through the nucleated liquid phase 3-iodothiophene monomers as it grew out from the solideliquid interface into the liquid at high supercooling. The supercooling seems to provide the driving force for the crystalline growth. However, the crystallized P3HT films of tree branch pattern are obtained in this experimental procedure. Generally, the charge-carrier mobility of solution-processed conjugated polymers is limited by the hopping process between the polymer chains in the disordered region of the materials. For effective charge transport, it is important for both polymer backbones to possess a high degree of intra-chain order and the different chains need to be aligned favorably to achieve large overlap of the p-orbital on the adjacent chains. In this study, highly crystalline films of polythiophene with a iodine side-group appeared to originate from the self-organized structure prepared readily from the vapor-phase polymerization technique. Recent, reports have shown that a high degree of region-regularity leads to more ordered microstructures because of supramolecular selforganization higher mobilities. However, Figs. 2 and 3 shows a crystalline backbone in the form of a netlike, and a lower surface roughness. These P3IT thin films were analyzed by grazing incidence X-ray diffraction (GIXRD) to determine the internal structures of the crystals. Fig. 4 shows the X-ray diffraction (XRD) patterns of the out-ofplane scattering geometries of the P3IT film samples. The XRD pattern of the P3IT film showed two large diffraction peaks at 2q ¼ 4.69 (100) and 15.87 (300), corresponding to the interlayer spacing, to be composed of stacked layers constructed by a side-byside arrangement of iodine side-chain. On the other hand, the
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Fig. 2. SEM surface images of self-assembled P3IT films. (I), (II), and (III) show surface forms grown when the 3-iodothiophene monomer is exposed in the VPP chamber for 5, 10, and 30 min at 80 C, respectively.
relatively very small intensity peak of (200) was at 2q ¼ 11.05 . These relatively peak intensities are caused by a periodic density modulation perpendicular to the surface and are indicative of the formation of an ordered phase. In the case of the P3IT film, however, the intensity of the (200) peaks quite weak. This is probably because of steric interference between the iodine molecules on the same repeating units of thiophene that twisted the substituted group moieties slightly out-of coplanarity, and sterically hindered the close packing of the lamellar layers. The diffraction peak at 2q ¼ 20.65 (010) corresponded to the p-p stacking structure between lamellar layers. Especially, the relatively large out-of-plane (010) peak and correspondingly large (100) peak were measured, and the (100) reflection was more intense and pronounced than the (300) and (010) reflections. In particular, the relatively larger (010) peak and correspondingly larger (300) peak than that of films cast from a solution process were measured. These results suggest that preferentially oriented p-p stacking structures were formed with their lamellar axis normal to the substrate. Therefore, the number of selforganized domains having an orientation with the (010) axis (the pp stacking distance) along the sample surface, is much larger than the number of domains having a random orientation along the backbone. According to previous literature, a self-crystalline or wellorganized lamellar structure of regioregular P3ATs was accomplished by annealing after wet coating/drying according in the cast films of the polymer solution with cyclohexaone as the solvent [22]. The characteristic strong (100) and very weak (200), (300) peaks were the only ones observed. Fig. 5 show the UVevisible absorption spectrum of P3IT in the solid films. A single broad UV absorption band was measured at a maximum absorption (lmax) of 502 nm. According to previous reports [19,20], in 3-substituted polythiophene derivatives, the polymers having regioregular structures have been reported as have a maximum absorption peak at up to 460 nm which is 20e30 nm longer than regiorandom structures with a characteristic lmax of w430 nm. Rieke et al. [21] reported that the regiorandom 3-alkyl
substituted polythiophene films (P3AT) showed the maximum absorption for the p-p* transition at w 438 nm, whereas the regioregular head-to-tail P3AT films showed a much longer maximum absorption peak at 560 nm. On the other hand, the lmax of the regioregular head-to-tail P3AT has an absorption peak that was significantly longer than that of the other P3ATs. In these results, the UV absorption lmax (502 nm) value in P3IT films in this report is significant to having a high regioregular head-to-tail structures than the regiorandom structures. Electrical properties and mobility are very important because many applications such as LEDs, TFTs, or OPVs are based on electrical behavior. Conjugate polymers are straightforwardly prepared by several methods, and their electronic states can be reversibly changed between insulating and conducting states by redox reactions. Conductivity is the product of two important factors: the number of carrier electrons or holes and carrier mobility, which in a loose sense is the case in which a carrier moves through a material. The electrical conductivities of most conductive polymers are in the same range as those of inorganic semiconductors, but there is some difference according to the degree of crystalline, purity, and a lack of defects in these materials. In his work, we obtained P3IT films that have a high crystalline and purity. For the demonstration of conductivity as a function of crystalline, it used a method employing a scanning probe microscope (SPM). In this measurement, an SPM equipped with a diamond tip cantilever was used to measure the electrical conductivity of a P3IT crystal, and the electrical contact on the other side of the template was achieved with a conducting gold paste. Fig. 3 (right) shows the change of the IeV characteristics at different contact points (one is a crystalline region (A) and another is an amorphous region (B)). All the IeV curves display exponential increases until they saturate at the highest fullscale current of 1000 nA. These curves represent the typical IeV traces obtained reproducibility depending on the doping states of the films. In particular, in local IeV measurements, shown in traces A and B, crystalline regions of the sample showed a higher current
Fig. 3. AFM surface images of self-assembled P3IT films grown for 30 min at 80 C. (I) and (II) is when the scan area was 10 10 and 40 40 mm, respectively. Right figure is IeV characteristics of P3IT films obtained from marking spots (A and B in Fig 1) In the film shown in an AFM image of the sample.
G.-J. Lee et al. / Current Applied Physics 12 (2012) 1148e1152
Fig. 4. X-ray diffraction spectra of the P3IT thin films.
response than the amorphous regions as the bias voltage increases. Thus, the crystalline regions have more vertical form than that of the amorphous or boundary regions. It was shown that a linear relationship between the voltage and the current was observed when the bias voltage was in the range from 1 to þ1 V, indicating that the conductivity is constant. However, when a relatively high voltage (1 V) is biased, the conductivity of the P3IT film increases with increasing bias voltage. That is, the P3IT films that have crystalline structures exhibit nonlinear IeV characteristics at high bias voltage. As mentioned above, the semi-conductive P3IT films made by VPP method shows high crystalline because they can be grown with the microstructure having compact morphology. The conductivity of the resulting film was lower than 1 S/cm, and the light transmittance in the visible range was lower than 70%. The mobility is also very important. Conjugated polymers, especially P3ATs, are being extensively studied because their high Hall mobility values make them suitable semiconductors for organic devices. However, the carrier mobility of polymer-based semiconductor films fabricated by a wet solution process was not as high as the values of amorphous silicon. The Hall mobility was measured for the P3IT thin films fabricated by the present process. The Hall effects is a useful technique for making some electrical property measurements related to transport, such as the carrier concentration, Hall mobility, resistivity, and conduction type. The hole-only device used in this experiment. In all of the devices, the polymer
Fig. 5. UVevisible absorption spectra of P3IT.
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film was polymerized in the gas-phase condition on a SiO2 coated silicon wafer with a four-contact electrodes set-up. The Hall effects measurement was carried out in liquid nitrogen under a low magnetic field (B). A current supplied by a constant source passed through the electrodes and voltages, such as the resistivity voltage VR and the transverse Hall voltage VH, were measured in the presence of a 5-kG magnetic field. After the VH was measured, the free-carrier concentration (cm3), the resistivity (U cm), and subsequently the mobility (cm2/Vs) were calculated. The Hall mobility of P3IT films fabricated by VPP method was plotted as a function of the resistivity and carrier concentration in Fig. 6 (I-c and II-c), respectively. For comparison, the Hall mobility data of poly(3-hexylthiophene) (P3HT) films prepared by the wet process at cyclohexane solvent condition and the same VPP process as this work are shown in (a) and (b) of Fig. 6, respectively. The resistivity and Hall mobility increased with increasing carrier concentration, and the Hall mobility was clearly higher for the P3IT film sample with a higher degree of crystalline. In particular, the Hall mobility was between 1 and 10 cm2/Vs when the resistivity was in the range of 0.1e80 U cm range and the carrier concentration was in the range of 0.05e600 (x 1017) cm3. The carrier concentration in P3IT thin films are controlled by polymerization conditions of P3IT such as concentration of oxidant used in synthesis and/or function of the exposed temperature and time of 3-iodothiophene monomer. This mobility value in P3IT film is the standard higher than that of P3HT and especially, it’s the very high standard more than a general wet coating process film. This implied that the mobility increased with the quantity of highly oriented regioregular structures. In general,
Fig. 6. Hall mobility as a function of the resistivity and carrier concentration: (a) poly(3-hexyl thiophene) (P3HT) film fabricated by wet process in cyclohexane solvent, (b) P3HT film fabricated by VPP method, and (c) P3IT film fabricated by VPP method.
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the charge-carrier mobility associated with the increased in-plane p stacking is actually caused by the improved grain boundaries of the highly oriented crystal, and therefore, only indirectly related to the in-plane p stacking. In conclusion, a vapor-phase deposited polymerization technique using the self-assembly of monomers was investigated in order to produce thin semi-conducting polymer films, and a systematically region controlled synthesis of P3IT was demonstrated. The regioregular P3IT was consistently characterized as polymers with a regiospecific head-to-tail conformation, significant conjugation length, self-organized structure of the polymer chain, and polycrystalline structure. These polymer-based semiconducting films had a Hall mobility about 10 cm2/Vs with a regioregularity reaching as high as 80%. This value was much higher than those reported in previous literature. This was probably due to the regular arrangement of the efficiently stacked layers constructed by the in-plane p-p stacking. Conventionally, pconjugated polymers, especially polythiophene substituted with alkyl or halogen atoms, synthesized in a wet solution process, have been used as the active materials for TFTs or OPVs. However, their mobility values were very low at a level of less than 1 cm;2/Vs. High conductivity and higher charge mobility would be beneficial for many the polymeric semiconductor applications. Acknowledgments This work was financially supported in part by the ERC program of MEST/KOSEF (R11-2005-048-0000-0), the Korea Research Foundation Grant funded by the Korean Government (2010-DK001084), and the Industrial Core Technology
Development Project through the Ministry of Knowledge and Commerce (10035644). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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C. Dimitrakopoulos, P. Malenfant, Adv. Mater. 14 (2002) 99. H. Katz, J. Bao, J. Phys. Chem. B 104 (2000) 671. G. Horowitz, J. Mater. Res. 19 (2004) 1946. C. Brabec, N. Sariciftci, J. Hummelen, Adv. Funct. Mater. 11 (2001) 15. W.U. Huynh, J.J. Dittmer, A.P. Alivisators, Science 295 (2002) 2425. K.M. Coakley, M.D. McGehee, Chem. Mater. 16 (2004) 4533. A. Heeger, Angew. Chem. Int. Ed. 40 (2001) 2591. R. Friend, R. Gymer, A. Holmes, J. Burroughes, R. Marks, C. Taliani, D. Bradley, D. Dos Santos, J. Bredas, M. Logdlund, W. Salaneck, Nature 397 (1999) 121. 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. S.H. Jhung, J.S. Chang, J.W. Yoon, J.M. Greneche, G. Ferey, A.K. Cheetham, Chem. Mater. 16 (2004) 4772. R.J. Kline, M.D. McGehee, E.N. Kadnikova, J. Liu, J.M.J. Frchet, M.F. Toney, Macromolecules 38 (2005) 3312. H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekaran, W. Wu, E.P. Woo, M. Grell, D.D.C. Bradley, Appl. Phys. Lett. 77 (2000) 406. R.J. Kline, M.D. McGehee, E.N. Kadnikova, J. Liu, J.M.J. Frechet, Adv. Mater. 15 (2003) 1519. Z. Bao, A. Dodabalapur, A. Lovinger, Appl. Phys. Lett. 69 (1996) 4108. C. Tanase, E.J. Meijer, P.W.M. Blom, D.M. de Leeuw, Phys. Rev. Lett. 91 (2003) 216601. H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. M. Raja, G. Lioyd, N. Sedghi, W. Eccleston, R. Di Lucrezia, S. Higgins, J. Appl. Phys. 92 (2002) 1441. G. Wang, J. Swenson, D. Moses, A.J. Heeger, J. Appl. Phys. 93 (2003) 6137. S. Hotta, S.D.D.V. Rughooputh, A.J. Heeger, F. Wudl, Macromolecules 20 (1987) 212. S.D.D.V. Rughooputh, S. Hotta, A.J. Heeger, F. Wudl, J. Polym. Sci. Part B: Polym. Phys. 20 (1987) 212. T.A. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233. S. Berson, R. De Bettignies, S. Bailly, S. Guillereez, Adv. Fun.Mater. 17 (2007) 1377.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Hall mobility and characteristics of gas-phase polymerized poly(3-iodothiophene) thin films Gum-Joo Lee, Sang-Hyun Jo, Jong-Won Yang, Jin-Yeol Kim* School of Advanced Materials Engineering, Kookmin University, 861-1 Jeongneung, Seongbuk-gu, Seoul 136-702, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2011 Received in revised form 4 February 2012 Accepted 9 February 2012 Available online 28 February 2012
The semi-conductive poly(3-iodothiophene)(P3IT) films were fabricated by gas-phase polymerization through a chemical vapor deposition process. The P3IT nanoscale films have a high crystalline morphologies, and possessed a high Hall mobility up to 10 cm2/Vs. The degree of crystalline and the mobility values measured through Scanning Electron Microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy with structural analysis. These conductive thin films, possessing polycrystalline structures, have a very high mobility and are capable of being applied to organic electronic layers for electrical devices such as the thin film transistors and organic photovoltaic cells. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Poly(3-iodothiophene) Conductive polymers Gas-phase polymerization Hall mobility
1. Introduction
p-electron conjugated polymers have attracted considerable attention in the past few decades because of their electronics/ physical properties and potential application in electronic devices. Recently, these polymers have been developed into very useful organic materials that can be used as an alternative to silicon for the active layer of thin film transistors (TFTs) [1e3] and organic photovoltaic cells (OPVs) [4e6] and for large-area and low-cost applications, including organic electroluminescence displays (OELDs) [7,8]. The electronic devices, especially OPVs and organic semiconductors etc., in which a high light transmittance, electronic conductivity, and electron/hall mobility are required as a important factor, need ultrathin films and crystalline structures for more improved performance of the devices. In polymeric materials, most research into microstructure formation during solidification has focused on crystal growth, ranging from faced crystals to symmetric dendrites. These growth forms can be perturbed by heterogeneities, yielding a rich variety of polycrystalline growth patterns. Among conjugated polymers, poly(3-alkylthiophene)s (P3ATs) have been most extensively studied as a conducting polymer for the development of electronic devices such as organic TFTs and OPVs from both a fundamental and technological point of view.
* Corresponding author. Tel.: þ82 2 910 4663; fax: þ82 2 943 1016. E-mail address: [email protected] (J.-Y. Kim). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.02.037
Obtaining conjugated polymers with high conductivity usually implies that a high carrier mobility is achieved, which results from better regularity of the extended conjugated chain structure and/or the chain packing. Controlling crystalline structure, previous studies have reported that the regioregularity and morphology in semi-conducting polymers are important factors for obtaining a high carrier mobility [9e12]. The charge-carrier mobility in polymers has also been related to the molecular weight [13], casting solvent [14], and carrier concentration [15]. Previous reports showed that most conjugated polymers have a carrier mobility in the range of 0.01 and 0.1 cm2/Vs and typically do not exceed 0.1 cm2/Vs [16e18], which is well below the typical level for amorphous silicon (about 1 cm2/Vs). We have developed a vapor-phase-deposited polymerization (VPP) technique using a self-assembly method to create conducting polymer ultrathin films and patterning at the microscale. Here, the vapor-phase polymerization technique is one of the nanofabrication techniques based on a bottom-up processing method which can utilize well the organic arrangement of macromolecules to produce ordered aggregates at the scale of a nano-layer. This process is the self-assembly polymerization progressing in the gasphase. Conventionally, the conducting polymers having p-conjugated bond have been synthesized by either an oxidative chemical or electrochemical polymerization of a monomer in the liquid phase. In this paper, we present studies on the structure and properties of the conductive ultrathin films prepared by selfassembling poly(3-iodothiophene), head-to-tail structured P3IT (Fig. 1), which is grown on the substrate film in the gas-phase and
G.-J. Lee et al. / Current Applied Physics 12 (2012) 1148e1152
Fig. 1. Head-to-tail molecular structure of poly(3-iodothiophene).
chemically deposited. Distinctive features of these conductive ultrathin films of p-doped conducting polymer include high carrier mobility and their having polycrystalline microstructures. Especially, these synthesized P3IT films had a high carrier mobility up to 10 cm2/Vs and degree of crystalline. 2. Experimental The P3IT films were synthesized by gas-phase deposit polymerization using a self-assembly method. The electrically semiconducting P3IT thin films were directly grown on the polymeric film substrates by polymerizing the 3-iodothiophene monomer in vapor-phase. Solutions of ferric chloride hexahydrate (FeCl3$6H2O; 10e20wt%) in methyl alcohol/isopropyl alcohol/2-methoxyethanol (3:1:1) mixed solvent were prepared as an oxidant. First, FeCl3$6H2O solution was pretreated on clean bare polymeric substrate films (PET, PI, PC etc.) by meyer bar or spin coating. In the second step, after drying at 60 C, these pretreated substrates films with an oxidant were exposed to a 3-iodothiophene monomer vapor at 80 C condition for 10e30 min in a vapor deposition chamber at ambient condition. Finally, after polymerization, the P3IT films produced directly through a washing process, using methanol to remove the impurities in the third step. The P3IT films were assembled on the substrate using this method, and the thickness of the assembled polymer layer or the height of the grown pattern were freely controlled between 200 and 600 nm by varying the conditions of deposition temperature and time. Materials such as 3-iodothiophene, ferric chloride, methanol, isopropyl alcohol, 2-methoxyethanol were obtained from Aldrich and used as received. The thickness and the conductivity of the semi-conducting P3IT thin films grown on the plastic substrates were measured using the scanning electron microscopy (SEM) (JSM-633F, Jeol) and a standard 4-probe technique (Loresta-GP, Mitsubishi Chemical) from the film samples, respectively. The polymer morphologies were observed with an AFM (Nanoscope Ⅲa DI, USA). UV/vis (UV-3150, SHIMADZU) spectroscopy and X-ray diffraction were used for the structural analysis. 3. Results and discussion Conventionally, semi-conducting polymers with a p-conjugated bond have been synthesized by either oxidative chemical or electrochemical polymerization of the monomer in the liquid phase, and these polymers are coated or casted for film formation under wet conditions. On the other hand, polymer semiconductors are more amenable to solution-based processing techniques, and their crystallinity can be achieved by annealing and/or cooling, but the carrier mobility of these films is not very high. In this study, regionregular P3ATs, which have a high molecular weight and an intrinsic tendency to form well-ordered polymer structures, achieve favorable alignment in the film simply by vapor-phase polymerization using a self-assembling technique in a vapor deposition chamber. The vapor-phase polymerization technique is a bottom-up
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processing method, which exploits the organic arrangement of macromolecules to produce ordered nanoscale aggregates. This method can also be used to prepare thin films of self-assembled molecules. The self-assembly of molecules or small clusters, that is, the spontaneous association of atomic or molecular building blocks under conditions of equilibrium, is emerging as a successful chemical strategy to create well-defined structures of nanometer dimensions, with potential applications in many areas of nanotechnology. In this work, region-regular P3ITs, which have a high molecular weight and an intrinsic tendency to form well-ordered polymer structures, attained a favorable alignment in the film simply by vapor-phase polymerization using the self-assembling technique in dry condition. Compared with traditional polymerization and film-forming procedures, this process increases the crystallinity for carrier mobility, increases the Hall mobility by several orders of magnitude. These conductive films, possessing polycrystalline structures, have a high level of carrier mobility. Highly region-regular P3IT films with a carrier mobility of 10 cm2/ Vs were actually prepared by in-situ gas-phase polymerization. Figs. 2 and 3 presents SEM and AFM topographic images of the P3IT film surfaces formed at several stages on insulating substrates (Si/SiO2). In Fig. 2, image I show the initial stage of growth when the 3-iodothiophene monomer was exposed in the VPP chamber for 5 min at 80 C condition. As shown in this figure, the thin, fine crystalline structured films were generated in the form of the netlike structure, and this fine crystalline was increased greatly with an increasing deposition time as shown in image III. The grown pattern was clearly observed at 30 min deposit time. The reticulum was clearly resolved in the AFM topography (Fig. 3). Fig 3-I show a two-dimensional AFM topographic images at a 10 10 mm scale. The surface of the crystallized P3IT films differed from that of the amorphous phase. The amorphous zones appeared dark in these images. On the contrary, crystalline zones appeared in white images. These crystalline structures, which exhibited a netlike structure, grew to a height of almost 400 nm in the P3IT film. This growth pattern can be explained by assuming that the crystalline probably advanced through the nucleated liquid phase 3-iodothiophene monomers as it grew out from the solideliquid interface into the liquid at high supercooling. The supercooling seems to provide the driving force for the crystalline growth. However, the crystallized P3HT films of tree branch pattern are obtained in this experimental procedure. Generally, the charge-carrier mobility of solution-processed conjugated polymers is limited by the hopping process between the polymer chains in the disordered region of the materials. For effective charge transport, it is important for both polymer backbones to possess a high degree of intra-chain order and the different chains need to be aligned favorably to achieve large overlap of the p-orbital on the adjacent chains. In this study, highly crystalline films of polythiophene with a iodine side-group appeared to originate from the self-organized structure prepared readily from the vapor-phase polymerization technique. Recent, reports have shown that a high degree of region-regularity leads to more ordered microstructures because of supramolecular selforganization higher mobilities. However, Figs. 2 and 3 shows a crystalline backbone in the form of a netlike, and a lower surface roughness. These P3IT thin films were analyzed by grazing incidence X-ray diffraction (GIXRD) to determine the internal structures of the crystals. Fig. 4 shows the X-ray diffraction (XRD) patterns of the out-ofplane scattering geometries of the P3IT film samples. The XRD pattern of the P3IT film showed two large diffraction peaks at 2q ¼ 4.69 (100) and 15.87 (300), corresponding to the interlayer spacing, to be composed of stacked layers constructed by a side-byside arrangement of iodine side-chain. On the other hand, the
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Fig. 2. SEM surface images of self-assembled P3IT films. (I), (II), and (III) show surface forms grown when the 3-iodothiophene monomer is exposed in the VPP chamber for 5, 10, and 30 min at 80 C, respectively.
relatively very small intensity peak of (200) was at 2q ¼ 11.05 . These relatively peak intensities are caused by a periodic density modulation perpendicular to the surface and are indicative of the formation of an ordered phase. In the case of the P3IT film, however, the intensity of the (200) peaks quite weak. This is probably because of steric interference between the iodine molecules on the same repeating units of thiophene that twisted the substituted group moieties slightly out-of coplanarity, and sterically hindered the close packing of the lamellar layers. The diffraction peak at 2q ¼ 20.65 (010) corresponded to the p-p stacking structure between lamellar layers. Especially, the relatively large out-of-plane (010) peak and correspondingly large (100) peak were measured, and the (100) reflection was more intense and pronounced than the (300) and (010) reflections. In particular, the relatively larger (010) peak and correspondingly larger (300) peak than that of films cast from a solution process were measured. These results suggest that preferentially oriented p-p stacking structures were formed with their lamellar axis normal to the substrate. Therefore, the number of selforganized domains having an orientation with the (010) axis (the pp stacking distance) along the sample surface, is much larger than the number of domains having a random orientation along the backbone. According to previous literature, a self-crystalline or wellorganized lamellar structure of regioregular P3ATs was accomplished by annealing after wet coating/drying according in the cast films of the polymer solution with cyclohexaone as the solvent [22]. The characteristic strong (100) and very weak (200), (300) peaks were the only ones observed. Fig. 5 show the UVevisible absorption spectrum of P3IT in the solid films. A single broad UV absorption band was measured at a maximum absorption (lmax) of 502 nm. According to previous reports [19,20], in 3-substituted polythiophene derivatives, the polymers having regioregular structures have been reported as have a maximum absorption peak at up to 460 nm which is 20e30 nm longer than regiorandom structures with a characteristic lmax of w430 nm. Rieke et al. [21] reported that the regiorandom 3-alkyl
substituted polythiophene films (P3AT) showed the maximum absorption for the p-p* transition at w 438 nm, whereas the regioregular head-to-tail P3AT films showed a much longer maximum absorption peak at 560 nm. On the other hand, the lmax of the regioregular head-to-tail P3AT has an absorption peak that was significantly longer than that of the other P3ATs. In these results, the UV absorption lmax (502 nm) value in P3IT films in this report is significant to having a high regioregular head-to-tail structures than the regiorandom structures. Electrical properties and mobility are very important because many applications such as LEDs, TFTs, or OPVs are based on electrical behavior. Conjugate polymers are straightforwardly prepared by several methods, and their electronic states can be reversibly changed between insulating and conducting states by redox reactions. Conductivity is the product of two important factors: the number of carrier electrons or holes and carrier mobility, which in a loose sense is the case in which a carrier moves through a material. The electrical conductivities of most conductive polymers are in the same range as those of inorganic semiconductors, but there is some difference according to the degree of crystalline, purity, and a lack of defects in these materials. In his work, we obtained P3IT films that have a high crystalline and purity. For the demonstration of conductivity as a function of crystalline, it used a method employing a scanning probe microscope (SPM). In this measurement, an SPM equipped with a diamond tip cantilever was used to measure the electrical conductivity of a P3IT crystal, and the electrical contact on the other side of the template was achieved with a conducting gold paste. Fig. 3 (right) shows the change of the IeV characteristics at different contact points (one is a crystalline region (A) and another is an amorphous region (B)). All the IeV curves display exponential increases until they saturate at the highest fullscale current of 1000 nA. These curves represent the typical IeV traces obtained reproducibility depending on the doping states of the films. In particular, in local IeV measurements, shown in traces A and B, crystalline regions of the sample showed a higher current
Fig. 3. AFM surface images of self-assembled P3IT films grown for 30 min at 80 C. (I) and (II) is when the scan area was 10 10 and 40 40 mm, respectively. Right figure is IeV characteristics of P3IT films obtained from marking spots (A and B in Fig 1) In the film shown in an AFM image of the sample.
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Fig. 4. X-ray diffraction spectra of the P3IT thin films.
response than the amorphous regions as the bias voltage increases. Thus, the crystalline regions have more vertical form than that of the amorphous or boundary regions. It was shown that a linear relationship between the voltage and the current was observed when the bias voltage was in the range from 1 to þ1 V, indicating that the conductivity is constant. However, when a relatively high voltage (1 V) is biased, the conductivity of the P3IT film increases with increasing bias voltage. That is, the P3IT films that have crystalline structures exhibit nonlinear IeV characteristics at high bias voltage. As mentioned above, the semi-conductive P3IT films made by VPP method shows high crystalline because they can be grown with the microstructure having compact morphology. The conductivity of the resulting film was lower than 1 S/cm, and the light transmittance in the visible range was lower than 70%. The mobility is also very important. Conjugated polymers, especially P3ATs, are being extensively studied because their high Hall mobility values make them suitable semiconductors for organic devices. However, the carrier mobility of polymer-based semiconductor films fabricated by a wet solution process was not as high as the values of amorphous silicon. The Hall mobility was measured for the P3IT thin films fabricated by the present process. The Hall effects is a useful technique for making some electrical property measurements related to transport, such as the carrier concentration, Hall mobility, resistivity, and conduction type. The hole-only device used in this experiment. In all of the devices, the polymer
Fig. 5. UVevisible absorption spectra of P3IT.
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film was polymerized in the gas-phase condition on a SiO2 coated silicon wafer with a four-contact electrodes set-up. The Hall effects measurement was carried out in liquid nitrogen under a low magnetic field (B). A current supplied by a constant source passed through the electrodes and voltages, such as the resistivity voltage VR and the transverse Hall voltage VH, were measured in the presence of a 5-kG magnetic field. After the VH was measured, the free-carrier concentration (cm3), the resistivity (U cm), and subsequently the mobility (cm2/Vs) were calculated. The Hall mobility of P3IT films fabricated by VPP method was plotted as a function of the resistivity and carrier concentration in Fig. 6 (I-c and II-c), respectively. For comparison, the Hall mobility data of poly(3-hexylthiophene) (P3HT) films prepared by the wet process at cyclohexane solvent condition and the same VPP process as this work are shown in (a) and (b) of Fig. 6, respectively. The resistivity and Hall mobility increased with increasing carrier concentration, and the Hall mobility was clearly higher for the P3IT film sample with a higher degree of crystalline. In particular, the Hall mobility was between 1 and 10 cm2/Vs when the resistivity was in the range of 0.1e80 U cm range and the carrier concentration was in the range of 0.05e600 (x 1017) cm3. The carrier concentration in P3IT thin films are controlled by polymerization conditions of P3IT such as concentration of oxidant used in synthesis and/or function of the exposed temperature and time of 3-iodothiophene monomer. This mobility value in P3IT film is the standard higher than that of P3HT and especially, it’s the very high standard more than a general wet coating process film. This implied that the mobility increased with the quantity of highly oriented regioregular structures. In general,
Fig. 6. Hall mobility as a function of the resistivity and carrier concentration: (a) poly(3-hexyl thiophene) (P3HT) film fabricated by wet process in cyclohexane solvent, (b) P3HT film fabricated by VPP method, and (c) P3IT film fabricated by VPP method.
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the charge-carrier mobility associated with the increased in-plane p stacking is actually caused by the improved grain boundaries of the highly oriented crystal, and therefore, only indirectly related to the in-plane p stacking. In conclusion, a vapor-phase deposited polymerization technique using the self-assembly of monomers was investigated in order to produce thin semi-conducting polymer films, and a systematically region controlled synthesis of P3IT was demonstrated. The regioregular P3IT was consistently characterized as polymers with a regiospecific head-to-tail conformation, significant conjugation length, self-organized structure of the polymer chain, and polycrystalline structure. These polymer-based semiconducting films had a Hall mobility about 10 cm2/Vs with a regioregularity reaching as high as 80%. This value was much higher than those reported in previous literature. This was probably due to the regular arrangement of the efficiently stacked layers constructed by the in-plane p-p stacking. Conventionally, pconjugated polymers, especially polythiophene substituted with alkyl or halogen atoms, synthesized in a wet solution process, have been used as the active materials for TFTs or OPVs. However, their mobility values were very low at a level of less than 1 cm;2/Vs. High conductivity and higher charge mobility would be beneficial for many the polymeric semiconductor applications. Acknowledgments This work was financially supported in part by the ERC program of MEST/KOSEF (R11-2005-048-0000-0), the Korea Research Foundation Grant funded by the Korean Government (2010-DK001084), and the Industrial Core Technology
Development Project through the Ministry of Knowledge and Commerce (10035644). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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