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Transition Between Band and Hopping Transport in Polymer Field-Effect Transistors Yu Yamashita, Junto Tsurumi, Felix Hinkel, Yugo Okada, Junshi Soeda, Wojciech Zaja˛czkowski, Martin Baumgarten, Wojciech Pisula, Hiroyuki Matsui,* Klaus Müllen, and Jun Takeya* A microscopic understanding of charge carrier transport in polymeric semiconductors, the most suited class of materials for printed and flexible semiconductor devices,[1,2] has been the central subject of organic electronics.[3] Due to the presence of inherent disorder and thermally activated structural fluctuation, it is a fundamental challenge to realize band transport within the range of charge accumulation accessible in field-effect transistors. To our knowledge, there has been no report proving band transport in polymer organic transistors. In this work, we demonstrate the Hall effect and a slightly negative temperature dependence of the charge carrier mobility in solid-gate polymeric transistors based on a polycyclopentadithiophenebenzothiadiazole donor-acceptor copolymer (CDT-BTZ).[4–6] We thereby employ a unique method for the elaborate orientation of polymer chains on the stable surface of an ionic liquid.[7] The Hall factor close to the ideal value, a maximum hole mobility of 5.6 cm2 V−1 s−1 at room temperature, and its temperature dependence indicate that the system is located in the transition region between the band and hopping transport. Charge transport mechanisms have been one of the most important issues in the field of organic semiconductors since they determine the theoretical limit of the charge carrier mobility in the system. At present, such studies have been focused particularly on small-molecule semiconductors because of the availability of single crystals as well as their high mobility.[8] Highly ordered single crystals, indeed, enabled us to Y. Yamashita, J. Tsurumi, Dr. Y. Okada, Dr. H. Matsui, Prof. J. Takeya Department of Advanced Materials Science The University of Tokyo 5–1–5 Kashiwanoha, Kashiwa 277–8561, Chiba, Japan E-mail: [email protected]; [email protected] F. Hinkel, W. Zaja˛czkowski, Prof. M. Baumgarten, Dr. W. Pisula, Prof. K. Müllen Max Planck Institute for Polymer Research Ackermannweg 10, 55128, Mainz, Germany J. Soeda Department of Applied Physics Osaka University 2–1 Yamadaoka, Suita 565–0871, Osaka, Japan J. Soeda Department of Advanced Materials Science The University of Tokyo 5–1–5 Kashiwanoha, Kashiwa 277–8561, Chiba, Japan
DOI: 10.1002/adma.201403767
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reach mobilities as high as 40 cm2 V−1 s−1.[9] Many groups have reported a negative temperature dependence of the mobility which is regarded as one of the evidences for band transport.[8] Furthermore, Hall measurements provided proof for intermolecular charge coherence.[10–17] The Hall effect appears as the result of the coupling between wavenumber k of delocalized electronic states and the vector potential A as k·A. Therefore, the ideal Hall coefficient RH = 1/nq, where n is the carrier density and q is the charge of each carrier, is guaranteed only if the wavenumber k is a good quantum number, that is, in band transport systems. In fact, the equation does not hold for many disordered systems such as amorphous silicon[18] and fullerene. The ratio of the charge density and the inverse Hall coefficient, i.e., α ≡ nq/(1/RH), is called Hall factor or scattering factor.[19] The values of α turned out to be almost unity for the high-mobility organic semiconductors such as rubrene,[13] dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT),[14] 3,11didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT-NW)[15], and 1,4,8,11-tetramethyl-6,13-triethylsilylethynyl pentacene (TMTES-pentacene),[16] where the conduction is expected to follow a band transport. On the other hand, the systems with lower mobility exhibited values smaller than unity, e.g. α ∼0.6 for pentacene,[17] α ∼0.5 for 6,13-bistriisopropyl-silylethynyl pentacene (TIPS-pentacene),[16] α < 0.2 for fullerene, and α ∼0.1 for amorphous silicon.[18] Thus, the Hall factor α appears as an important parameter in discussing the transport mechanisms of organic semiconductors. Since the calculation of α is based on the charge carrier density, it is mandatory to measure the Hall coefficient and the density at the same time upon applying field-effect transistors. In contrast to small molecules, band transport in polymeric semiconductors has not been reported so far.[20] The only examples of band-like transport are heavily-doped polymers such as polyacetylene and polyaniline[21] which, however, exhibit metallic rather than semiconducting behavior. S. Wang and his coworkers have reported Hall measurements of poly(3-hexylthiophene) films with an extremely high charge carrier density gated electrochemically through ionic liquids whereby the temperature dependence of the mobility still remained in the hopping regime.[22] One of the reasons for the difficulties in realizing a band transport for polymeric systems is that the polymers intrinsically involve some kind of disorder. This follows from the occurrence of different molecular weights and chemical defects formed during polymerization.[3,23–26] Nevertheless, the recent development of novel polymer semiconductors has produced several kinds of
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renders the free surface of the liquid more stable even at high temperature owing to the S S low vapor pressure of ionic liquids compared parallel 0.3 n perpendicular to water. As a further advantage, amphiphilic N N 0.2 modification of CDT-BTZ is not necessary as S C16H33 C16H33 in the case of LB. The film was then trans0.1 ferred onto a SiO2/Si wafer previously treated 0 with 1H,1H,2H,2H-perfluorodecyltriethoxCDT-BTZ 1 2 3 4 ysilane (F-SAM) which served as gate insub Photon energy (eV) lator and gate electrode. The remaining ionic liquid was removed carefully by dipping the d substrate in dehydrated acetonitrile. The gold B V1 V2 source, drain and voltage-probe electrodes were deposited in vacuum through a shadow 100 µm Ionic liquid mask on top of the CDT-BTZ film. The CDT-BTZ films were then patterned by YAG laser etching in order to define the channel Source Drain length and width precisely (Figure 1c). glass blade The orientation and crystallinity of the films were examined by polarized optical V3 V4 absorption spectroscopy (Figure 1d), polymer compress main chain cross-polarized optical microscopy (POM, ID Figure S2, Supporting Information) and VD IG grazing-incidence X-ray wide-angle scatVG Gate tering (GIWAXS, Figure S3, Supporting Figure 1. (a) Molecular structure of CDT-BTZ polymer. (b) Compression of polymer film at the Information). Particularly, the high anisotsurface of an ionic liquid [EMIM][TFSI] to align the polymer main chains in the same direction. ropy of the optical absorptions at 1.56 eV (c) Polarized optical absorption spectra with the polarizations parallel and perpendicular to the and 2.83 eV implies the high degree of orichannel direction measured on quartz substrates. (d) Photograph of the device after the laser etching. The substrate is a SiO2/n-Si wafer treated with F-SAM. The arrow indicates the direc- entation. Since the strong absorption was tion of polymer main chains. The magnetic field B was applied perpendicularly to the substrate. observed with the polarization parallel to the channel, polymer chains are considered mostly parallel to the current flow. The dichroic ratio defined polymeric semiconductors which possess high mobilities of by the ratio of two absorbances in parallel and perpendicular 1−5 cm2 V−1 s−1.[4–6,27–29] CDT-BTZ (Figure 1a) is one of the polarization was 4.6, which is as high as typical values reported high-mobility donor-acceptor copolymers, which exhibited for LB films. The birefringence in the POM images indicated 4.6 cm2 V−1 s−1 in a fiber-based transistor.[6] The transition that the polymer chains are oriented uniformly over the area between the hopping and band transport is expected to comof 300 µm × 400 µm, which is large enough to cover one FET mence at around 1 cm2 V−1 s−1 and therefore this polymer device. The structural analysis revealed a typical assembly of holds promise for band-like transport characteristics. edge-on arranged polymer chains in layered structures (intraSince the mobility of CDT-BTZ strongly depends on the layer distance of 2.64 nm) with as evident from the appearmolecular weight,[5] a polymer with a relatively high molecance of a meridional small-angle reflection. In spite of the high ular weight of Mn = 36 kg mol−1 and a PDI of 2.6 has been degree of orientation, the polymer crystallinity was surprisingly selected as proven by detailed GPC analysis (Figure S1, Suppoor as higher order scattering intensities were missing. The porting Information). By intense workup via Soxhlet extraction lack of a characteristic π-stacking reflection expected in the with chlorinated solvents this polymer has no small molecular weight fractions. In order to maximize the structural order, equatorial wide-angle range, suggested a disordered polymer the polymer film was first deposited on the surface of an packing within the unixially oriented layers. Due to the appearionic liquid, where the polymer molecules can move relatively ance of only one reflection located on the meridional plane, the freely and rearrange their packing mode.[7] A small amount of patterns obtained parallel and perpendicular to the compression direction were identical and did not provide information CDT-BTZ solution in o-dichlorobenzene was dropped onto about the orientation of the polymer. The combined analysis of the ionic liquid, [EMIM][TFSI], which was kept at 120 °C in POM and GIWAXS implied that the edge-on arranged polymer a 1 × 1 cm metal trough. After most of the solvent had dried, chains were poorly packed in uniaxial oriented layer structures the polymer film was compressed horizontally by a glass as presented schematically in Figure S4, Supporting Informablade moving at a speed of 1 mm s−1 (Figure 1b). The comtion. The poor intralayer order of CDT-BTZ is in agreement pression is considered to induce an uniaxial orientation of the with our earlier studies.[4] polymer chains along the glass blade and hence to decrease the number of packing defects and domain boundaries. Although The source and drain electrodes of the transistor were fabrithe method looks quite similar to the Langmuir-Blodgett (LB) cated so that the current flowed along the polymer main chain. technique,[30] the film thickness of ca. 180 nm after compresHereafter the data of three devices labeled as devices A, B and C, all of which were fabricated under the similar conditions, are sion is significantly higher. In addition, the use of ionic liquid
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have exhibited a positive temperature dependence and have been understood by a hopping model. We confirmed that the temperature dependence of the mobility is not an artifact due to aging, by measuring the mobility in both directions of cooling and heating for device C. The gradual transitions from negative to temperature independent mobility indicated that these polymeric systems are located in the marginal region between hopping and band transport. The temperature dependence is consistent with the high mobility of 1–6 cm2 V−1 s−1 which is almost at the upper limit of the hopping transport model. As a reference, twoterminal mobility is also plotted in Figure S7. Because of the acceptor layer between channel layer and contact electrodes in device C, the device exhibited almost the same magnitude and temperature dependFigure 2. (a) Typical transfer characteristics of device A in the linear region at VD = −2 V at room ence for two- and four-terminal mobilities. temperature. (b) Typical output characteristics of device A at room temperature. (c) Temperature For device B without acceptor layer, on the other hand, two-terminal mobility was much dependence of the four-terminal field-effect mobility for devices B and C. lower than the four-terminal one and its temperature dependence became nearly constant as a result of the compensation by contact resistance which presented in order to clarify the reproducibility and the sampleusually increases at low temperature. to-sample variation. The detailed protocol for device fabrication For further investigation of the transport mechanism, we is summarized in the method section below. performed Hall measurements of the devices B and C by use of The transport properties of the uniaxially-oriented films were the superconducting magnet which generates a magnetic field firstly evaluated by two- and four-terminal measurements at up to 12 T. The magnetic field was swept between +12 T and room temperature. Figure 2a shows the two-terminal transfer −12 T at a slow speed of ∼0.6 cycles h−1 while the gate voltage characteristics of the device A at room temperature. The device exhibited a two-terminal mobility as high as 4.5 cm2 V−1 s−1 was swept relatively quickly at a speed of ∼24 cycles h−1. The which is one order higher than that obtained for drop-cast thin drain voltage was kept constant at −2 V during the measurefilms, 0.5 cm2 V−1 s−1, at almost same source-drain electric field ment. Although the Hall voltage should, in principle, be proportional to the magnetic field as represented by V1–V3 = of 100–130 V cm−1. The results imply that orienting polymers by mechanical compression improved the mobility significantly. RHB(t)I(t), where RH is the Hall coefficient, B(t) is the magnetic The mobility for oriented films is also higher than the reported field and I(t) is the drain current, it had a background voltage of mobility, 3.3 cm2 V−1 s−1, for drop-cast films at very high electric V1–V3 ∼200 mV even at B = 0 T due to the small inhomogeneity field, 30 kV cm−1, even though the mobility usually increases at of the film conductivity. In order to eliminate the influence of the background voltage we repeated more than three cycles of high electric field.[5] The devices B and C also gave high two-terthe sweep of the magnetic field and subtracted the background minal mobilities of 3.0 cm2 V−1 s−1 and 1.4 cm2 V−1 s−1, respecby a linear line fit. Figure 3a shows the Hall voltage (open circle) tively. The mobilities were further increased to 5.6 cm2 V−1 s−1 between V1 and V3 probes and the magnetic field (solid line) and 1.6 cm2 V−1 s−1 in four-terminal measurements owing to the elimination of the effect of contact resistance. All devices measured at VG = −45 V at 320 K for device C. The Hall voltage revealed Ohmic contact as seen in the output characteristics in clearly followed the oscillation of the magnetic field over the Figure 2b which is important for precise Hall measurements at three cycles. Device B also exhibited a quite similar Hall voltage voltage probes. as seen in Figure S5. It was also confirmed that the device charThe temperature dependence of the four-terminal mobility acteristics were stable enough during the Hall measurements was measured for devices B and C (Figure 2c). The mobility over more than five hours (Figure S6). Such observation of Hall of device B increased by 27% as the temperature decreased effect was possible for the first time by using the high-mobility from 300 K to 260 K, and became saturated below 260 K. The materials, CDT-BTZ, and the method of uniaxial alignment for mobility of device C increased by 11% as the temperature non-liquidcrystalline polymers. decreased from 320 K to 285 K, and then turned to decrease. The Hall coefficients RH were estimated by the fitting analAt the same time, the increase of sheet conductivity with ysis based on the above equation. Figure 3b displays the gate decreasing temperature was observed in high temperature voltage dependence of the reciprocal Hall coefficient at various regime for both devices (Figure S8). These results are surtemperatures for device C. The magnitudes of the Hall coefprising because all the mobilities reported for polymeric tranficients have important implications for the transport mechasistors so far, including electrochemically-gated transistors,[22] nism of the system. Although the Hall coefficient is expected
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Figure 3. (a) Hall voltage (V1 −V3) and magnetic field measured at 320 K for device C. The Hall voltage is plotted after subtracting a linear base line. Each point represents the average of successive five measured values, and the error bar shows the standard deviation of the five values. Gate and drain voltages were −45 V and −2 V, respectively. (b) Gate voltage dependence of reciprocal Hall coefficient at various temperatures for device C. Each point stands for the average of six measured values, and the error bar gives the standard deviation of the six values. The solid line shows the slope of the charge density calculated from the capacitance C = 6.75 nF cm−2.
to equal 1/nq according to the simple band theory, it has been found much smaller than expected for many disordered systems such as amorphous silicon and fullerene. To describe this effect phenomenologically, a correction factor α is introduced to the Lorentz force:[17] α·q(v × B), which finally gives the Hall coefficient RH = α/(nq). We evaluated the factor α by comparing the slopes of 1/RH and nq = C(VG – Vth) in the gate voltage dependence from Figure 3b. The data reveal that the Hall factor α is close to unity above 300 K where the carriers can feel the Lorentz force fully because of the high carrier coherence. At the same time, the temperature dependence of mobility is negative in the temperature range. These two observations consistently imply that the charge transport is near the regime of band transport. In contrast, the Hall factor α decreases down to ∼0.5 at 280 K where the carrier coherence is not enough for the carriers to feel the full Lorentz force. The temperature dependence of mobility turns to hopping-like at the same time. Based on the observations above, it can safely be concluded that the uniaxially-oriented but poorly packed CDT-BTZ transistors lie in the transitional region between hopping and band transports. As a reference, the temperature dependence of the Hall mobility defined as µH = RHσ, where σ is sheet conductance, was plotted in Figure S7. However, the Hall mobility is not intrinsic in the systems where the Hall factor α is not unity. Because RH 4
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is proportional to α and α decreases at low temperatures, the positive temperature dependence of Hall mobility should be ascribed to the temperature dependence of α. The obtained Hall factors are comparable to those of the single crystals of pentacene (α ∼ 0.5)[17] and rubrene (α = 1)[13] in spite of the much lower crystallinity of the CDT-BTZ films. We note that the temperature dependence of α in CDT-BTZ films is opposite to that for pentacene, indicating a different mechanism of the decoherence in the present polymers. In fact, the ordering of the small-molecule single crystals is quite high as observed by X-ray diffraction measurements, and is reduced by the thermal fluctuation, which results in lower α at high temperature. By contrast, the polymer films possess a high degree of inherent static disorder, so that the effect of disorder-induced decoherence appears with reduced thermal activation at lower temperature. We believe that the results of the Hall effect as well as the temperature dependence of mobility discussed above will contribute to abandon the common understanding that polymer semiconductors as disordered systems are dominated by a hopping mechanism. This breakthrough will then open new possibilities for realizing further high performance donor-acceptor type polymer semiconductors. In conclusion, we have demonstrated band-like transport characteristics of uniaxially-oriented, but weakly ordered polymeric field-effect transistors by the temperature dependence of the charge carrier mobility and Hall measurement. The mobility of the high molecular weight CDT-BTZ-based transistors was as high as 5 cm2 V−1 s−1, and further increased to 6.5 cm2 V−1 s−1 as the temperature decreased. This clearly indicates the possibility of band transport in polymeric semiconductors. Due to the poor polymer packing these values are quite surprising. The Hall voltage was detected beyond doubt and found comparable to the ideal value. The ratio of the observed Hall voltage to the ideal value, or the coherence factor, was the largest among a variety of polymeric semiconductors and even comparable to that of pentacene single crystals. These are the first observations of a band-like transport in polymeric semiconductors, and hold promise for further improvement of the performance by inducing higher order beyond the limitations of hopping transport.
Experimental Section Synthesis: The polymer was obtained by a literature-known procedure, including endcapping with bromobenzene and benzene boronic acid, standard workup and removal of residual amounts of metal ions by a metal-organic framework. Sample Preparation: Sample preparation was performed inside a nitrogen-filled glove box. The solution of CDT-BTZ in o-dichlorobenzene was prepared at 0.008 wt% for Device A and C, and 0.025 wt% for Device B. A 20 µL droplet of the solution was put on the ionic liquid of [EMIM] [TFSI], which was kept at 120 °C in a 1 × 1 cm metal trough. After most of the solvent had dried (∼70 s), the film of CDT-BTZ was compressed horizontally by the edge of a glass blade moving at the speed of 1 mm s−1. The floating film was then transferred onto the SiO2/n-Si substrate by placing the substrate horizontally on the surface of the ionic liquid. The surfaces of the substrates were treated with 1H,1H,2H,2Hperfluorodecyltriethoxysilane (F-SAM), and the thickness of the SiO2 layer was 100 nm for Device A and B, and 500 nm for Device C. The films were then cleaned carefully by dehydrated acetonitrile to get rid
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was partly supported by the Center of Innovation Program from Japan Science and Technology Agency (JST), JSPS Core-toCore Program A Advanced Research Networks, JSPS Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation, and Grant-in-Aid in Scientific Research (No. 26246011) from MEXT, Japan. W. Z. acknowledges the ERC Advanced Grant NANOGRAPH (AdG-2010–267160). Received: August 17, 2014 Revised: September 23, 2014 Published online:
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of the remaining ionic liquid, and annealed at 120 °C (Device A and B) or 200 °C (Device C) for 1 h. The gold source, drain and four voltage probes were fabricated by vacuum deposition through a shadow mask. Before the gold evaporation, an acceptor layer of 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was deposited for Device A and C in order to reduce the contact resistance. Finally the CDT-BTZ layers were etched by a laser in order to define the channel length and width, and to form Hall bars. Measurements: Two-terminal and four-terminal mobilities were estimated by the equations: µ2T = (L/W)(1/CiVD)(∂ID/∂VG) and µ4T = (1/Ci)(∂σ/∂VG). Here Ci is the capacitance per unit area and σ is sheet conductance measured by four-terminal measurement. Temperaturedependent measurements of the four-terminal mobility and Hall effect were performed in vacuum in a commercial cryostat/superconducting magnet system. Prior to the Hall measurements, the transfer characteristics were measured in the cryostat for more than six hours until the characteristics were stabilized. The magnetic field was swept between −12 T and 12 T at a slow speed of ∼0.6 cycles h−1 while the gate voltage was swept relatively quickly at a speed of ∼24 cycles h−1. The drain voltage was kept constant at −2 V. In order to eliminate the drift of background voltage, which is due to the small asymmetry of the Hall bar geometry and/or the inhomogeneity of the electric field in the film, we repeated more than three cycles of the sweep of the magnetic field and subtracted the background voltage by a linear line fit. Hall mobility was estimated by µH = RHσ.
[6] S. Wang, M. Kappl, I. Liebewirth, M. Müller, K. Kirchhoff, W. Pisula, K. Müllen, Adv. Mater. 2012, 24, 417. [7] J. Soeda, H. Matsui, T. Okamoto, I. Osaka, K. Takimiya, J. Takeya, Adv. Mater. 2014, 26, 6430. [8] M. E. Gershenson, V. Podzorov, Rev. Mod. Phys. 2006, 78, 973. [9] J. Takeya, M. Yamagishi, Y. Tominari, R. Hirahara, Y. Nakazawa, T. Nishikawa, T. Kawase, T. Shimoda, S. Ogawa, Appl. Phys. Lett. 2007, 90, 102120. [10] V. Podzorov, E. Menard, J. Rogers, M. Gershenson, Phys. Rev. Lett. 2005, 95, 226601. [11] J. Takeya, K. Tsukagoshi, Y. Aoyagi, T. Takenobu, Y. Iwasa, Jpn. J. Appl. Phys. 2005, 44, L1393. [12] T. Sekitani, Y. Takamatsu, S. Nakano, T. Sakurai, T. Someya, Appl. Phys. Lett. 2006, 88, 253508. [13] Y. Okada, K. Sakai, T. Uemura, Y. Nakazawa, J. Takeya, Phys. Rev. B 2011, 84, 245308. [14] M. Yamagishi, J. Soeda, T. Uemura, Y. Okada, Y. Takatsuki, T. Nishikawa, Y. Nakazawa, I. Doi, K. Takimiya, J. Takeya, Phys. Rev. B 2010, 81, 161306. [15] C. Mitsui, T. Okamoto, M. Yamagishi, J. Tsurumi, K. Yoshimoto, K. Nakahara, J. Soeda, Y. Hirose, H. Sato, A. Yamano, T. Uemura, J. Takeya, Adv. Mater. 2014, 26, 4546. [16] J.-F. Chang, T. Sakanoue, Y. Olivier, T. Uemura, M.-B. Dufourg-Madec, S. G. Yeates, J. Cornil, J. Takeya, A. Troisi, H. Sirringhaus, Phys. Rev. Lett. 2011, 107, 066601. [17] T. Uemura, M. Yamagishi, J. Soeda, Y. Takatsuki, Y. Okada, Y. Nakazawa, J. Takeya, Phys. Rev. B 2012, 85, 035313. [18] L. Friedman, J. Non-Cryst. Solids 1971, 6, 329. [19] J. F. Lin, S. S. Li, L. C. Linares, K. W. Teng, Solid State Electron. 1981, 24, 827. [20] 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 1999, 401, 685. [21] C. Chiang, C. Fincher, Y. Park, A. Heeger, H. Shirakawa, E. Louis, S. Gau, A. MacDiarmid, Phys. Rev. Lett. 1977, 39, 1098. [22] S. Wang, M. Ha, M. Manno, C. Daniel Frisbie, C. Leighton, Nat. Commun. 2012, 3, 1210. [23] D. P. McMahon, D. L. Cheung, L. Goris, J. Dacuña, A. Salleo, A. Troisi, J. Phys. Chem. C 2011, 115, 19386. [24] B. Watts, T. Schuettfort, C. R. McNeill, Adv. Funct. Mater. 2011, 21, 1122. [25] C. R. McNeill, J. Polym. Sci. Part B: Polym. Phys. 2011, 49, 909. [26] R. Noriega, J. Rivnay, K. Vandewal, F. P. V Koch, N. Stingelin, P. Smith, M. F. Toney, A. Salleo, Nat. Mater. 2013, 12, 1038. [27] H. Bronstein, D. S. Leem, R. Hamilton, P. Woebkenberg, S. King, W. Zhang, R. S. Ashraf, M. Heeney, T. D. Anthopoulos, J. de Mello, I. McCulloch, Macromolecules 2011, 44, 6649. [28] J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney, Z. Bao, J. Am. Chem. Soc. 2011, 133, 20130. [29] Z. Chen, M. J. Lee, R. Shahid Ashraf, Y. Gu, S. Albert-Seifried, M. Meedom Nielsen, B. Schroeder, T. D. Anthopoulos, M. Heeney, I. McCulloch, H. Sirringhaus, Adv. Mater. 2012, 24, 647. [30] J. Paloheimo, P. Kuivalainen, H. Stubb, E. Vuorimaa, P. Yli-Lahti, Appl. Phys. Lett. 1990, 56, 1157.
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Transition Between Band and Hopping Transport in Polymer Field-Effect Transistors Yu Yamashita, Junto Tsurumi, Felix Hinkel, Yugo Okada, Junshi Soeda, Wojciech Zaja˛czkowski, Martin Baumgarten, Wojciech Pisula, Hiroyuki Matsui,* Klaus Müllen, and Jun Takeya* A microscopic understanding of charge carrier transport in polymeric semiconductors, the most suited class of materials for printed and flexible semiconductor devices,[1,2] has been the central subject of organic electronics.[3] Due to the presence of inherent disorder and thermally activated structural fluctuation, it is a fundamental challenge to realize band transport within the range of charge accumulation accessible in field-effect transistors. To our knowledge, there has been no report proving band transport in polymer organic transistors. In this work, we demonstrate the Hall effect and a slightly negative temperature dependence of the charge carrier mobility in solid-gate polymeric transistors based on a polycyclopentadithiophenebenzothiadiazole donor-acceptor copolymer (CDT-BTZ).[4–6] We thereby employ a unique method for the elaborate orientation of polymer chains on the stable surface of an ionic liquid.[7] The Hall factor close to the ideal value, a maximum hole mobility of 5.6 cm2 V−1 s−1 at room temperature, and its temperature dependence indicate that the system is located in the transition region between the band and hopping transport. Charge transport mechanisms have been one of the most important issues in the field of organic semiconductors since they determine the theoretical limit of the charge carrier mobility in the system. At present, such studies have been focused particularly on small-molecule semiconductors because of the availability of single crystals as well as their high mobility.[8] Highly ordered single crystals, indeed, enabled us to Y. Yamashita, J. Tsurumi, Dr. Y. Okada, Dr. H. Matsui, Prof. J. Takeya Department of Advanced Materials Science The University of Tokyo 5–1–5 Kashiwanoha, Kashiwa 277–8561, Chiba, Japan E-mail: [email protected]; [email protected] F. Hinkel, W. Zaja˛czkowski, Prof. M. Baumgarten, Dr. W. Pisula, Prof. K. Müllen Max Planck Institute for Polymer Research Ackermannweg 10, 55128, Mainz, Germany J. Soeda Department of Applied Physics Osaka University 2–1 Yamadaoka, Suita 565–0871, Osaka, Japan J. Soeda Department of Advanced Materials Science The University of Tokyo 5–1–5 Kashiwanoha, Kashiwa 277–8561, Chiba, Japan
DOI: 10.1002/adma.201403767
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reach mobilities as high as 40 cm2 V−1 s−1.[9] Many groups have reported a negative temperature dependence of the mobility which is regarded as one of the evidences for band transport.[8] Furthermore, Hall measurements provided proof for intermolecular charge coherence.[10–17] The Hall effect appears as the result of the coupling between wavenumber k of delocalized electronic states and the vector potential A as k·A. Therefore, the ideal Hall coefficient RH = 1/nq, where n is the carrier density and q is the charge of each carrier, is guaranteed only if the wavenumber k is a good quantum number, that is, in band transport systems. In fact, the equation does not hold for many disordered systems such as amorphous silicon[18] and fullerene. The ratio of the charge density and the inverse Hall coefficient, i.e., α ≡ nq/(1/RH), is called Hall factor or scattering factor.[19] The values of α turned out to be almost unity for the high-mobility organic semiconductors such as rubrene,[13] dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT),[14] 3,11didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT-NW)[15], and 1,4,8,11-tetramethyl-6,13-triethylsilylethynyl pentacene (TMTES-pentacene),[16] where the conduction is expected to follow a band transport. On the other hand, the systems with lower mobility exhibited values smaller than unity, e.g. α ∼0.6 for pentacene,[17] α ∼0.5 for 6,13-bistriisopropyl-silylethynyl pentacene (TIPS-pentacene),[16] α < 0.2 for fullerene, and α ∼0.1 for amorphous silicon.[18] Thus, the Hall factor α appears as an important parameter in discussing the transport mechanisms of organic semiconductors. Since the calculation of α is based on the charge carrier density, it is mandatory to measure the Hall coefficient and the density at the same time upon applying field-effect transistors. In contrast to small molecules, band transport in polymeric semiconductors has not been reported so far.[20] The only examples of band-like transport are heavily-doped polymers such as polyacetylene and polyaniline[21] which, however, exhibit metallic rather than semiconducting behavior. S. Wang and his coworkers have reported Hall measurements of poly(3-hexylthiophene) films with an extremely high charge carrier density gated electrochemically through ionic liquids whereby the temperature dependence of the mobility still remained in the hopping regime.[22] One of the reasons for the difficulties in realizing a band transport for polymeric systems is that the polymers intrinsically involve some kind of disorder. This follows from the occurrence of different molecular weights and chemical defects formed during polymerization.[3,23–26] Nevertheless, the recent development of novel polymer semiconductors has produced several kinds of
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renders the free surface of the liquid more stable even at high temperature owing to the S S low vapor pressure of ionic liquids compared parallel 0.3 n perpendicular to water. As a further advantage, amphiphilic N N 0.2 modification of CDT-BTZ is not necessary as S C16H33 C16H33 in the case of LB. The film was then trans0.1 ferred onto a SiO2/Si wafer previously treated 0 with 1H,1H,2H,2H-perfluorodecyltriethoxCDT-BTZ 1 2 3 4 ysilane (F-SAM) which served as gate insub Photon energy (eV) lator and gate electrode. The remaining ionic liquid was removed carefully by dipping the d substrate in dehydrated acetonitrile. The gold B V1 V2 source, drain and voltage-probe electrodes were deposited in vacuum through a shadow 100 µm Ionic liquid mask on top of the CDT-BTZ film. The CDT-BTZ films were then patterned by YAG laser etching in order to define the channel Source Drain length and width precisely (Figure 1c). glass blade The orientation and crystallinity of the films were examined by polarized optical V3 V4 absorption spectroscopy (Figure 1d), polymer compress main chain cross-polarized optical microscopy (POM, ID Figure S2, Supporting Information) and VD IG grazing-incidence X-ray wide-angle scatVG Gate tering (GIWAXS, Figure S3, Supporting Figure 1. (a) Molecular structure of CDT-BTZ polymer. (b) Compression of polymer film at the Information). Particularly, the high anisotsurface of an ionic liquid [EMIM][TFSI] to align the polymer main chains in the same direction. ropy of the optical absorptions at 1.56 eV (c) Polarized optical absorption spectra with the polarizations parallel and perpendicular to the and 2.83 eV implies the high degree of orichannel direction measured on quartz substrates. (d) Photograph of the device after the laser etching. The substrate is a SiO2/n-Si wafer treated with F-SAM. The arrow indicates the direc- entation. Since the strong absorption was tion of polymer main chains. The magnetic field B was applied perpendicularly to the substrate. observed with the polarization parallel to the channel, polymer chains are considered mostly parallel to the current flow. The dichroic ratio defined polymeric semiconductors which possess high mobilities of by the ratio of two absorbances in parallel and perpendicular 1−5 cm2 V−1 s−1.[4–6,27–29] CDT-BTZ (Figure 1a) is one of the polarization was 4.6, which is as high as typical values reported high-mobility donor-acceptor copolymers, which exhibited for LB films. The birefringence in the POM images indicated 4.6 cm2 V−1 s−1 in a fiber-based transistor.[6] The transition that the polymer chains are oriented uniformly over the area between the hopping and band transport is expected to comof 300 µm × 400 µm, which is large enough to cover one FET mence at around 1 cm2 V−1 s−1 and therefore this polymer device. The structural analysis revealed a typical assembly of holds promise for band-like transport characteristics. edge-on arranged polymer chains in layered structures (intraSince the mobility of CDT-BTZ strongly depends on the layer distance of 2.64 nm) with as evident from the appearmolecular weight,[5] a polymer with a relatively high molecance of a meridional small-angle reflection. In spite of the high ular weight of Mn = 36 kg mol−1 and a PDI of 2.6 has been degree of orientation, the polymer crystallinity was surprisingly selected as proven by detailed GPC analysis (Figure S1, Suppoor as higher order scattering intensities were missing. The porting Information). By intense workup via Soxhlet extraction lack of a characteristic π-stacking reflection expected in the with chlorinated solvents this polymer has no small molecular weight fractions. In order to maximize the structural order, equatorial wide-angle range, suggested a disordered polymer the polymer film was first deposited on the surface of an packing within the unixially oriented layers. Due to the appearionic liquid, where the polymer molecules can move relatively ance of only one reflection located on the meridional plane, the freely and rearrange their packing mode.[7] A small amount of patterns obtained parallel and perpendicular to the compression direction were identical and did not provide information CDT-BTZ solution in o-dichlorobenzene was dropped onto about the orientation of the polymer. The combined analysis of the ionic liquid, [EMIM][TFSI], which was kept at 120 °C in POM and GIWAXS implied that the edge-on arranged polymer a 1 × 1 cm metal trough. After most of the solvent had dried, chains were poorly packed in uniaxial oriented layer structures the polymer film was compressed horizontally by a glass as presented schematically in Figure S4, Supporting Informablade moving at a speed of 1 mm s−1 (Figure 1b). The comtion. The poor intralayer order of CDT-BTZ is in agreement pression is considered to induce an uniaxial orientation of the with our earlier studies.[4] polymer chains along the glass blade and hence to decrease the number of packing defects and domain boundaries. Although The source and drain electrodes of the transistor were fabrithe method looks quite similar to the Langmuir-Blodgett (LB) cated so that the current flowed along the polymer main chain. technique,[30] the film thickness of ca. 180 nm after compresHereafter the data of three devices labeled as devices A, B and C, all of which were fabricated under the similar conditions, are sion is significantly higher. In addition, the use of ionic liquid
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have exhibited a positive temperature dependence and have been understood by a hopping model. We confirmed that the temperature dependence of the mobility is not an artifact due to aging, by measuring the mobility in both directions of cooling and heating for device C. The gradual transitions from negative to temperature independent mobility indicated that these polymeric systems are located in the marginal region between hopping and band transport. The temperature dependence is consistent with the high mobility of 1–6 cm2 V−1 s−1 which is almost at the upper limit of the hopping transport model. As a reference, twoterminal mobility is also plotted in Figure S7. Because of the acceptor layer between channel layer and contact electrodes in device C, the device exhibited almost the same magnitude and temperature dependFigure 2. (a) Typical transfer characteristics of device A in the linear region at VD = −2 V at room ence for two- and four-terminal mobilities. temperature. (b) Typical output characteristics of device A at room temperature. (c) Temperature For device B without acceptor layer, on the other hand, two-terminal mobility was much dependence of the four-terminal field-effect mobility for devices B and C. lower than the four-terminal one and its temperature dependence became nearly constant as a result of the compensation by contact resistance which presented in order to clarify the reproducibility and the sampleusually increases at low temperature. to-sample variation. The detailed protocol for device fabrication For further investigation of the transport mechanism, we is summarized in the method section below. performed Hall measurements of the devices B and C by use of The transport properties of the uniaxially-oriented films were the superconducting magnet which generates a magnetic field firstly evaluated by two- and four-terminal measurements at up to 12 T. The magnetic field was swept between +12 T and room temperature. Figure 2a shows the two-terminal transfer −12 T at a slow speed of ∼0.6 cycles h−1 while the gate voltage characteristics of the device A at room temperature. The device exhibited a two-terminal mobility as high as 4.5 cm2 V−1 s−1 was swept relatively quickly at a speed of ∼24 cycles h−1. The which is one order higher than that obtained for drop-cast thin drain voltage was kept constant at −2 V during the measurefilms, 0.5 cm2 V−1 s−1, at almost same source-drain electric field ment. Although the Hall voltage should, in principle, be proportional to the magnetic field as represented by V1–V3 = of 100–130 V cm−1. The results imply that orienting polymers by mechanical compression improved the mobility significantly. RHB(t)I(t), where RH is the Hall coefficient, B(t) is the magnetic The mobility for oriented films is also higher than the reported field and I(t) is the drain current, it had a background voltage of mobility, 3.3 cm2 V−1 s−1, for drop-cast films at very high electric V1–V3 ∼200 mV even at B = 0 T due to the small inhomogeneity field, 30 kV cm−1, even though the mobility usually increases at of the film conductivity. In order to eliminate the influence of the background voltage we repeated more than three cycles of high electric field.[5] The devices B and C also gave high two-terthe sweep of the magnetic field and subtracted the background minal mobilities of 3.0 cm2 V−1 s−1 and 1.4 cm2 V−1 s−1, respecby a linear line fit. Figure 3a shows the Hall voltage (open circle) tively. The mobilities were further increased to 5.6 cm2 V−1 s−1 between V1 and V3 probes and the magnetic field (solid line) and 1.6 cm2 V−1 s−1 in four-terminal measurements owing to the elimination of the effect of contact resistance. All devices measured at VG = −45 V at 320 K for device C. The Hall voltage revealed Ohmic contact as seen in the output characteristics in clearly followed the oscillation of the magnetic field over the Figure 2b which is important for precise Hall measurements at three cycles. Device B also exhibited a quite similar Hall voltage voltage probes. as seen in Figure S5. It was also confirmed that the device charThe temperature dependence of the four-terminal mobility acteristics were stable enough during the Hall measurements was measured for devices B and C (Figure 2c). The mobility over more than five hours (Figure S6). Such observation of Hall of device B increased by 27% as the temperature decreased effect was possible for the first time by using the high-mobility from 300 K to 260 K, and became saturated below 260 K. The materials, CDT-BTZ, and the method of uniaxial alignment for mobility of device C increased by 11% as the temperature non-liquidcrystalline polymers. decreased from 320 K to 285 K, and then turned to decrease. The Hall coefficients RH were estimated by the fitting analAt the same time, the increase of sheet conductivity with ysis based on the above equation. Figure 3b displays the gate decreasing temperature was observed in high temperature voltage dependence of the reciprocal Hall coefficient at various regime for both devices (Figure S8). These results are surtemperatures for device C. The magnitudes of the Hall coefprising because all the mobilities reported for polymeric tranficients have important implications for the transport mechasistors so far, including electrochemically-gated transistors,[22] nism of the system. Although the Hall coefficient is expected
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Figure 3. (a) Hall voltage (V1 −V3) and magnetic field measured at 320 K for device C. The Hall voltage is plotted after subtracting a linear base line. Each point represents the average of successive five measured values, and the error bar shows the standard deviation of the five values. Gate and drain voltages were −45 V and −2 V, respectively. (b) Gate voltage dependence of reciprocal Hall coefficient at various temperatures for device C. Each point stands for the average of six measured values, and the error bar gives the standard deviation of the six values. The solid line shows the slope of the charge density calculated from the capacitance C = 6.75 nF cm−2.
to equal 1/nq according to the simple band theory, it has been found much smaller than expected for many disordered systems such as amorphous silicon and fullerene. To describe this effect phenomenologically, a correction factor α is introduced to the Lorentz force:[17] α·q(v × B), which finally gives the Hall coefficient RH = α/(nq). We evaluated the factor α by comparing the slopes of 1/RH and nq = C(VG – Vth) in the gate voltage dependence from Figure 3b. The data reveal that the Hall factor α is close to unity above 300 K where the carriers can feel the Lorentz force fully because of the high carrier coherence. At the same time, the temperature dependence of mobility is negative in the temperature range. These two observations consistently imply that the charge transport is near the regime of band transport. In contrast, the Hall factor α decreases down to ∼0.5 at 280 K where the carrier coherence is not enough for the carriers to feel the full Lorentz force. The temperature dependence of mobility turns to hopping-like at the same time. Based on the observations above, it can safely be concluded that the uniaxially-oriented but poorly packed CDT-BTZ transistors lie in the transitional region between hopping and band transports. As a reference, the temperature dependence of the Hall mobility defined as µH = RHσ, where σ is sheet conductance, was plotted in Figure S7. However, the Hall mobility is not intrinsic in the systems where the Hall factor α is not unity. Because RH 4
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is proportional to α and α decreases at low temperatures, the positive temperature dependence of Hall mobility should be ascribed to the temperature dependence of α. The obtained Hall factors are comparable to those of the single crystals of pentacene (α ∼ 0.5)[17] and rubrene (α = 1)[13] in spite of the much lower crystallinity of the CDT-BTZ films. We note that the temperature dependence of α in CDT-BTZ films is opposite to that for pentacene, indicating a different mechanism of the decoherence in the present polymers. In fact, the ordering of the small-molecule single crystals is quite high as observed by X-ray diffraction measurements, and is reduced by the thermal fluctuation, which results in lower α at high temperature. By contrast, the polymer films possess a high degree of inherent static disorder, so that the effect of disorder-induced decoherence appears with reduced thermal activation at lower temperature. We believe that the results of the Hall effect as well as the temperature dependence of mobility discussed above will contribute to abandon the common understanding that polymer semiconductors as disordered systems are dominated by a hopping mechanism. This breakthrough will then open new possibilities for realizing further high performance donor-acceptor type polymer semiconductors. In conclusion, we have demonstrated band-like transport characteristics of uniaxially-oriented, but weakly ordered polymeric field-effect transistors by the temperature dependence of the charge carrier mobility and Hall measurement. The mobility of the high molecular weight CDT-BTZ-based transistors was as high as 5 cm2 V−1 s−1, and further increased to 6.5 cm2 V−1 s−1 as the temperature decreased. This clearly indicates the possibility of band transport in polymeric semiconductors. Due to the poor polymer packing these values are quite surprising. The Hall voltage was detected beyond doubt and found comparable to the ideal value. The ratio of the observed Hall voltage to the ideal value, or the coherence factor, was the largest among a variety of polymeric semiconductors and even comparable to that of pentacene single crystals. These are the first observations of a band-like transport in polymeric semiconductors, and hold promise for further improvement of the performance by inducing higher order beyond the limitations of hopping transport.
Experimental Section Synthesis: The polymer was obtained by a literature-known procedure, including endcapping with bromobenzene and benzene boronic acid, standard workup and removal of residual amounts of metal ions by a metal-organic framework. Sample Preparation: Sample preparation was performed inside a nitrogen-filled glove box. The solution of CDT-BTZ in o-dichlorobenzene was prepared at 0.008 wt% for Device A and C, and 0.025 wt% for Device B. A 20 µL droplet of the solution was put on the ionic liquid of [EMIM] [TFSI], which was kept at 120 °C in a 1 × 1 cm metal trough. After most of the solvent had dried (∼70 s), the film of CDT-BTZ was compressed horizontally by the edge of a glass blade moving at the speed of 1 mm s−1. The floating film was then transferred onto the SiO2/n-Si substrate by placing the substrate horizontally on the surface of the ionic liquid. The surfaces of the substrates were treated with 1H,1H,2H,2Hperfluorodecyltriethoxysilane (F-SAM), and the thickness of the SiO2 layer was 100 nm for Device A and B, and 500 nm for Device C. The films were then cleaned carefully by dehydrated acetonitrile to get rid
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was partly supported by the Center of Innovation Program from Japan Science and Technology Agency (JST), JSPS Core-toCore Program A Advanced Research Networks, JSPS Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation, and Grant-in-Aid in Scientific Research (No. 26246011) from MEXT, Japan. W. Z. acknowledges the ERC Advanced Grant NANOGRAPH (AdG-2010–267160). Received: August 17, 2014 Revised: September 23, 2014 Published online:
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of the remaining ionic liquid, and annealed at 120 °C (Device A and B) or 200 °C (Device C) for 1 h. The gold source, drain and four voltage probes were fabricated by vacuum deposition through a shadow mask. Before the gold evaporation, an acceptor layer of 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was deposited for Device A and C in order to reduce the contact resistance. Finally the CDT-BTZ layers were etched by a laser in order to define the channel length and width, and to form Hall bars. Measurements: Two-terminal and four-terminal mobilities were estimated by the equations: µ2T = (L/W)(1/CiVD)(∂ID/∂VG) and µ4T = (1/Ci)(∂σ/∂VG). Here Ci is the capacitance per unit area and σ is sheet conductance measured by four-terminal measurement. Temperaturedependent measurements of the four-terminal mobility and Hall effect were performed in vacuum in a commercial cryostat/superconducting magnet system. Prior to the Hall measurements, the transfer characteristics were measured in the cryostat for more than six hours until the characteristics were stabilized. The magnetic field was swept between −12 T and 12 T at a slow speed of ∼0.6 cycles h−1 while the gate voltage was swept relatively quickly at a speed of ∼24 cycles h−1. The drain voltage was kept constant at −2 V. In order to eliminate the drift of background voltage, which is due to the small asymmetry of the Hall bar geometry and/or the inhomogeneity of the electric field in the film, we repeated more than three cycles of the sweep of the magnetic field and subtracted the background voltage by a linear line fit. Hall mobility was estimated by µH = RHσ.
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