Organic Semiconductor Designed for Lamination Transfer between

Oct 15, 2005 - Meguro, Tokyo, Japan, Bell Laboratories, Lucent Technologies, 600 Mountain AVenue,. Murray Hill, New Jersey 07974, and Departments of ...
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Chem. Mater. 2005, 17, 5748-5753

Organic Semiconductor Designed for Lamination Transfer between Polymer Films Masato Ofuji,†,‡ Andrew J. Lovinger,‡ Christian Kloc,‡ Theo Siegrist,‡ Ashok J. Maliakal,‡ and Howard E. Katz*,‡,§ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S8-42 Ookayama, Meguro, Tokyo, Japan, Bell Laboratories, Lucent Technologies, 600 Mountain AVenue, Murray Hill, New Jersey 07974, and Departments of Materials Science and Engineering and of Chemistry, Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed July 22, 2005. ReVised Manuscript ReceiVed September 2, 2005

Organic field-effect transistors (FETs) may be cost-effective alternatives to amorphous-silicon transistors in certain applications. Vacuum-deposited polycrystalline organic semiconductor films exhibit equivalent mobilities to amorphous silicon and often outperform polymeric semiconductors. Here we discuss a lamination-based method for manipulating such films, featuring a thermoplastic “receiver” polymer layer that captures semiconductor films from “donor” substrates, and also acts as a gate insulator. The compound 5,5′-bis(4-isopropylphenyl)-2,2′-bithiophene (diPr-PTTP) is shown to have favorable solid-state properties for this process. Its X-ray structure is determined, and its performance and those of pentacene and copper phthalocyanine are evaluated. The influence of surface properties of the donor and receiver is examined. diPr-PTTP retained a mobility after transfer of one-third the value of the same material vapor-deposited directly onto the receiver layer.

Introduction Organic electronics is an emerging field of technology that aims to realize low-cost and environmentally friendly fabrication of electronic devices. It has drawn many researchers’ interest both from academia and industry and has seen a remarkable evolution in the past few years.1,2 Organic field-effect transistors (FETs) are now regarded as realistic alternatives to amorphous silicon transistors for relatively low speed devices, such as pixel drivers of active matrix displays3 and short-range radio frequency identification.4 Crystallographic data and procedures are given in the CIF. There are some potential advantages to organic FETs over silicon transistors, including large-area and low-temperature fabrication, which may help enable electronics such as display drivers on flexible plastic substrates. Patterning different component materials (electrodes, dielectrics, and semiconductors) is one of the key requirements for forming useful circuits from organic FETs. For example, pixel driver FETs in a backlit matrix display benefit from higher current dynamic range (on/off ratio), lower crosstalk among pixels, and higher transparency, as a result * To whom correspondence should be addressed: Department of Materials Science and Engineering, Johns Hopkins University, 103 Maryland Hall, 3400 North Charles Street, Baltimore, MD 21218. E-mail: [email protected]. † Tokyo Institute of Technology. ‡ Lucent Technologies. § John Hopkins University.

(1) Forrest, S. R. Nature 2004, 428, 911-918. (2) Katz, H. E. Chem. Mater. 2004, 16, 4748-4756. Also related articles in the same issue. (3) Gelnick G. H.; Huitema, H. E. A.; Van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; Van der Putten, Jbph.; Geuns, T. C. T.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B. H.; Meijer, E. J.; Benito, E. M.; Touwslager, F. J.; Marsman, A. W.; Van Rens, B. J. E.; De Leeuw, D. M. Nat. Mater. 2004, 3, 106-110. (4) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4412-4422.

of patterned semiconductor layers.5 Processing conditions for patterning must be carefully chosen, or else semiconductor performance can be compromised. For example, degraded semiconductor or residual solvent may give rise to increased off-current or lower mobility. Shadow masking,4 cold welding,9 soft lift-off,10-11 and surface energy-controlled nucleation12 are possible means of patterning molecular solid films. However, they require either pre-patterned and aligned masks or pre-embossed parts to define the patterning area, which can complicate the fabrication procedure. Another attractive process for patterning the organic semiconductor is the solventless movement of portions of fully formed films onto the substrate on which devices will ultimately be fabricated. This kind of process, which involves lamination of a semiconductor-coated “donor” substrate onto a “receiver” layer, and effecting the transfer of the semiconductor, has the advantage of utilizing a semiconductor film that was already formed under conditions and on smooth substrates13 that can provide desirable electrical properties, and manipulating it without exposure to solvents or reagents. This kind of process, with resolution limited either by the relief pattern of a stamp (sub(5) Kymissis, I.; Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, I. J. Vac. Sci. Technol. B 2002, 20, 956-959. (6) Sirringhaus, H.; Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 21232126. (7) Paul, K. E.; Wong, W. S.; Ready, S. E.; Street, R. A. Appl. Phys. Lett. 2003, 83, 2070-2072. (8) Afzali, A.; Dimitrakopoulos, C. D.; Graham, T. O. AdV. Mater. 2003, 15, 2066-2069. (9) Kim, C., Burrows, P. E.; Forrest, S. R. Science 2000, 288, 831-833. (10) Wang, Z.; Xing, R.; Zhang, J.; Yuan, J.; Yu, X.; Han, Y. Appl. Phys. Lett. 2004, 85, 831-833. (11) Wang, Z.; Zhang, J.; Xing, R.; Yuan, J.; Yan, D.; Han, Y. J. Am. Chem. Soc. 2003, 125, 15278-15279. (12) Steudel, S.; Janssen, D.; Verlaak, S.; Genoe, J.; Heremans, P. Appl. Phys. Lett. 2004, 85, 5550.

10.1021/cm051616w CCC: $30.25 © 2005 American Chemical Society Published on Web 10/15/2005

Lamination Transfer between Polymer Films

micrometer) or by the definition of heat and pressure regions (ten-micrometers) has been demonstrated for inorganic materials,14 electroluminescent organics,15 polymeric conductors,16 polymeric semiconductors,17 and molecular semiconductors.17a The methods used for the polymeric conductors16 further illustrate the possibility of locally directed thermal energy as a means of transferring regions of films selectively. Molecular solid films are less morphologically continuous over large areas than polymers, with distinct grain boundaries that might influence the boundaries of the transferred regions. There would be a larger barrier to breaking off a grain in a bulk region than at a predetermined boundary. There may also be a larger morphological and even electronic difference between the interfacial film region bordering the “donor” substrate and the film-air interfacial region (the “free” surface), which ultimately would end up on the “receiver” substrate and could act as the channel region of an FET built on that substrate. Pentacene devices have been made with channels incorporating both surfaces, without transferring the pentacene or comparing the surfaces separately.18 Here we discuss the crystal structure, film morphology, behavior under lamination, and FET performance of an organic semiconductor, 5,5′-bis(4-isopropylphenyl)-2,2′bithiophene (diPr-PTTP), designed for effective laminationinduced patterning for FET fabrication. The low surface energy of the isopropyl groups at the boundaries of the layers of the molecular crystal of diPr-PTTP was anticipated to be advantageous in promoting clean layer transfer. Although it is possible that medium chain n-alkyl groups would also confer low surface energies to organic semiconductors, the isopropyl group was also expected to allow easier growth of single crystals for structural analysis. Lamination behavior of diPr-PTTP was compared to two unsubstituted, commonly used vapor-deposited organic semiconductors, pentacene and copper phthalocyanine (CuPc). The isopropyl compound showed cleaner lamination transfer and a higher proportion of retained hole mobility after the transfer. Structural Analysis of iPr-PTTP A single-crystal X-ray structure of diPr-PTTP, synthesized analogously to our previously published dihexyl analogue,19 is given in Figure 1. The crystals were grown by physical vapor transport, as detailed in the Experimental Section. Two (13) Fritz, S. E.; Kelley, T. W.; Frisbie, C. D. J. Phys. Chem. B 2005, 109, 10574-10577. (14) Hur, S.-H.; Kang, D.-Y.; Kocabas, C.; Rogers, J. A. Appl. Phys. Lett. 2004, 85, 5730-5732. Sun, Y.; Rogers, J. A. Nano Lett. 2004, 4, 1953-1959. (15) Kim, C.; Cao, Y.; Soboyejo, W. G.; Forrest, S. J. Appl. Phys. 2005, 97, 113512. (16) Blanchet, G. B.; Loo, Y. L.; Rogers, J. A.; Gao, F.; Fincher, C. R. Appl. Phys. Lett. 2003, 82, 463-5.; Lee, J. Y.; Lee, S. T. AdV. Mater. 2004, 16, 51. (17) (a) Hines, D. R.; Mezhenny, S.; Breban, M.; Williams, E. D.; Ballarotto, V. W.; Esen, G.; Southard, A.; Fuhrer, M. S. Appl. Phys. Lett. 2005, 86, 163101. (b) Park, J.; Shim, S. O.; Lee, H. H. Appl. Phys. Lett. 2005, 86, 073505. (c) Chabinyc, M. L.; Salleo, A.; Wu, Y.; Liu, P.; Ong, B. S.; Heeny, M.; McCulloch, I. J. Am. Chem. Soc. 2004, 126, 12938-12939. (18) Cui, T. H.; Liang, G. R. Appl. Phys. Lett. 2005, 86, 064102. Iba, S.; Sekitani, T.; Kato, Y.; Someya, T.; Kawaguchi, H.; Takayama, M.; Sakurai, T.; Tagaki, S. Appl. Phys. Lett. 2005, 87, 023541.

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Figure 1. (Top) Chemical structure and crystallographic data of diPr-PTTP. (Bottom) Crystal structure viewed along the (left) b- and (right) c-axes, respectively. Hydrogen atoms are omitted.

Figure 2. 10 × 10 µm2 AFM image of a transferred diPr-PTTP film.

phenylenes are tilted in the same sense by 32.2° with respect to the central dithiophene plane. The crystal (film) faces, expected to be parallel to the ab surface, would be rich in low surface energy methyl groups, facilitating clean release of crystallites following lamination. Two-dimensional growth of diPr-PTTP is clearly seen in a nominally 50 nm thick film vapor-deposited on a hard, flat substrate. The top left panel in Figure 2 is an AFM image of diPr-PTTP grown on a cover glass slip. The top right panel stands for the height distribution of all pixels in the image measured from the highest pixel. The image reveals twodimensional layers equally spaced by ∼2 nm, in close agreement with the thin film XRD pattern of this sample, which shows sharp reflections with a d spacing of 1.88 nm, observable up to 6th order. However, this d spacing does not exactly match any spacing from the single crystal structure, and therefore, the thin film must crystallize in a different polymorphic form, though still clearly in a “layered solid” packing motif. All the “fingers” in the depth distribution lie within ∼20 nm from the surface, less than one-third (19) Mushrush, M.; Facchetti, A., Lefenfeld, M.; Katz, H. E.; Marks T. J. J. Am. Chem. Soc. 2003, 125, 9414-9423.

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The surfaces were joined together on a hotplate (∼110 °C). A partial contact formed, which spreads on the application of moderate pressure. The whole system was then cooled to room temperature. Because the semiconductor layer adheres more strongly to the receiving layer than to the donor layer, when the two surfaces were pulled apart, the semiconductor was found to have transferred onto the receiving layer. The transfer process can be done in ∼30 s or less. Films transferred during lamination showed well-defined edges on a micrometer scale. Figure 2, as already mentioned, shows an AFM image and cross section taken at the edge of a transferred diPr-PTTP layer. Figure 4 shows AFM images of semiconductor films both on the donor before transfer and on the receiver after transfer. The first set of images shows the top, or “free” surface of the film, while the second set shows the growth interface, originally with the donor substrate, but now exposed as the new top surface of the film. diPr-PTTP films were transferred reproducibly from TF donor layer without any critical film fracture, while other donor layers gave little or no transfer; in those latter cases the diPr-PTTP films were wetted by receiving layers, but were hardly transferred onto receiving layers. Here, the very low surface energies of both the semiconductor and the TF donor layer synergistically promote transfer of the entire layer. In our initial demonstration of the transfer process, receiving layers are heated evenly on a hotplate without attempting to define specific regions for transfer. However, we were able to induce wetting and transfer selectively in regions where localized pressure was applied, resulting in an area-selective transfer as shown in Figure 3b. Figure 3. (a) A schematic of hot-melt transfer. (b) A photograph of donor (left) and receiving (right) layers. The transfer of a 50 nm thick pentacene film occurred only where the pressure was applied with a jig (∼7 × 7 mm2). (c) Test device geometry.

of the film thickness determined by Figure 5 ()70 nm). This suggests that the film is composed of domains highly interconnected with other domains laterally. Lamination Experiments on diPr-PTTP Figure 3a shows a schematic of our lamination experiment. First, small molecules are vacuum-deposited (50 nm thick) on donor substrates to make semiconductor transfer layers. Substrates can act as donors if they have lower tackiness and higher glass transition temperature (Tg) relative to the receiving layer. We tried three different polymer films as donor layers: poly(trifluoroethyl methacrylate) (TF), poly(cyclohexyl methacrylate) (CH), and poly(4-methylstyrene) (4M). Receiving layers were prepared by spin-casting poly(butyl methacrylate) (PBMA) films 1-2 µm thick on ITO/ glass surfaces. We selected PBMA because its surface energy and mechanical properties change significantly on modest heating. This polymer is also suitable for use as a gate dielectric in an FET. Hardening of this kind of “soft” dielectric could be accomplished by adding a thermal or photocuring functionality to the polymer and performing a post-transfer curing step.

Electronic Properties and Comparison with Pentacene and Copper Phthalocyanine (CuPc) Test FETs were completed by vacuum deposition of gold source-drain electrodes on top of semiconductor films, where the initial substrate had a conductive surface to serve as the gate, as in Figure 3c. For semiconductor films transferred by lamination, the PBMA receiver layer served as gate dielectric. Transfer characteristics of FETs using laminated pentacene, CuPc, and diPr-PTTP are shown in Figure 5, and data are summarized in Table 1. On/off ratios ranged from 103 to 104 and could well be higher if devices were isolated. Lamination-transferred diPr-PTTP films retained relatively high mobility (µ ) 4.5 × 10-3 cm2/V s) compared to control FETs directly vapor-deposited onto PBMA (µ ) 1.4 × 10-2 cm2/V s). On the other hand, laminated pentacene and CuPc had mobilities 1 order of magnitude less than their control FETs. As mentioned above, diPr-PTTP shows clear twodimensional growth, with a flatter as-grown “free” surface than those of the other semiconductor films. The transferred film has similar topography to the original film; presumably this is the reason the laminated diPr-PTTP FETs retain higher proportional mobilities to originally deposited controls. By way of contrast, Figure 6 shows optical micrographs of the receiving layers that captured the other two semiconductor films. Laminated pentacene and CuPc had vague boundaries, composed of small (∼10 µm) islands of transferred films

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Figure 4. 5 × 5 µm2 AFM images of the semiconductor films. Upper row: donor films imaged before transfer showing the as-grown, or free, surfaces of the films. Lower row: receiver films imaged after transfer showing the growth interfaces exposed as the new top surface of the films.

Figure 5. Transfer characteristics of the laminated FETs. (Top) Pentacene (donor layer ) 4M), (center) CuPc (donor layer ) glass, vacuum-deposited at 150 °C), and (bottom) diPr-PTTP (donor layer ) TF). VD ) -100 V in all cases, ensuring the FETs are in saturation regime throughout the VG sweep range. Chemical structures are also shown. Table 1. Mobilities of Semiconductor Films (cm2/V s, Saturation Mode) compound pentacene CuPc, sublimation on ambient substrate CuPc, sublimation on 150-deg substrate diPrPTTP

donor surface

µ(laminated)

µ(vapor-deposited on PBMA)

4M glass

0.024 0.00046

0.29 0.0076

glass

0.0039

0.0003

TF

0.0045

0.014

scattered around main bodies of the laminated films. DiPrPTTP was the only semiconductor of the three to make defined edges reproducibly. Transferred pentacene films showed the best mobility (measured in saturation mode) when they were laminated

Figure 6. Optical micrographs of the edge of the semiconductor films transferred on receiving layers. (Top) Pentacene, (center) CuPc (deposited at 150 °C), and (bottom) diPr-PTTP.

from 4M donor layer (µ ) 2.4 × 10-2 cm2/V s), compared to those from other donor layers (µ ) 5-8 × 10-3 cm2/V s for glass, CH, and TF donor layers). This is consistent with XRD results we obtained on as-deposited pentacene films on different donor layers (i.e., samples before transfer) where 4M sample showed stronger (001) reflection intensity by 40% than TF and CH counterparts, indicating an improved (001) texture in the film. However, these mobilities in transferred layers were 1 order of magnitude less than those in control FETs, whose pentacene layers were directly vacuumdeposited on the receiving layers (µ ) 0.29 cm2/V s). Experiments with CuPc point out how the lamination process can enable use of films that cannot be formed by direct vapor deposition. CuPc grown at ambient temperature

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and processed by lamination had a mobility of 4.6 × 10-4 cm2/V s. Use of a donor substrate temperature held at 150 °C during deposition gave a mobility after transfer to a receiver substrate of 3.9 × 10-3 cm2/V s. The temperaturedependent grain size of as-deposited CuPc observed by AFM was consistent with literature.13 Control FETs had mobilities of 7.6 × 10-3 cm2/V s (grown at room temperature) and ∼3 × 10-4 cm2/V s (150 °C), showing that at 150 °C the surface of the receiving layer was not stable enough to support a continuous CuPc layer on it. This implies that this transfer method can introduce some flexibility in fabrication of FET; i.e., a semiconductor film was formed on PBMA by lamination that could not have been formed by direct vapor deposition because the PBMA substrate was not compatible with the necessary deposition conditions. Still, the absolute and proportional retained mobilities were lower than those for diPr-PTTP. It is notable that pentacene21-23 and CuPc20,24 show similar profiles after transfer to upper surface profiles that have been published elsewhere for as-deposited thin films. They have flatter donor layer/semiconductor interfaces ()newly exposed surfaces by lamination) than as-grown top surfaces. This is consistent with the fact that most organic (and also inorganic) compounds deposited on a flat, inert substrate show twodimensional layers along the substrate at the first stage of growth, followed by three-dimensional domain growth.25 In our laminated FETs the top surfaces of as-deposited films become conducting channels. When an organic FET is in accumulation mode, the carriers are strongly confined in the semiconductor within a few monolayers of the interface with the dielectric.26-28 Therefore, a rougher semiconductor/ dielectric interface should make more localized carrier traps and lead to poorer FET mobility. Of course, there are other factors that limit carrier mobility in FETs, for example, defect density in the semiconductor film28 and intrinsic mobility of the semiconductor. (Degraded performance in pentacene FETs sometimes results from partial phase transition of pentacene from thin-film phase to bulk phase,29,30 but XRD shows that this is not the case in our system; i.e., there was no trace of a heat-induced transition after transfer.) By contrast, the lower right panel of Figure 7 shows that in pentacene films most of the film volume lies close to the substrate, with significant penetration of the AFM needle through a large fraction of the film thickness. This means that its domains connect to each other only through several molecular layers on their bottom, suggesting poor cohesion (20) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996, 69, 3066-3068. (21) Lin, Y.-Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Trans. Electron DeV. 1997, 44, 1325-1331. (22) Ruiz, R.; Choudhary, D.; Nickel, B.; Toccoli, T.; Chang, K. C.; Mayer, A. C.; Clancy, P.; Blakely, J. M.; Headrick, R. L.; Iannotta, S.; Malliaras, G. G. Chem. Mater. 2004, 16, 4497-4508. (23) Kato, Y.; Iba, S.; Teramoto, R.; Sekitani, T.; Someya, T.; Kawaguchi, H.; Sakurai, T. Appl. Phys. Lett. 2004, 84, 3789-3791. (24) Ofuji, M.; Inaba, K.; Omote, K.; Hoshi, H.; Takanishi, Y.; Ishikawa, K.; Takezoe, H. Jpn. J. Appl. Phys. 2003, 42, 7520. (25) Forrest, S. R. Chem. ReV. 1997, 97, 1793-1896. (26) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270-271. (27) Alam, M. A.; Dodabalapur, A.; Pinto, M. R. A. IEEE Trans. Electron. DeV. 1997, 44, 1332-1337. (28) Horowitz, G.; Hajlaoui, M. E. AdV. Mater. 2000, 12, 1046-1050. (29) Gundlach, D. J.; Jackson, T. N.; Schlom D. G.; Nelson S. F. Appl. Phys. Lett. 1999, 74, 3302-3304.

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Figure 7. (Top left) A 5 × 5 µm2 AFM image of a diPr-PTTP film vacuumdeposited on the glass surface. (Top right) The histogram showing depth distribution of each pixel in the left image. (Bottom left and right) A 5 × 5 µm2 AFM image and the histogram of a pentacene film vacuum-deposited on the glass surface.

among domains. This observation is consistent with a very recent study of dependence of pentacene morphology and mobility on a number of layers in the film.31 The film volume is better distributed in diPr-PTTP. The transfer of significant amounts of donor polymer along with the semiconductor film is probably not a factor in the transfer mechanism. First, pentacene was transferred from a glass substrate coated with only a monolayer of octadecylsilyl groups, a situation where no donor material was available for transfer. Second, root-mean-square roughnesses of transferred films can be as small as 1-2 nm, below the hydrodynamic radius of even single chains of the donor polymers. Third, the transferred films were tested as FETs with top contacts; substantial thicknesses of resistive material between the films and contacts would have greatly diminished the observed currents. The clarity of the edge is related to the cohesion of the grains in the semiconductor film. For example, a film with high cohesion among domains would break along a definite line in the film. For example, the edge of diPr-PTTP is welldefined even on a microscopic scale, as seen in Figure 6. On the other hand, a film with poor grain cohesion such as pentacene and CuPc can make a more complex edge, breaking along grain boundaries. Lamination and related dry transfer techniques could lead to new methods for manufacturing organic FETs at low cost and high processing speed. Our results along with others obtained in parallel, as discussed in the Introduction, suggest that dry transfer methods can in principle be used for organic semiconductor-based technologies. Many existing semiconductor cores could easily be terminated with simple, low surface energy substituents such as isopropyl and trifluoromethyl to facilitate transfer of planar polycrystalline films. (30) Laudise, R. A.; Kloc, Ch.; Simpkins, P. G.; Siegrist, T. J. Cryst. Growth 1998, 187, 449-454. (31) Ruiz, R.; Papadimitratos, A.; Mayer, A. C.; Malliaras, G. AdV. Mater. 2005, 17, 1795-1798.

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Processes that rely on an “upper” surface of a vapordeposited organic semiconductor to form the channel can provide mobility on the same order as the “lower” surface, provided the semiconductor is judiciously chosen and the surfaces are smooth enough.32 Experimental Section Synthesis of 5,5′-Bis(4-isopropylphenyl)-2,2′-bithiophene (diPrPTTP). A solution of 1.8 g of 5,5′-bis(tributylstannyl)-2,2′bithiophene, 1.3 g of 1-bromo-4-isopropylbenzene, and 80 mg of Pd(PPh3)4 in 35 mL of N,N-dimethylformamide was heated at 70 °C for 2 days under nitrogen. After cooling, 0.6 g of yellow-green solid was obtained after washing with methanol and ether, pure by NMR, and used directly for film and crystal studies. 1H NMR (CDCl3): δ 7.55 (d, J ) 8.4 Hz, 4H), 7.27 (d, J ) 8.4 Hz, 4H), 7.21 (d, J ) 3.86 Hz, 2H), 7.16 (d, J ) 3.86 Hz, 2H), 2.95 (septet, J ) 6.81 Hz, 2H), 1.30 (d, J ) 6.81 Hz, 12H) ppm. 13C NMR (CDCl3): δ 148.5, 143.2, 136.3, 131.7, 127.0, 125.6, 124.3, 123.3, 33.8, 23.9 ppm. UV-Vis spectrum in THF: λmax ) 378 nm. HRMS Calcd for C26H26S2: 402.14759. Found: 402.14708. Crystal Studies. DiPr-PTTP single crystals were grown by a horizontal physical vapor transport in a stream of argon gas.23 The evaporating material was heated to 240 °C in the hot zone of a two-zone furnace. The second zone was held at room temperature. DiPr-PTTP single crystals spontaneously grew on a glass tube wall between the two zones. The crystal structure of these needles was investigated using an Oxford-Diffraction Xcalibur-2 diffractometer using graphite monochromated molybdenum radiation. Crystallographic data were deposited with the Cambridge Crystallographic Data Centre, CCDC 259208. Synthesis of Poly(trifluoromethyl methacrylate) (TF). TF was synthesized by polymerizing 16 g of inhibitor-free 2,2,2-trifluoroethyl methacrylate with 0.1 g of azobis(isobutyronitrile) initiator in 50 mL of acetone heated nearly to reflux under nitrogen, adding the initiator as a solution in 10 mL of acetone over 3 h, heating 3 additional hours, and precipitating into 300 mL of 50% aqueous methanol.) (32) Gelinck, G. H,; van Veenendaal, E.; Coehoorn, R. Appl. Phys. Lett. 2005, 87, 073508.

Chem. Mater., Vol. 17, No. 23, 2005 5753 Preparation of Donor Layers. TF, poly(4-methylstylene) (4M) (Aldrich), and poly(cyclohexyl methacrylate) (Aldrich) were spincoated from 5 to 10 wt % cyclopentanone (Aldrich, HPLC grade) solution on RIE-cleaned microscope cover glass slides (VWR #2) and baked at 150 °C for >12 h. As-cleaned slides were used as glass donor layers. Organic semiconductors (pentacene (Aldrich, as-received), CuPc (Aldrich, 3 times sublimed), diPr-PTTP (assynthesized)) were vacuum-deposited under background pressure of 12 h, yielding 0.6 µm thick films, typical capacitance of Ci ) 4.9 nF/ cm2. Lamination (Performed in Air) and FET Fabrication. A receiving substrate, a donor substrate, and another slab of glass (1 mm thick, for more even pressure) were stacked on a hotplate (110120 °C). Pressure was applied (1.4 kg/cm2 for 30 s f 8 kg/cm2 for 5 s) with a brass rod tipped with a rubber sheet (area ∼0.5 cm2). Both substrates were removed immediately from the hotplate onto the tabletop to quench and mechanically pulled apart. The transfer area was strongly dependent on the temperature of the hotplate; there seemed to be a “threshold” temperature below which the transfer hardly occurred. Gold was vacuum-deposited through a shadow mask which defined channel width and length: W ) 2.5-5 mm, L ) 200 um. FET characteristics were measured with tungsten probes and HP4155B semiconductor parameter analyzer.

Acknowledgment. This work was done during M.O.’s internship at Bell Laboratories, supported by TokyoTech COE21 program “Nanomaterial Frontier Cultivation for Industrial Collaboration”. The authors acknowledge E. Reichmanis at Bell Labs, Geoffrey Nunes at Dupont, and Michael Kane and Arthur Firester at Sarnoff for thoughtful discussions. This work was done as a contribution to a collaboration with Sarnoff and DuPont Corporations, supported by NIST, ATP cooperative Agreement number 70NANB2H3032. CM051616W