Orthogonal Ambipolar Semiconductor Nanostructures for

Aug 22, 2016 - Films of these aligned crystalline semiconductors were used to fabricate complementary inverter devices, which exhibited good switching...
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Orthogonal Ambipolar Semiconductor Nanostructures for Complementary Logic Gates Weiguo Huang,† Jens C. Markwart,†,‡ Alejandro L. Briseno,*,† and Ryan C. Hayward*,† †

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States Department of Chemistry, Johannes Gutenberg University Mainz, 55128 Mainz, Germany

‡

S Supporting Information *

ABSTRACT: We report orthogonal ambipolar semiconductors that exhibit hole and electron transport in perpendicular directions based on aligned films of nanocrystalline “shishkebabs” containing poly(3-hexylthiophene) (P3HT) and N,N′-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PDI) as p- and n-type components, respectively. Polarized optical microscopy, scanning electron microscopy, and X-ray diffraction measurements reveal a high degree of in-plane alignment. Relying on the orientation of interdigitated electrodes to enable efficient charge transport from either the respective p- or n-channel materials, we demonstrate semiconductor films with high anisotropy in the sign of charge carriers. Films of these aligned crystalline semiconductors were used to fabricate complementary inverter devices, which exhibited good switching behavior and a high noise margin of 80% of 1/2 Vdd. Moreover, complementary “NAND” and “NOR” logic gates were fabricated and found to exhibit excellent voltage transfer characteristics and low static power consumption. The ability to optimize the performance of these devices, simply by adjusting the solution concentrations of P3HT and PDI, makes this a simple and versatile method for preparing ambipolar organic semiconductor devices and high-performance logic gates. Further, we demonstrate that this method can also be applied to mixtures of PDI with another conjugated polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]) (PBTTT), with better hole transport characteristics than P3HT, opening the door to orthogonal ambipolar semiconductors with higher performance. KEYWORDS: orthogonal ambipolar semiconductor, shish-kebab nanocrystal, anisotropic charge carrier transport, complementary logic gates, organic electronics

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and current on/off ratios. However, as these materials rely on transport of both electrons and holes within the same spatial domain, it is fundamentally challenging to cleanly separate and independently optimize the p- and n-type behaviors. Indeed, most reported ambipolar semiconductors show unbalanced hole and electron mobilities, much better p-type switching than n-type switching, and high mobilities only at the expense of decreased current on/off ratios.19−21 Therefore, the fabrication of complementary logic gates that takes advantage of both pand n-type behavior within the same material remains a key challenge. For example, when used in complementary inverters, the inability for most ambipolar transistors to be fully switched off (because the electron current rises before the hole current completely disappears) tends to lead to incomplete voltage switching,8,19−21 as well as excessive leakage and power dissipation,24 while unbalanced charge transport behavior

mbipolar semiconductors have become increasingly attractive in recent years,1−8 as their ability to transport both holes and electrons within the conducting channel opens possibilities for fabricating more compact complementary metal-oxide semiconductor (CMOS) circuits, as well as enabling new optoelectronic device technologies (e.g., ambipolar light-emitting transistors).5,8 However, the number of intrinsically ambipolar semiconductors is small, with Cu- and Fe-phthalocyanine,9 rubrene single crystals,10 and carbon nanotubes11,12 representing the bulk of reports to date. Additionally, their charge carrier transport behavior is highly dependent on the work function of the metal electrodes, the interface between semiconductor crystal and gate dielectric layer, and the test conditions such as oxygen concentration and humidity.8 There have been numerous reports of approaches to obtain ambipolar semiconductors by co-evaporation or solution blending of p- and n-type semiconductors13−16 or copolymerization of electron-donor and acceptor building blocks,17−23 resulting in high-performance materials with high mobilities © 2016 American Chemical Society

Received: June 14, 2016 Accepted: August 15, 2016 Published: August 22, 2016 8610

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Scheme 1. Schematic Illustrations of the Capillary-Assisted Drop Casting Method and the Resulting Aligned P3HT/PDI ShishKebab Nanocrystal Film Exhibiting Anisotropic Hole and Electron Mobility

type transistors using the same active layer, simply by changing the orientation of the channel connecting the source and drain electrodes. In this way, the operation of each channel does not interfere with the other, overcoming one of the main drawbacks associated with traditional ambipolar semiconductors. Additionally, only a single drop-casting step is needed to form the shish-kebab film, avoiding the need for multiple fabrication steps to separately pattern both p- and n-type materials as for traditional unipolar semiconductors. Thus, these orthogonal ambipolar semiconductors combine the best features of traditional unipolar and ambipolar semiconductors. Furthermore, the electron and hole mobilities, and threshold voltages of both p and n channels, can be readily tuned simply by adjusting the concentrations of P3HT and PDI in solution. Based on the directionally separated channels for electron and hole transport and the easily balanced charge transport properties and switching behaviors, we demonstrate a simple route to complementary logic gates (inverters, NAND and NOR logic gates) with low leakage current and excellent switching behavior, using only a single semiconducting active layer, thus providing a powerful tool for the fabrication of organic electronic devices.

pushes the switching voltage away from its ideal value of half of the drain voltage, 0.5 Vdd.20 Further, the current on/off ratios for both holes and electrons are low under the operating conditions typically used for complementary logic devices, resulting in low gain values in ambipolar organic field-effect transistor (OFET)-based inverters.25,26 Notably, the use of ambipolar semiconductors as active layers in more advanced complementary logic gates is even more limited, likely due to the same issues described above for inverters. One report has described dicyanomethylene-substituted quinoidal quaterthiophene [QQT(CN)4] (p-typedominant, ambipolar semiconductor) as a single active layer for complementary NAND and NOR logic gates, however, thermal annealing or laser irradiation was required to convert selected regions of the film from showing p- to n-type behavior.27 So far, only carbon nanotubes have been successfully used to fabricate NAND and NOR logic gates using a single region of an ambipolar semiconducting material as the active layer, but these devices have been restricted to topgate configurations,11,12,28 thereby limiting device fabrication conditions and material selection;29−31 or have required a separate patterning step to dope selected regions of the material.7 In addition, the necessity to separate mixtures of carbon nanotubes to achieve those with a single type of behavior is time-consuming and material inefficient.32−35 Here, we introduce an ‘orthogonal ambipolar semiconductor’, capable of transporting holes and electrons in perpendicular directions, based on heteronanocrystals of the p-type semiconductor polymer poly(3-hexyl thiophene) (P3HT) and the n-type semiconductor small molecule N,N′di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PDI). Recently, our group described how casting from solutions containing a mixture of these components gives rise to “shish-kebab” structures with long crystalline PDI nanowires flanked by perpendicularly oriented crystalline P3HT fibers.36,37 Here, we study the electrical properties of such shish-kebab structures in OFET devices and show that films of these heterostructures with a high degree of alignment can be obtained, allowing for electron, but not hole, transport along the PDI shish direction, compared to hole, but not electron, transport along the orthogonal direction (Scheme 1). Significantly, this enables the construction of either p- or n-

RESULTS AND DISCUSSION As detailed previously, P3HT/PDI shish-kebabs can be readily prepared by casting from mixed solutions in o-dichlorobenzene (o-DCB).36,37 P3HT modifies the growth of PDI crystals, leading to formation of extended PDI nanowires with narrow cross-sectional dimensions, which in turn serve as heterogeneous nucleation sites for P3HT fibrils (Scheme 1).36−38 The nanostructure of the resulting shish-kebabs is determined by the concentrations of P3HT and PDI, with higher P3HT concentrations giving a thinner shish and longer kebabs, and vice versa for PDI. As the long axes of both PDI nanowires and P3HT fibrils correspond to directions of good overlap between π-conjugated systems of neighboring molecules, these shish-kebab nanostructures are expected to show highly anisotropic hole and electron mobilities. However, achieving macroscopic anisotropy in transport behavior requires a high degree of global alignment. Motivated by this, we employed a capillary-assisted drop casting method, reported by Pyo and co-workers to yield 8611

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Figure 1. SEM images of nanocrystalline films grown by capillary-assisted drop casting of solutions containing 2.0 mg/mL PDI in o-DCB (a) without P3HT and with (b) 0.1 mg/mL, (c) 0.2 mg/mL and (d) 0.5 mg/mL P3HT (black arrows indicate the location of P3HT kebabs). (e,f) Polarized optical micrographs of an aligned shish-kebab film (0.2 mg/mL P3HT and 2.0 mg/mL PDI) with the PDI crystals oriented at (e) 0 and (f) 45° relative to the polarizer (crossed arrows represent the polarization directions of polarizer and analyzer).

highly oriented films of bis(triisopropylsilylethynyl)pentacene.39 Specifically, a cleaned Si/SiO2 substrate was heated to 80 °C, a dust-free clean glass capillary tube with an outer diameter of 1.2 mm was placed on the substrate, the P3HT/PDI solution in o-DCB was added dropwise to the capillary tube (spreading quickly across the sides and length of the tube), and finally, solvent was dried at 80 °C for 1 h. This process resulted in a well-aligned compactly packed shish-kebab film, with PDI nanowires oriented normal to the long axis of the capillary tube and P3HT fibrils oriented parallel (Scheme 1). As shown in Figure S1, the resulting film is not continuous, but instead shows periodic bands of dense structure separated by nearly bare substrate regions, due to pinning and depinning of the solution contact line (i.e., the

“coffee ring effect”), as reported previously.16,39−41 The morphology of the resulting film is highly dependent on P3HT concentration. For a solution of PDI alone in o-DCB (2.0 mg/mL), a nearly featureless film of PDI was formed (Figure 1a); interestingly, however, adding only 0.1 mg/mL of P3HT to the solution yields a pronounced change of morphology to compactly packed and well-aligned fibers with typical widths of ∼220 nm (Figure 1b). Increases in P3HT concentrations to 0.2 and 0.5 mg/mL (Figure 1c,d) led to further reductions in PDI fiber size to 140 and 110 nm, respectively. As the solution contact line draws closer to the capillary tube, the pinning and depinning period becomes shorter, and thus the lengths of PDI nanocrystals gradually decrease as the distance between capillary and band decreases, 8612

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Figure 2. 2D GIXD patterns for nanocrystal films cast from solutions containing 0.1 mg/mL P3HT and 2.0 mg/mL PDI: (a) without capillaryassisted drop casting; (b,c) with capillary-assisted drop casting and X-rays incident (b) parallel and (c) perpendicular to the PDI crystal long axis. The black indices represent reflections assigned to PDI crystals and red to P3HT. The film plane is oriented vertically.

long axes of PDI nanowires reveals a high degree of in-plane orientation. In particular, Figure 2b shows the 010 reflection nearly parallel to the substrate (with a sequence of 01l reflections moving along qz), while these reflections are completely absent from Figure 2c, as expected for a highly oriented sample of PDI crystals (see Figure S6 for simulated single crystal diffraction patterns). In contrast, Figure 2c shows the 100 and 110 reflections nearly along the film plane (and associated sequence of 10l and 11l reflections moving along qz), consistent with highly oriented films with X-rays nearly perpendicular to the long axis of PDI nanowires (Figure S6). In addition to the relatively strong scattering from PDI, weaker diffraction spots along qz (marked in red) corresponding to the {h00} planes of P3HT oriented parallel to the substrate can be seen. Since the P3HT concentration is lower than that of PDI, the intensities of diffraction peaks from P3HT are considerably weaker that those from PDI, which may make the peaks from P3HT appear narrower (with respect to out-of-plane orientation) in Figure 2. However, the azimuthal intensity profiles from the GIXD pattern for the 001 diffraction arc of PDI and the 100 arc of P3HT have almost equal breadths (fullwidths-at-half-maximum of 7−8°, Figure S10), indicating a similar level of out-of-plane alignment for both materials. Because the long axes of P3HT and PDI crystals are almost completely in-plane, we can thus express their degree of inplane orientation using the 2D order parameter S2 = 2⟨cos2(φ)⟩ − 1, where φ is the misorientation angle of a given crystal from the average alignment direction. For perfectly oriented samples, S2 = 1, and for unoriented samples, S2 = 0. By casting an aligned shish-kebab film on a thin Kapton substrate and measuring wide-angle X-ray diffraction in transmission mode, we used the azimuthal intensity variations of the PDI 100 peak and P3HT 020 peaks (which are perpendicular to the long axes of PDI and P3HT crystals, respectively) to determine S2, as described in more detail in Figures S7−S9.44 We obtained reasonably high values of S2 = 0.80 for PDI and 0.82 for P3HT, consistent with the high degree of alignment seen from cross-polarized optical microscopy and GIXD measurements. Having established a route to well-oriented shish-kebab films, we proceeded to study the electrical properties of these heteronanocrystals using OFET devices. Balancing electron and hole mobility is a fundamental challenge to obtaining highperformance organic semiconductor logic circuits.39,45 Gen-

whereas their widths show negligible changes (Table S1). As seen in Figure 1c, P3HT crystals nucleated from the PDI fibers also become apparent at 0.2 mg/mL (Figure 1c) and grow in length at 0.5 mg/mL to give long P3HT kebabs oriented perpendicular to the PDI shish (Figure 1c,d, black arrows). Importantly, the P3HT fibrils nucleated from a given PDI crystal are long enough that they can cross over (and possibly under) neighboring PDI crystals, as also confirmed by energydispersive X-ray spectroscopy measurements indicating the presence of large amounts of sulfur in these regions (Figure S2) and therefore bundle together with other fibrils to create a wellconnected pathway for hole transport. Crossed-polarized optical microscopy provides further evidence for the high degree of crystal alignment; as shown in Figure 1e,f, with the polarizer oriented parallel to the PDI crystal axis, we observed an almost completely dark field, while at 45°, the sample showed a bright and uniform striated texture. The alignment of shish-kebab nanocrystals in the band farthest from the capillary is typically not as good as others, because the shape of this band is affected by the initial solution contact line with the substrate upon spreading of the drop (Figure S3). However, the nanocrystals in all other bands are well aligned, with no clear differences in the degree of alignment, as evidenced by both polarized optical microscopy (Figure S4) and the similar characteristics for devices prepared using different bands (Figure S5). Further information on the structure of aligned P3HT/PDI shish-kebab films was obtained using 2D grazing-incidence Xray diffraction (GIXD). The diffraction pattern for a film prepared by drop casting without a capillary is shown in Figure 2a; the presence of broad arcs for diffraction spots corresponding to many different planes reveals a high degree of misorientation.42 Most peaks can be assigned as denoted, based on the triclinic crystal structure with unit cell parameters a = 4.68 Å, b = 8.50 Å, c = 19.72 Å, α = 85.99°, β = 88.43°, γ = 82.79°, previously established for this PDI molecule.43 In contrast, GIXD patterns for films prepared by capillary-assisted drop casting (Figure 2b,c) show shorter arcs, indicating greater orientation with respect to the surface normal. For the aligned films, the PDI nanowires are seen to orient with {00l} crystal planes preferentially parallel to the substrate. Of greater importance in terms of anisotropic charge mobility, the clear difference in observed diffraction spots when the X-rays are incident parallel (Figure 2b) vs perpendicular (Figure 2c) to the 8613

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Figure 3. Schematic view of bottom-contact bottom-gate orthogonal ambipolar transistor (W = 100 μm and L = 10 μm) based on P3HT/PDI (0.2 mg/mL P3HT + 2.0 mg/mL PDI in o-DCB) shish-kebab crystal film (top); charge transport behavior in the parallel direction (a, c) and perpendicular direction (b, d).

while P3HT concentrations above 0.6 mg/mL gave p-type dominant ambipolar behavior (Figure S11). Further, as shown in Table S2, the electron and hole mobilities could be continuously tuned by adjusting the concentration of P3HT. Optimal performance was found at 0.2 mg/mL P3HT and 2.0 mg/mL PDI, and thus we fixed these values for the remainder of the experiments described here. In the “parallel” direction (along the long axes of PDI nanocrystals), we observed electron transport behavior but no hole transport behavior, as evidenced by the efficient n-type switching (Figure 3a,c) and lack of p-type switching (Figure S12a). Electron mobility values of 1.09 ± 0.18 × 10−3 cm2/(V s), current on/off ratios of 103, and threshold voltages of 8.0 ± 7.2 V were observed. Along the perpendicular direction (Figure 3b,d), values of hole mobilities of 4.8 ± 0.5 × 10−4 cm2/(V s), current on/off ratios of 102, and threshold voltages of 8.5 ± 6.7 V were determined, with no electron transport behavior (Figure S12b and Table S3). As an explanation for these electrical characteristics, we suggest that in the parallel direction, individual PDI nanocrystals are sufficiently long to span from source to drain electrodes (Figure S1), thus giving typical electron transport behavior, while the P3HT fibrils are discontinuous along this direction, preventing hole transport. Along the perpendicular direction,

erally, it is very difficult to match two unipolar p- and n-type semiconductors due to their different electron and hole mobilities. To overcome this obstacle, different channel widths are typically used for p- and n-type semiconductors to match their mobilities and therefore source/drain currents. However, matching these characteristics through variations in photolithographically patterned features can be time-consuming and furthermore can increase the size of the circuit and complicate device fabrication.7,46 For ambipolar polymers, obtaining balanced electron and hole mobility requires subtle adjustments to donor and acceptor building block combinations, requiring extensive chemical synthesis.17−21 In the current work, we can readily tune the electron and hole mobilities of the shish-kebab films over a wide range simply by adjusting the concentrations of P3HT and PDI (Table S2). Gold electrodes were fabricated by photolithography to yield a channel width of W = 100 μm and length L = 10 μm for devices in bottom-contact bottom-gate configuration (Figure 3). Additionally, to tune and balance the drain current and charge carrier mobilities in the two orthogonal directions, a series of concentrations of P3HT (from 0.01 to 1.0 mg/mL) and PDI (from 1.0 to 3.0 mg/mL) in o-DCB were evaluated. At a fixed PDI concentration of 2.0 mg/mL, P3HT concentrations below 0.1 mg/mL gave n-type dominant ambipolar behavior, 8614

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Figure 4. Schematic illustration of a complementary integrated inverter based on an aligned P3HT/PDI shish-kebab film as an orthogonal ambipolar semiconductor; (a) voltage transfer characteristics; (b) absolute gain.

PDI nanocrystals are physically separated by the P3HT fibrils, and thus there are no pathways for electron transport, while the bundling together of P3HT fibrils nucleated from neighboring crystals provides a continuous P3HT phase, thereby enabling hole transport. We also verified that devices prepared with a top-contact bottom-gate architecture (using gold electrodes with W = 37 μm and L = 25 μm patterned with a TEM grid as a shadow mask) gave similar levels of anisotropic charge carrier transport behavior, as shown in Figure S13. The square root of drain current vs gate voltage and output curves for anisotropic transistors prepared in this way are shown in Figures S14 and S15. We also investigated the hysteresis behavior for both charge carriers, as shown in Figure S16. In the perpendicular direction, hole transport shows negligible hysteresis, with the forward and reserve sweeps almost completely coinciding. In the parallel direction, electron transport shows only a slight hysteresis, with a variation in threshold voltage by 98%) was purchased from TCI. Each material was used without further purification. Hexamethyldisilazane (>97%) was purchased from Strem Chemicals, negative resist NR9 1000PY and RD6 developer were purchased from Futurrex, Inc., and Cr and Au metal pellets were purchased from ACI Alloys, Inc. The precursor for HfO2 atomic layer deposition (ALD) is tetrakis(dimethylamino)hafnium, 98+% (99.99+%-Hf, < 0.2%-Zr) TDMAH, PURATREM, 72−8000, contained in 50 mL Swagelok cylinder for CVD/ALD, purchased from STREM. Trimethoxy(octadecyl)silane (OTS) Treatment. Twenty-four μL of OTS was added to a mixture of 4 mL of CHCl3 and 1 mL of hexane, followed by soaking of the SiO2/Si wafer in this solution for 30 min at room temperature, annealing of the wafer at 150 °C in N2 for 1 h, and finally rinsing of the SiO2/Si wafer surface with CHCl3 and hexane, and blowing dry with N2. Capillary-Assisted Drop Casting. An Si/SiO2 substrate was cleaned (by sonicating in soap water for 15 min, rinsing three times with DI water, sonicating in acetone for 15 min, drying with N2 flow, sonicating in isopropanol for 15 min, and drying again with N2 flow) and then heated to 80 °C, followed by placement of a dust-free clean glass capillary tube with a diameter of 1.2 mm. Next, P3HT/PDI blend solution in o-DCB (0.1 to 0.5 mg/mL P3HT, 1.0 to 3.0 mg/mL PDI) was added dropwise to the capillary tube and allowed to spread across the sides and length of the tube, followed by drying at 80 °C for 1 h. This process was conducted in a standard wet chemical laboratory, not in a clean room, and thus some samples were contaminated by pieces of dust or other defects during solvent evaporation, leading to mixed orientation of the shish-kebab nanostructures. In practice, the yield of samples exhibiting orthogonal ambipolar behavior was 65% (out of 104 samples). Photolithography. Hexamethyldisilazane (HMDS) was spin-coated on a thoroughly cleaned silicon wafer (with 300 nm SiO2) at 3K rpm for 30 s, then negative resist NR9 1000py was spin-coated on top of the HMDS layer at 3K rpm for 30 s, followed by a soft baking of the silicon wafer at 150 °C for 60 s. 8617

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03942. SEM and optical microscope images of orthogonal ambipolar OFET and logic gates, simulated diffraction pattern of PDI crystal, orthogonal ambipolar OFET characterizations (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was primarily supported by the Department of Energy, Basic Energy Sciences, through grant DE- SC0006639 (semiconductor film preparation, structural analysis, and device characterization), with additional support from the National Science Foundation (DMR-1508627) (interdigitated electrode device design and fabrication). The work made use of facilities supported by the National Science Foundation through grants CMMI-1025020 and DBI-0923105. REFERENCES (1) Muccini, M. A Bright Future for Organic Field-Effect Transistors. Nat. Mater. 2006, 5, 605−613. (2) Martel, R.; Derycke, V.; Lavoie, C.; Appenzeller, J.; Chan, K. K.; Tersoff, J.; Avouris, P. Ambipolar Electrical Transport in Semiconducting Single-Wall Carbon Nanotubes. Phys. Rev. Lett. 2001, 87, 256805. (3) Chesterfield, R. J.; Newman, C. R.; Pappenfus, T. M.; Ewbank, P. C.; Haukaas, M. H.; Mann, K. R.; Miller, L. L.; Frisbie, C. D. High Electron Mobility and Ambipolar Transport in Organic Thin-Film Transistors Based on a π-Stacking Quinoidal Terthiophene. Adv. Mater. 2003, 15, 1278−1282. (4) Meijer, E. J.; de Leeuw, D. M.; Setayesh, S.; van Veenendaal, E.; Huisman, B. H.; Blom, P. W. M.; Hummelen, J. C.; Scherf, U.; Klapwijk, T. M. Solution-Processed Ambipolar Organic Field-Effect Transistors and Inverters. Nat. Mater. 2003, 2, 678−682. (5) Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296−1323. (6) Dinelli, F.; Capelli, R.; Loi, M. A.; Murgia, M.; Muccini, M.; Facchetti, A.; Marks, T. J. High-Mobility Ambipolar Transport in Organic Light-Emitting Transistors. Adv. Mater. 2006, 18, 1416−1420. (7) Wang, H.; Wei, P.; Li, Y.; Han, J.; Lee, H. R.; Naab, B. D.; Liu, N.; Wang, C.; Adijanto, E.; Tee, B. C.-K.; et al. Tuning the Threshold Voltage of Carbon Nanotube Transistors By n-Type Molecular Doping for Robust and Flexible Complementary Circuits. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 4776−4781. (8) Bisri, S. Z.; Piliego, C.; Gao, J.; Loi, M. A. Outlook and Emerging Semiconducting Materials for Ambipolar Transistors. Adv. Mater. 2014, 26, 1176−1199. (9) de Boer, R. W. I.; Stassen, A. F.; Craciun, M. F.; Mulder, C. L.; Molinari, A.; Rogge, S.; Morpurgo, A. F. Ambipolar Cu- and FePhthalocyanine Single-Crystal Field-Effect Transistors. Appl. Phys. Lett. 2005, 86, 262109. (10) Takahashi, T.; Takenobu, T.; Takeya, J.; Iwasa, Y. Ambipolar Organic Field-Effect Transistors Based on Rubrene Single Crystals. Appl. Phys. Lett. 2006, 88, 033505. (11) Ha, M.; Xia, Y.; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam, M. C.; Frisbie, C. D. Printed, Sub-3V Digital Circuits on 8618

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