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Jonathan D. Yuen , Rajeev Kumar , Dante Zakhidov , Jason Seifter , Bogyu Lim , Alan J. Heeger , and Fred Wudl *

Ambipolarity in Benzobisthiadiazole-Based Donor–Acceptor Conjugated Polymers

The application of solution processible semiconducting poly-mers in the fi eld of plastic electronics offers interesting poten-tial applications requiring light weight, fl exibility, large area and low-cost, such as portable displays, radiofrequency identi-fi cation (RFID) tags and sensors. [ 1 ] Recent progress in the fi eld of polymer fi eld effect transistors (FETs) has yielded materials which have mobilities that reliably exceeded 0.1 cm 2 V − 1 s − 1 , [ 2 ] even reaching 1 cm 2 V − 1 s − 1 , [ 3 ] the target mobility required for numerous applications requiring thin fi lm FETs. In particular, there is interest in the application of ambipolar polymer FETs, [ 4 ] capable of hole and electron transport, in CMOS-like digital integrated circuits, [ 5 ] with simpler processing requirements compared to separately patterned p- and n-type transistors. Spe-cifi cally, ambipolar transistors with a single active layer [ 6 ] offer processing simplicity and fl exibility compared to dual unipolar-component systems, such as bilayers or bulk-heterojunction blends. Until recently, development of conjugated polymers for single-component ambipolar transistors has been sporadic. Currently, however, the maturation of the donor–acceptor (DA) approach in the synthesis of conjugated co-polymers with tai-lored electronic properties has led to a jump in reports of ambi-polar behavior in FETs fabricated from these materials.

The DA polymers consist of a combination of π -electron rich (donors) and π -electron defi cient (acceptors) conjugated moieties arrayed along the polymer chain. The DA repeat units determine the electronic behavior of the resulting polymer. [ 7 ] While p-type behavior has been observed in almost all DA poly-mers, ambipolar behavior is dependent on the type of acceptor used. While there are many DA polymers that exhibit ambipolar behavior, the types of acceptor units remain scant, possibly less than 10 different types as of this publication. [ 8 ] Examples of include diketopyrrolopyrrole, [ 9 ] the rylene bisimides [ 10 ] and benzobisthiadiazole (BBT). [ 11 ] Despite their different chemical structures, a common feature of all polymers containing these acceptor moieties is that the electron affi nities of the original monomers are high, hence their rarity; the lowest unoccu-pied molecular orbitals (LUMO) of the monomers lie below –3.5 eV with respect to vacuum. An example is shown from

© 2011 WILEY-VCH Verlag Gwileyonlinelibrary.com

DOI: 10.1002/adma.201101134

Dr. J. D. Yuen , D. Zakhidov , J. Seifter , B. Lim , Prof. A. J. Heeger , Prof. F. Wudl Center for Polymers and Organic SolidsUniversity of CaliforniaSanta Barbara, CA 93106, USA E-mail: wudl@chem.ucsb.edu Dr. R. Kumar Nano-Terra Inc., Brighton, MA 02135, USA

cyclic voltammetric data of the BBT monomer in Figure S1 in the Supporting Information. We observe that the LUMO of BBT is around 4.1 eV below the vacuum. While diketopyrrolopyrrole and the rylene bisimides have been extensively explored, little work has been done on electronic devices with polymers con-taining BBT. This is mainly because the monomer typically used in the DA coupling reaction, benzobisthiadiazole-bithiophene results in polymers that are intractable. We have overcome this problem by adding hexyl side-groups to the neighboring bithiophene heterocycles, resulting in polymers which are sol-uble enough to be solution processible. Thus, we were able to synthesize a family of four new DA polymers based on BBT, in which BBT was paired with four different donor moieties: dithienopyrrole (PD), [ 11 ] dithienocyclopentane (CD), dithienosi-lole (SiD) and fl uorene (FL).

In this report, we describe the FET performance of these BBT-based polymers. All members of the family exhibited balanced ambipolar behavior, despite the wide range of mobilities. Stille coupling was used to copolymerize BBT with PD, CD and SiD to form the black polymers polybenzobisthiadiazole-dithienopyr-role (PBBTPD), polybenzobisthiadiazole-dithienocyclopentane (PBBTCD) and polybenzobisthiadiazole-dithienosilole (PBBTSiD), respectively. Polybenzobisthiadiazole-fl uorene (PBBTFL), a dark green fl uorene-based BBT polymer was synthesized using Suzuki coupling. All synthetic procedures are sum-marized in Figure 1 . The molecular weights and PDI, deter-mined by gel permeation chromatography performed at room temperature in tetrahydrofuran or chloroform, are shown in Table 1 . Poly merization with Stille coupling yielded polymers with higher molecular weights, with M n ≥ 14 kD, compared to Suzuki coupling, in which the number molecular weight of PBBTFL was around 11 kD. It should be noted that while processiblity was improved with the modifi cation, solubility remains limited to the point where accurate determination of molecular weight is diffi cult. The solution (in chloroform) and the solid state UV-vis-IR absorption spectra of the polymers are shown in Figure 1 . The optical energy gaps for all the polymers are very low, below 0.6 eV for the polymers containing strong electron donating moieties: PBBTPD, PBBTCD and PBBTSiD. The fl uorene-containing polymer, PBBTFL, has a wider energy gap of about 1.2 eV since the fl uorene moiety is essentially a biphenyl unit, it therefore is a weaker electron donor. The absorption spectra do not differ signifi cantly between solution and fi lm; there is no observable red-shift on going from solu-tion to fi lm.

Bottom-gate, bottom contact transistors were fabricated with all four polymers as described in the experimental section. It was found that an optimal annealing temperature for each

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Figure 1 . (top) Synthesis scheme of the BBT family of polymers. UV-vis-NIR Spectra of the BBT family of polymers in fi lm (bottom left) and solution (bottom right).

S

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C12H25O

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polymer, listed as T o on Table 1 , was required to optimize the device. All devices were annealed prior to testing. Output curves of three polymers (PBBTCD, PBBTSiD and PBBTFL) are shown in Figure 2 a, 2 b, and 2 c respectively, with the transfer curves in the insets. A similar fi gure describing the transistor behavior of PBBTPD has been included in the Supporting Information as Figure S2. As stated above, all transistors exhibit behavior typical

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 3780–3785

of ambipolar FETs. At high gate voltages (V G ), with source-drain voltage (V SD ) at the same polarity, charge transport is unipolar, with electron transport at positive gate bias and hole transport at negative bias. Output curves in these regimes show typical FET behavior. At low gate voltages or when the gate voltage and drain voltage are at opposite polarity, the ambipolar regime occurs due to the overlap in the onset of hole and electron

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levels, and electrical parameters of transistors–optimized annealing temperature (T o ), average hole ( μ h,avg ) and electron ( μ e,avg ) mobilities and max-imum hole ( μ h,max ) and electron mobilities ( μ e,max ) at the optimized annealing temperature.

Name M n [kD]

PDI HOMO [eV]

LUMO [eV]

T o [ ° C]

μ h,avg [cm 2 V − 1 s − 1 ]

μ e,avg [cm 2 V − 1 s − 1 ]

μ h,max [cm 2 V − 1 s − 1 ]

μ e,max [cm 2 V − 1 s − 1 ]

PBBTCD 17 2.8 4.8 4.0 240 1.1E-01 7.4E-02 1.3E-01 1.0E-01

PBBTPD 14 2.8 4.8 4.2 260 9.6E-03 8.2E-03 1.1E-02 1.1E-02

PBBTFL 11 2.8 5.1 3.9 160 5.6E-03 7.0E-04 1.7E-03 7.0E-03

PBBTSiD 18 2.8 4.8 4.1 240 1.9E-03 1.1E-02 2.5E-03 1.6E-02

accumulation. Thus, n-type and p-type regions are formed in the channel, resulting in diode-like current-voltage characteris-tics in the output curves. This behavior is also refl ected in the transfer curves, with the transition from unipolar to ambipolar occurring at the trough of the “V”.

Transistor performance varied greatly between polymers, with an increase in almost three orders of magnitude in cur-rent from PBBTFL to the highest performing polymer PBBTCD under the same operating conditions. To more accurately quantify and compare device performance, charge-carrier mobilities were calculated using the standard equation used to describe metal-oxide FETs operating in the saturation regime: I DS = ½(W/L) μ C i (V G –V T ) 2 , with mobility determined

© 2011 WILEY-VCH Verlag Gwileyonlinelibrary.com

Figure 2 . Typical transistor output characteristics of (a) PBBTCD, (b) PBMobilities were determined to be 0.076 cm 2 V − 1 s − 1 for electrons and 0.100.007 cm 2 V − 1 s − 1 for holes for PBBTSiD, and 0.00015 cm 2 V − 1 s − 1 for electroteristics for a complementary inverter with PBBTCD as the active layer, anne

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from ∂ | I DS | 1/2 / ∂ V G . Since in the ambipolar regime the elec-tron and hole contributions cannot be clearly distinguished, only the unipolar hole and electron transport regimes could be used as comparison. Table 1 summarizes the average and maximum p-type ( μ h ) and n-type ( μ e ) mobilities for all the polymers, and the optimal annealing temperature used to achieve the highest mobilities. In general, we observe that μ h,e (PBBTCD) > μ h (PBBTPD) > μ h (PBBTSiD) > μ h,e (PBBTFL), with an exception that μ e (PBBTPD) < μ e (PBBTSiD). Both hole and electron mobilities span around three orders of magnitude, indicating that the BBT moiety plays no signifi cant role in lim-iting the mobility. Despite this large variation, hole and electron mobilities for each polymer remain close, within an order of

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BTSiD and (c) PBBTFL. Transfer characteristics are shown in the inset. cm 2 V − 1 s − 1 for holes for PBBTCD, 0.016 cm 2 V − 1 s − 1 for electrons and ns and 0.004 cm 2 V − 1 s − 1 for holes for PBBTFL. d) Voltage transfer charac-aled at 240 ° C. The maximum gain was determined to be 20.

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magnitude for the highest performing three polymers and just over an order of magnitude for PBBTFL. This is most likely due to the strong electron accepting nature of the BBT moiety and the low bandgaps of the polymers, allowing both electrons and holes to be easily injected. The ambipolar performance of the polymers is comparable to many polymers reported in litera-ture, particularly with PBTTCD, which had p-type and n-type mobilities above 0.1cm 2 V − 1 s − 1 , almost reaching the current record mobilities of a similarly balanced ambipolar polymer of around 0.5 cm 2 V − 1 s − 1 . [ 9d ] Moreover, by improving the solubility of PBBTPD via addition of alkyl chains, we have improved the transistor performance of a PBBTPD polymer of similar mole-cular weight previously reported by Reynolds et al . [ 11 ]

The balanced ambipolarity of the BBT family of polymers makes them good candidates for logic circuit applications. We integrated the highest performing polymer, PBBTCD, into an inverter confi guration, the most basic device in complementary logic circuits. Despite the lack of optimization, such as the use of unequal channel lengths, we report good inverter perform-ances with gain close to 20, as shown in Figure 2d, comparable to ambipolar transistors used in inverters. [ 5 , 9c ] The successful implementation of an inverter with PBBTCD highlights the promise of the BBT family of polymers in the implementa-tion of CMOS-like logic applications. The stability of PBBTPD has been tested in ambient condition and a description of the experiment has been included in the Supporting Information.

While we have shown the applicative potential of the BBT family of polymers, transistor performance remains a great issue due to the great variation in mobilities between polymers. In order to better synthesize higher performing derivatives of BBT-containing polymers, determining factors that affect tran-sistor performance will be important as feedback. We consider several polymer properties that affect transistor performance: energy levels, crystalline order, morphology and molecular weight. The position of the highest occupied molecular orbital (HOMO) and the LUMO determines charge injection and

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 3780–3785

Figure 3 . AFM images (top) and optical microscopy (bottom) images of P

DCTBBP PBBTS

therefore transistor performance. We can observe that the energy levels do not affect transistor performance signifi cantly since PBBTCD, PBBTPD and PBBTSiD have similar energy levels, but a wide variation of mobilities. A hole injection barrier may exist for PBBTFL due to its comparatively higher HOMO level, but this does not explain the low electron transport as the LUMO level of PBBTFL is close to that of PBBTCD, which has the highest electron transport.

Crystalline ordering of the polymers may also affect tran-sistor performance. However, based on X-ray diffraction spectra of the polymers (as shown in Figure S4 in the Supporting Information), we observe no correlation between ordering and transistor performance. Both high performing PBBTCD and low performing PBBTFL lack XRD peaks, indicating little or no ordering. In contrast, the mid-performers, PBBTPD and PBBTSiD show peaks around 2.75 ° and 5 ° , respectively, indi-cating ordering in the range of 3 nm and 2 nm, respectively. Thus, crystalline ordering and energetics probably do not play a signifi cant role in performance.

Comparing the properties of the polymers listed on Table 1, we observe a possible correlation between mobility and molec-ular weight for PBBTCD, PBBTPD and PBBTFL. For increasing molecular weight: M n (PBBTCD) > M n (PBBTPD) > M n (PBBTFL), a corresponding increase in mobility, μ h,e (PBBTCD) > μ h,e (PBBTPD) > μ h,e (PBBTFL), is observed. An exception to this correlation is PBBTSiD. While the number average molec-ular weight of PBBTSiD is comparable to PBBTCD, around 17–18 kD, both electron and hole mobilities are about 2 orders of magnitude lower than that of PBBTCD.

This discrepancy can be attributed to the difference in mor-phology between PBBTSiD and the rest of the BBT family. Figure 3 shows images of three polymers in the BBT family, PBBTCD, PBBTSiD and PBBTFL, taken from both optical micro-scopy and atomic-force microscopy (AFM). Similar images of PBBTPD are in Figure S3 in the Supporting Information. From both optical microscopy and AFM, we can see a clear contrast

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BBTCD (left), PBBTSiD (center) and PBBTFL (right).

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of the morphology of PBBTSiD as compared to the other poly-

mers. The optical microscopy images of PBBTSiD reveal a highly granular fi lm surface, indicating formation of large iso-lated grains, compared to the smoothly continuous, almost fea-tureless fi lms of PBBTCD, PBBTPD and PBBTFL. This effect is also mirrored in the AFM images of the three polymers, with the fi lms of PBBTCD, PBBTPD and PBBTFL each consisting of a fi ne-grained, tightly-packed microstructure, resulting in a surface with low roughness, in contrast to the much rougher surface of the coarser-grained, more highly-aggregated fi lm of PBBTSiD. We hypothesize that formation of coarser grains, resulted in a more loosely-connected, less coherent micro-structure, and hence, the reduced mobility is probably due to poor fi lm continuity. The high aggregation of PBBTSiD is most likely due to the poor solubility of PBBTSiD. While the other polymers dissolved easily, PBBTSiD solutions had large quanti-ties of particulates, resulting in powdery fi lms, despite fi ltration with 0.2 μ m fi lters. To conclude, we fi nd that while transistor performance is generally affected by the molecular weight of the polymer, with higher molecular weight resulting in higher mobility, the polymer must also be solution processible to ensure good fi lm quality and thus good transport.

Another observation from the AFM images of BBT family is the granular nature of the fi lms, indicating lack of obvious long range order, with grain sizes around 10 nm for all polymers, except PBBTSiD, which has grain sizes around 50 nm. Com-pared to most other high performing polymer materials, both ambipolar and unipolar, the BBT family of polymers lacks the microscopic signatures of interchain ordering, such as nano-fi brils or terracing. This is interesting particularly in context of the high performing PBBTCD, indicating that strong ordering may not be a prerequisite for high performance.

This observation of high performance in disordered fi lm structures seems closely aligned to the idea espoused by McCullough of high performing semiconducting polymers consisting of non-fi brillar structures with small granular mor-phologies known as “transistor paints”. [ 12 ] The performance of these materials relies on long-range chain to chain π -connections for transport, rather than grain-boundary limited transport via nano-fi brils. Recent work on ‘transistor paint’ materials by the McCullough group have yielded transistors with mobilities as high as 0.4 cm 2 V − 1 s − 1 with high reproducibility, with morpholo-gies similar to our materials. Hence, we believe that this is the reason behind the high performance of PBBTCD and with further material improvement, even better performance can be achieved.

In conclusion, we have synthesized a family of donor–acceptor semiconducting polymers, all containing the strongly accepting benzobisthiadiazole moiety. All polymers in the family showed narrow bandgaps from 0.6 to 1.2 eV. Integrating the polymers into transistors resulted in evenly-balanced ambi-polar properties, with mobilities ranging from 10 − 4 cm 2 V − 1 s − 1 to 10 − 1 cm 2 V − 1 s − 1 . We have found that mobility is possibly dependent on both molecular weight and fi lm quality, but interestingly not obviously dependent on crystalline order. The formation of a tightly-packed fi ne-grained, non-fi brillar mor-phology is observed for all the polymers, indicating that origin of high transport performance of these polymers is most likely different from that of many other high performance polymers which rely on high crystallinity for good transport.

© 2011 WILEY-VCH Verlag Gwileyonlinelibrary.com

Experimental Section

Synthesis of PBBTPD : Poly[(4,7-bis(3-hexylthien-2-yl)-2 λ 4 δ 2 - benzo[1,2-c;4,5-c ′ ]bis[1,2,5]thiadiazole)-alt-(N-(3,4,5-tris(dodecyloxy)phenyl)-dithieno[3,2-b:2 ′ ,3 ′ -d]pyrrole)] was synthesized by a modifi ed procedure described in literature. [ [ 11 ] A 5 mL microwave tube was charged with 4,7-bis(4-bromo-3-hexylthien-2-yl)-2 λ 4 δ 2 -benzo[1,2-c;4,5-c ′ ]bis[1,2,5]thiadiazole (0.23 g, 0.33 mmol), N-(3,4,5-tris(dodecyloxy)phenyl)-dithieno[3,2-b:2 ′ ,3 ′ -d]pyrrole (0.374 g, 0.33 mmol), Pd(PPh 3 ) 4 (0.006 g, 0.016 mmol), degassed dry chlorobenzene (2.5 mL), and sealed with a tefl on cap. The reaction mixture was heated with stirring to 150 ° C for 50 min, using a Biotage microwave reactor. Upon cooling, the crude reddish brown polymer was precipitated in methanol (500 mL) and the black solid was collected by fi ltration. The crude product was purifi ed by Soxhlet extraction with methanol, acetone and hexanes each for 1 day, and fi nally polymer was extracted with chloroform. After the second precipitation of the chloroform extracts into methanol, a black solid (0.29 g, 92%) was obtained. solution UV-vis-NIR: λ max : 480 nm, 1200 nm and onset at 2200 nm. Thin fi lm: λ max : 505 nm, 1220 nm and onset at 2250 nm ( E g = 0.55 eV).

Synthesis of PBBTCD and PBBTSiD : Poly[(4,7-bis(3-hexylthien-2-yl)-2 λ 4 δ 2 -benzo[1,2-c;4,5-c ′ ]bis[1,2,5] thiadiazole)-alt-(3,3-bis(2-ethyl hexyl) 4H-cyclopenta[2,1-b:3,4-b ′ ]dithiophene)] and poly[(4,7-bis(3- hexyl thien-2-yl)-2 λ 4 δ 2 -benzo[1,2-c;4,5-c ′ ]bis[1,2,5] thiadiazole)-alt-(3,3′-Di-n-octylsilylene-2,2 ′ -bithiophene)] were synthesized similarly to that of PBBTPD. The crude product was purifi ed by Soxhlet extraction with methanol, acetone and hexanes each for 1 day, and fi nally polymer was extracted with chloroform to produce a black solid. PBBTCD solution UV-vis-NIR: λ max : 500 nm, 1120 nm and onset at 2030 nm. PBBTCD thin fi lm: λ max : 510 nm, 1160 nm and onset at 2060 nm (E g = 0.6 eV). PBBTSiD solution UV-vis-NIR: λ max : 500 nm, 1170 nm and onset at 2030 nm. PBBTSiD thin fi lm: λ max : 500 nm, 1110 nm and onset at 2070 nm (E g = 0.6 eV).

Synthesis of PBBTFL : Poly[(4,7-bis(3-hexylthien-2-yl)-2 λ 4 δ 2 -benzo[1,2-c;4,5-c ′ ]bis[1,2,5] thiadiazole)-alt-(9,9-Di-n-octyl-fl uorene)] was synthe-sized via a Suzuki coupling procedure. A three-necked round bottomed fl ask (25 mL), equipped with condenser, magnetic stirrer and nitrogen-inlet, was charged with 2,7-diboronic ester of 9,9-dioctyl-fl uorene (0.2 g, 0.31 mmol), bis(triphenylphosphine) palladium dichloride (0.01 g, 0.015 mmole), dry degassed THF (8 mL) and 2 mL degassed water. Dibromo-BBT monomer (0.21 g, 0.31 mmol) was added, and the reaction mixture was then refl uxed for 72 h with vigorous stirring under N 2 -atmosphere. The formed dark green color polymer was precipitated in methanol. The crude product was purifi ed by Soxhlet extraction with methanol, acetone and hexanes each for 12 h, and fi nally polymer was extracted with chloroform. Solution UV-vis-NIR: λ max : 422 nm, 850 nm and onset at 1060 nm. Thin fi lm: λ max : 402 nm, 840 nm and onset at 1040 nm ( E g = 1.2 eV).

Synthesis of Monomers and Polymers NMR data : See Supporting Information.

Fabrication and characterization of FET devices : Transistors were fabricated in the bottom gate, bottom contact confi guration on heavily-doped n-type Si substrates as the gate and thermally grown 200 nm silicon dioxide as the dielectric layer (Silicon Quest, dry oxide). The source and drain electrodes were patterned using standard photolithography and were formed on SiO 2 with e-beam evaporation of 3 nm of nickel and 47 nm of gold. Prior to deposition, devices were cleaned in acetone and isopropanol and dried in an oven at 120 ° C for 15 min, then exposed for 1.5 h to UV light in air. The devices were surface treated with decyltrichlorosilane (DTS) for 0.5 h by immersion in a 1% by volume of DTS in toluene. The devices were then cleaned by rinsing with toluene and dried under nitrogen fl ow followed by spin-coating of the polymer. Polymer fi lms were spin-cast at 4000–5000 RPM in a glovebox from a 10 mg/mL solution in chloroform or chlorobenzene. Samples were dried at 60 ° C for 0.5 h and then annealed at the optimized temperature for the polymer for 10 min in a glove box. FET mobility measurements were performed in a glovebox using a Keithley 4200 Semiconductor Parametric Analyzer and a Signotone Micromanipulator S-1160 probe

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station. Samples measured had channel lengths of 5 μ m and channel widths of 1 mm.

Optical Microscopy : Samples consisting of n-type Si substrates with thermally grown 200 nm silicon dioxide were cut, subject to the cleaning, UV-treatment, DTS-treatment, spin-casting and annealing as listed above. Optical microscopy images of the samples were taken under a Nikon Eclipse L300D camera microscope with image capture software at 20X magnifi cation.

Tapping Mode Atomic Force Microscopy (TMAFM) : Samples consisting of n-type Si substrates with thermally grown 200 nm silicon dioxide were cut, subject to the cleaning, UV-treatment, DTS-treatment, spin-casting and annealing treatment as listed above. TMAFM studies were carried out in room-temperature air with an MFP-3D-SA Atomic Force Microscope made by Asylum Research. The probes were FORTA probes from Applied Nanostructures, which have a silicon cantilever of nominal spring constant 3.0 Nm − 1 , a nominal resonance frequency of ∼ 60 kHz, and were tuned to an RMS cantilever oscillation amplitude of ∼ 1000 mV. Acquired images were 1 μ m × 1 μ m, taken with a scan frequency of 1 Hz.

Acknowledgements J.D.Y. and R.K. contributed equally to this paper. Support for research was provided by the Air Force Offi ce of Scientifi c Research (Charles Lee, Program Offi cer), the National Science Foundation Polymer program (Grant No. NSF-DMR-0856060) and by the Korean Ministry of Science via a Global Research Laboratory (GRL) award. Support for R Kumar was through the Center for Energy Effi cient Materials (CEEM) supported by the DOE as an EFRC. Partial Support by DARPA through CBRITE Corporation is gratefully acknowledged. A portion of this work was done in the UCSB nanofabrication facility, part of the NSF funded NNIN network. J.D. Yuen thanks S. Valouch, N. Banerji, J. H. Seo, C. Takacs and D. Moses for useful discussions. R.K. thanks K. Brosten for helping in synthesis.

Received: March 26, 2011 Revised: May 14, 2011

Published online: July 18, 2011

[ 1 ] a) S. Allard , M. Forster , B. Souharce , H. Thiem , U. Scherf , Angw. Chem. Int. Ed. 2008 , 47 , 4070 – 4098 ; b) G. Gelinck , P. Heremans , K. Nomoto , T. D. Anthopoulos , Adv. Mater. 2010 , 22 , 3778 – 3798 .

[ 2 ] a) I. McCulloch , M. Heeney , C. Bailey , K. Genevicius , I. MacDonald , M. Shkunov , D. Sparrowe , S. Tierney , R. Wagner , W. Zhang , M. Chabinyc , J. Kline , M. McGehee , M. Toney , Nat. Mater. 2006 , 5 , 328 – 333 ; b) J. Liu , R. Zhang , G. Sauve , T. Kowalewski , R. McCullough , J. Am. Chem. Soc. 2008 , 130 , 13167 – 13176 ; c) T. Nelson , T. Young , J. Liu , S. Mishra , J. Belot , C. Balliet , A. Javier , T. Kowalewski , R. McCullough , Adv. Mater. 2010 , 22 , 4617 – 4621 .

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 3780–3785

[ 3 ] a) B. H. Hamadani , D. J. Gundlach , I. McCulloch , M. Heeney , Appl. Phys. Lett. 2007 , 91 , 243512 – 3 ; b) H. Yan , Z. Chen , Y. Zheng , C. Newman , J. Quinn , F. Dotz , M. Kastler , A. Facchetti , Nature 2009 , 457 , 679 – 686 ; c) Y. Li , S. Singh , P. Sonar , Adv. Mater. 2010 , 22 , 4862 – 4866 .

[ 4 ] a) L. Chua , J. Zaumseil , J. Chang , E. Ou , P. Ho , H. Sirringhaus , R. Friend , Nature 2005 , 434 , 194 – 199 ; b) J. Zaumseil , H. Sirringhaus , Chem. Rev. 2007 , 107 , 1296 – 1323 .

[ 5 ] a) E. Meijer , D. de Leeuw , S. Setayesh , E. van Veenendaal , B. H. Huisman , P. Blom , J. Hummelen , U. Scherf , T. Klapwijk , Nat. Mater. 2003 , 2 , 678 – 682 ; b) T. Anthopoulos , D. de Leeuw , E. Cantatore , S. Setayesh , E. Meijer , C. Tanase , J. Hummelen , P. Blom , App. Phys. Lett. 2004 , 85 , 4205 – 4207 ; c) T. Anthopoulos , S. Setayesh , E. Smits , M. Cölle , E. Cantatore , B. de Boer , P. Blom , D. de Leeuw , Adv. Mater. 2006 , 18 , 1900 – 1904 ; d) J. Bijleveld , A. Zoombelt , S. Mathijssen , M. Wienk , M. Turbiez , D. de Leeuw , R. Janssen , J. Am. Chem. Soc. 2009 , 131 , 16616 – 16617 .

[ 6 ] a) J. Swensen , C. Soci , A. Heeger , App. Phys. Lett. 2005 , 87 , 253511 ; b) R. Mondal , N. Miyaki , H. Becerril , J. Norton , J. Parmer , A. Mayer , M. Tang , J. Bredas , M. McGehee , Z. Bao , Chem. Mater. 2009 , 21 , 3618 – 3628 ; c) Z. Chen , H. Lemke , S. Albert-Seifried , M. Caironi , M. Nielsen , M. Heeney , W. Zhang , I. McCulloch , H. Sirringhaus , Adv. Mater. 2010 , 22 , 2371 – 2375 .

[ 7 ] a) Q. Zhang , J. Tour , J. Am. Chem. Soc. 1998 , 120 , 5355 – 5362 ; b) Q. Zhang , J. Tour , J. Am. Chem. Soc. 1997 , 119 , 5065 – 5066 ; c) S. Jenekhe , L. Lu , M. Alam , Macromolecules 2001 , 34 , 7315 – 7324 .

[ 8 ] a) H. Usta , A. Facchetti , T. Marks , J. Am. Chem. Soc. 2008 , 130 , 8580 – 8581 ; b) J. Letizia , M. Salata , C. Tribout , A. Facchetti , M. Ratner , T. Marks , J. Am. Chem. Soc. 2008 , 130 , 9679 – 9694 ; c) H. Usta , C. Risko , Z. Wang , H. Huang , M. Deliomeroglu , A. Zhukhovitskiy , A. Facchetti , T. Marks , J. Am. Chem. Soc. 2009 , 131 , 5586 – 5608 .

[ 9 ] a) L. Bürgi , M. Turbiez , R. Pfeiffer , F. Bienewald , H. Kirner , C. Winnewisser , Adv. Mater. 2008 , 20 , 2217 – 2224 ; b) A. Zoombelt , M. Fonrodona , M. Turbiez , M. Wienk , R. Janssen , J. Mater. Chem. 2009 , 19 , 5336 – 5342 ; c) J. Bijleveld , V. Gevaerts , D. Di Nuzzo , M. Turbiez , S. Mathijssen , D. de Leeuw , M. Wienk , R. Janssen , Adv. Mater. 2010 , 22 , E242 – E246 ; d) P. Sonar , S. Singh , Y. Li , M. Soh , A. Dodabalapur , Adv. Mater. 2010 , 22 , 5409 – 5413 .

[ 10 ] a) X. Guo , M. Watson , Org. Lett. 2008 , 10 , 5333 – 5336 ; b) F. Kim , X. Guo , M. Watson , S. Jenekhe , Adv. Mater. 2010 , 22 , 478 – 482 .

[ 11 ] a) T. Steckler , X. Zhang , J. Hwang , R. Honeyager , S. Ohira , X. Zhang , A. Grant , S. Ellinger , S. Odom , D. Sweat , D. Tanner , A. Rinzler , S. Barlow , J. Bredas , B. Kippelen , S. Marder , J. Reynolds , J. Am. Chem. Soc. 2009 , 131 , 2824 – 2826 ; b) X. Zhang , T. T. Steckler , R. R. Dasari , S. Ohira , W. J. Potscavage , S. P. Tiwari , S. Coppee , S. Ellinger , S. Barlow , J. Brédas , B. Kippelen , J. R. Reynolds , S. R. Marder , J. Mater. Chem. 2010 , 20 , 123 – 134 .

[ 12 ] J. Liu , R. Zhang , I. Osaka , S. Mishra , A. Javier , D. Smilgies , T. Kowalewski , R. McCullough , Adv. Funct. Mater. 2009 , 19 , 3427 – 3434 .

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