Scalable Synthesis of Fused Thiophene ... - ACS Publications

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Scalable Synthesis of Fused Thiophene-Diketopyrrolopyrrole Semiconducting Polymers Processed from Nonchlorinated Solvents into High Performance Thin Film Transistors James R. Matthews,† Weijun Niu,† Adama Tandia,† Arthur L. Wallace,† Jieyu Hu,† Wen-Ya Lee,‡ Gaurav Giri,‡ Stefan C. B. Mannsfeld,§ Yingtao Xie,∥ Shucheng Cai,∥ Hon Hang Fong,*,∥ Zhenan Bao,*,‡ and Mingqian He*,† †

Corning Incorporated, One River Front Plaza, Corning, New York 14831, United States Department of Chemical Engineering, Stanford University, 381 North South Mall, Stanford, California 94305, United States § Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025, United States ∥ Department of Electronic Engineering, Shanghai Jiao Tong University, 800 DongChuan Rd., Shanghai 200240, China ‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of a fused thiophene-diketopyrrolopyrrole based semiconducting polymer PTDPPTFT4 is presented. A number of synthetic challenges have been overcome in the development of a practical scalable synthesis. Characterization by Gel Permeation Chromatography (GPC) over a range of temperatures has revealed the tendency of this polymer to aggregate even at elevated temperatures and confirmed that the molecular weight values obtained are for nonaggregated material. This polymer meets a number of important requirements for potential industrial applications, such as scalable synthesis, solubility in industrially suitable solvents, and material stability and processability into stable high performance thin film transistor devices. Computational modeling has been used to help explain the structure property relationships contributing to the high performance. Grazing incidence X-ray of the thin films showed out of plane lamellar packing and in plane π−π stacking, both good indicators of a preferentially oriented thin film, desirable for high charge carrier mobility. Hole mobilities in excess of 2 cm2/V·s, on/off ratio of >106, and threshold voltage 100g) are not currently practical in commercially available microwave reaction systems. In such cases thermal 783

dx.doi.org/10.1021/cm303953e | Chem. Mater. 2013, 25, 782−789

Chemistry of Materials

Article

DPP based polymers has previously been observed.9,17,25 A study of the effect of temperature on the apparent molecular weight of PTDPPTFT4 showed that the bimodal distribution was a result of aggregation. Figure 1 shows GPC traces from a

Scheme 3. Synthesis of PTDPPTFT4 by Stille Coupling of Ditin-FT4 with Bromothienyl-DPP

Figure 1. GPC traces for PTDPPTFT4 run at 1 mg/mL in 1,2,4trichlorobenzene over a range of temperatures.

based batch reactions may be used to synthesize the polymer. Removal of oxygen is required to make high quality polymer. Reactions run in air generally yielded lower molecular weights and broader polydispersities. Purification of the polymer was achieved by precipitation into a stirred mixture of methanol and acetyl acetone, followed by Soxhlet extractions with acetone and then hexanes, before the polymer itself was extracted from the Soxhlet with chloroform, leaving behind any insoluble material. Other solvents (e.g., nonchlorinated solvents) may be substituted for the extraction if so desired. Toluene was tested for use in the extraction process and found to be suitable. Gel Permeation Chromatography (GPC). GPC analysis of the different polymer batches of PTDPPTFT4 made under different conditions, using different solvents, catalysts, temperatures, and durations, provided interesting observations. Previous FT4 based polymers, such as P2TDC17FT4,4 show aggregation even at relatively high temperatures, as does this new polymer. Increasing temperature can help break up aggregates and allow measurements to be made on the nonaggregated polymer chains. Solutions of P2TDC17FT4 in di- or trichlorobenzene are a deep purple-red at room temperature, but are a pale orange at elevated temperatures when the aggregates have been fully broken up. The GPC for P2TDC17FT4 was carried out in 1,2,4-trichlorobenzene at 160 °C, to avoid the influence of aggregates on the apparent molecular weight. However, PTDPPTFT4 does not show obvious changes in color, remaining dark green from room temperature up to the boiling point of 1,2,4-trichlorobenzene (214 °C). Using o-tolyl phosphine ligands in the synthesis of PTDPPTFT4 gave higher molecular weights. However, GPC analyses run at 160 °C showed unusually high polydispersities, and higher molecular weight samples showed bimodal distributions of molecular weights. This is unexpected from a highly efficient polymerization reaction between molar equivalents of two divalent monomers. This suggests, even though 160 °C was sufficient to disrupt aggregates in P2TDC17FT4, this was not the case in higher weight samples of PTDPPTFT4. Another significant indicator of the presence of aggregation in higher molecular weight samples was the marked shift in change of refractive index (n) with concentration (c) (dn/dc) observed when comparing lower molecular weight samples of PTDPPTFT4 exhibiting a monomodal molecular weight distribution with higher molecular weight samples exhibiting a bimodal molecular weight distribution. The effect of temperature on aggregation of

variable temperature GPC study on a single 1 mg/mL solution of PTDPPTFT4. The aggregates were finally eliminated when the system temperature was raised to 200 °C. Table 1 shows Table 1. Apparent Molecular Weights for a Single Sample of PTDPPTFT4 from GPC at Different Temperatures temp (°C)

Mn

Mw

PDI

140 150 160 170 180 190 200

21900 22200 21900 23600 22600 20500 22800

148000 192000 158000 164000 111000 66700 44200

6.75 8.67 7.20 6.97 4.93 3.25 1.94

molecular weight data relative to polystyrene standards from 140 to 200 °C. The polydispersity dropped from a peak value of almost nine, down to slightly less than two. A polydispersity of less than the theoretical value of two is reasonable, because the lower weight fractions were removed by solvent extraction, post-synthesis, but before GPC measurements. The molecular weight determined by using an elevated GPC system temperature (200 °C) is providing a more accurate value (within the restrictions imposed by the use of polystyrene standards), by eliminating the influence of aggregation on the results. Fused thiophene and DPP based copolymers tend to aggregate, with different specific species aggregating to various degrees. It is therefore important to measure molecular weight at elevated temperatures for those polymers exhibiting high PDI at lower temperatures. Spectroscopic and Electrochemical Characterization. The polymer was characterized by UV−vis spectroscopy to determine its optical properties and estimate the optical band gap. Figure 2 shows the spectra of PTDPPTFT4 in chloroform solution and as a drop-cast film from that solution. The optical band-gaps determined from the absorption onsets are 1.28 and 1.32 eV respectively for the film and solution states of the polymer. The electrochemical properties of PTDPPTFT4 were investigated by cyclic voltammetry (CV) to determine the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Figure 3 shows both oxidation and reduction cycles for the polymer. The oxidation of the polymer is fully reversible. The reduction 784



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ing polymers, PTDPPTFT4 can be dissolved and processed in many chlorinated solvents, such as chloro-alkanes and chlorinated benzene derivatives. However, unlike many other materials, this polymer can also be dissolved into many nonchlorinated hydrocarbon solvents, such as, toluene, xylenes, mesitylene, cyclooctane, tetrahydronaphthalene, decahydronaphthalene, and so forth. This is a significant benefit for industrial applications because of the regulatory restrictions on the use of chlorinated solvents as compared with nonchlorinated hydrocarbons. Typical conditions for device processing make use of concentrations between 1 and 5 mg/mL, a range that is easily achievable with this polymer in most aromatic and cyclic aliphatic solvents. Solutions with concentrations in excess of 25 mg/mL have been prepared in some solvent combinations and used at room temperature. Even higher concentrations may be realized when elevated temperatures are used. Device Characterization. Thin film transistors were made by spin-coating a 5 mg/mL solution of PTDPPTFT4 dissolved in p-xylene. Bottom gate, top contact devices were made on silicon wafer substrates as a common gate with a 200 nm thermal oxide dielectric layer and gold source and drain electrodes. The silica gate oxide layer was treated via soaking in a p-xylene solution of octyltrichlorosilane (OTS-C8) at room temperature for 1 h. Table 2 shows device performance

Figure 2. UV−vis spectra of a solution and a drop-cast film of PTDPPTFT4.

Table 2. OTFT Performance Measured on OTS-C8 Modified Si/SiO2(200 nm) Wafers after Annealing at Different Temperatures

Figure 3. Cyclic voltammetry redox potentials of PTDPPTFT4.

shows a shift to less negative potentials following the initial reduction cycle, while subsequent reduction cycles appear to be reversible. The HOMO was determined from the onset of the oxidation cycles to be −5.3 eV and the LUMO was determined from the onset of the reduction cycles to be −3.95 eV. This gives an electrochemical band gap of 1.35 eV. Thermal Stability. The polymer (PTDPPTFT4) exhibits excellent thermal stability. Thermal gravimetric analysis (TGA) shows that degradation does not begin until 310 °C in air and not until 407 °C in nitrogen. This is comparable to the thermal stability of previously published fused thiophene polymer semiconductors,4 with the added advantage of the increased solubility of this new polymer. Polymer thermal behavior was also evaluated by differential scanning calorimetry (DSC). PTDPPTFT4 shows a melting transition around 40−45 °C, attributed to side-chain melting. The presence of a side-chain melting transition is an indication that the side-chains possess sufficient overlap to form crystalline regions. PTDPPTFT4 has a slightly higher side-chain melting temperature than P2TDC17FT4, which exhibits side-chain melting at 0−20 °C (see Supporting Information for DSC traces). At higher temperatures the DSC of PTDPPTFT4 is featureless, similar to other FT4 based polymers, unlike other previously reported polymers, such as pBTCT-C10, pBTTT-C10, or pADBT-C14, which each show at least one significant transition between 100 and 300 °C.26 Solubility. The two long alkyl chains on both the fused thiophene and the DPP in the repeat unit impart good solubility to the resulting polymers in a number of solvents suitable for industrial applications. Most DPP based devices have been reported as being processed from chlorinated solvents.7,8,12−17,27,28 Similar to most well-known semiconduct-

a

OSC annealing temp. (°C)

mobility (cm2/V·s)

100 130 150 190 210

0.47 0.61 0.98 2.1 0.94

a

VTH (V)

9.7 9.7 12.2 13.6 (2.0)b 15.0

on/off ratio 3 2 4 3 1

× × × × ×

106 106 106 106 106

Measured freshly prepared. bMeasured after initial cycling.

characteristics after different annealing temperatures. Higher annealing temperatures result in higher mobilities, up to an optimum temperature of 190 °C, where hole mobilities of >2 cm2/V·s and on/off ratios of >106 were observed. Figure 4 shows a transfer curve for one such device. The threshold voltage of these devices was observed to drop after initial cycling and stabilize as low as 2 V. Further increase of annealing temperature does not further benefit the device performance. Nonchlorinated solvents have been used successfully to process OPV devices,29 but have not generally been used successfully for OTFT processing up to now. However, good performance has been produced from PTDPPTFT4 devices using either chlorinated or nonchlorinated solvents to process the semiconductor. This is in contrast to previous polymers, such as P2TDC17FT4, where although nonchlorinated solvents can be used, the performance produced from nonchlorinated solvents is not as great as that produced when using chlorinated solvents. Further details of device development and characterization will be published separately in the near future. Film Characterization. Figure 5 shows the grazing-incident X-ray diffraction (GIXD) patterns of a PTDPPTFT4 film spincoated on a crystalline octadecyltrimethoxylsilane (OTS)treated silicon substrate.30,31 The polymer film exhibited welldefined (100), (200), and (300) diffraction peaks along the out785



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Figure 6. (200) diffraction peaks of PTDPPTFT4 in the out-of-plane Qz axis as a function of annealing temperature.

diffraction peaks of the films annealed at different temperatures. It was found that the diffraction intensity increased with annealing temperature, with a significant increase at 230 °C, while the full width at a half-maximum reduces when elevating the annealing temperatures. This suggests that the crystallinity of polymer chains is significantly enhanced through thermal treatment. To investigate the coherence length of the polymer orientation in the thin films, the full width at half-maximum (FWHM) of the out-of-plane (200) peaks was calculated. Figure 7 shows the correlation between the FWHM and

Figure 4. Transfer characteristics of a bottom-gate top-contact thin film transistor of PTDPPTFT4 fabricated on OTS-C8 treated Si/ SiO2(200 nm) wafer.

Figure 5. GIXD image of a PTDPPTFT4 film on an OTS-modified substrate annealed at 190 °C.

of-plane Qz axis. This reveals that the polymer film possesses a highly ordered lamellar structure. Besides the out of plane peaks, the film also displayed two in-plane diffraction peaks at 1.48 and 1.69 Å−1 on the Qxy axis, assigned to the crystalline OTS molecules and the (010) Bragg peak of the polymer, respectively. Since the (010) diffraction peak was attributed to the π−π stacking of the polymer, this strong in-plane (010) diffraction peak indicates that the π−π stacking of the polymer is preferentially parallel to the substrates.20,32 Therefore, the GIXD patterns of PTDPPTFT4 suggest a highly ordered lamellar packing with an edge-on orientation.17 This edge-on molecular packing may facilitate charge carrier transport because either the π−π interchain stacking, the intrachain conjugation, or a combination of the two will be parallel to the direction of the drain current in the field-effect transistors. The calculated lamellar spacing and π−π stacking of PTDPPTFT4 are 26.0 and 3.71 Å, respectively. The lamellar spacing is consistent with an interdigitated packing for the side-chains. The π−π stacking distance is comparable to other DPP-based copolymers.15,17,33−35 Charge transport of conjugated polymers is highly dependent on the intermolecular overlap integral.36 Therefore, this polymer, having a small π−π stacking distance, favorable to improve the intermolecular overlap integral, would be expected to show high charge carrier mobility. To investigate the influence of temperature on the molecular orientation of PTDPPTFT4 films, the GIXD patterns of a PTDPPTFT4 film were measured. Figure 6 shows the

Figure 7. FWHMs of PTDPPTFT4 films as a function of annealing temperature.

annealing temperature. The films annealed at 230 °C showed a narrower FWHM (0.056 Å−1) for the Qz = (200) peaks, compared to that without thermal annealing (0.072 Å−1). This suggests that the high annealing temperature reduces the orientation distribution of the polymer chains. According to the Scherrer equation,37 the out-of-plane coherence length (36.2 Å) of the polymer annealed in 230 °C is larger than that of the polymer without thermal annealing (26.7 Å), indicating the increase of the coherence length of the crystalline domain upon thermal treatment. The increased annealing temperature may allow the movement of polymer chains, leading to better molecular ordering.38 Molecular Modeling. Calculations have been performed to understand the origin of the higher hole mobility of PTDPPTFT4, as compared to those of P2TDC17FT4 reported earlier.3,4 This study makes reference to the charge transfer rate as described by Marcus theory in eq 1, for equivalent sites. λ is the Reorganization Energy (RE), t is the transfer integral (TI), 786



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bonded oxygen is facing a β-hydrogen to correspond to the lowest energy conformation. To confirm this result, the structures with the oxygen atom facing a β-hydrogen and the oxygen atom facing a sulfur were considered, to further equilibrate their respective structures and compute their energies with Density Functional Theory (DFT). For this study, DMol3 program, another Accelrys module, was used with the double numerical plus d-functions basis set (DND) coupled with the PW91 exchange correlation function of the General Gradient Approximation (GGA). It was found that the structure with the oxygen facing a β-hydrogen has the lowest energy, with higher ESP (partial atomic charges derived from ElectroStatic Potential fitting) absolute charge differences between O−H than O−S. These findings support the existence of intramolecular hydrogen bonds in thiophene substituted DPP species, which strengthen coplanarity of the main chain and promote strong π−π stacking. Furthermore, the modeled HOMO and LUMO of PTDPPTFT4 (Figure 8) shows a skewed electron distribution

h and kB are respectively the Planck and Boltzmann constants, and T is the temperature: kct =

4π 2 h

1 t 2e−λ /4kBT 4πλkBT

(1)

As can be seen from the charge transfer rate equation (eq 1), at constant temperature, there are two ways to affect the intrinsic mobility: the RE and the TI. The reorganization energies of three model compounds (DC17FT4, TDPPTFT4, and TDPPT) have been computed separately using VAMP, an Accelrys module, with the PM6 Hamiltonian. As shown in Table 3, TDPPT has the highest Table 3. Comparison of Reorganization Energies

Reorganization Energy of the set, followed by TDPPTFT4 and DC17FT4. Therefore the increased hole mobility of PTDPPTFT4 as compared with P2TDC17FT4 cannot be explained by a lowering of the RE. Differences in degree of overlap of the orbitals of neighboring molecules could be a significant factor contributing to the differences in hole mobilities. In polymeric materials, many factors can contribute to the enhancement of such orbital overlap, including coplanarity, extended conjugation, and π−π stacking. Enhancements in these parameters lead to an increase in the transfer integral, which is a likely explanation for the increase in the hole mobility for FT4-DPP copolymers, as compared with FT4 polymers with no DPP component. Accurate calculation of transfer integrals for a material requires precise information about packing, such as the crystal structure. For polymers, where the crystal structure is not available, the ability to make such precise calculations is limited. However, Transfer Integrals are generally enhanced by increasing planarity of the conjugated polymer backbone, which extends the conjugation field within a single chain, and increases the intermolecular π−π stacking. It has been reported in the literature that intra molecular hydrogen bonds between oxygen atoms in the DPP units and β-hydrogen atoms of the neighboring thiophenes contribute significantly to the increase of the coplanarity, hence the intermolecular π−π stacking.8 Conformation analysis of TDPPTFT4 with the Accelrys Conformer Tool, based on the Compass force field, found the conformation where the doubly

Figure 8. Optimized electron delocalization Molecular Orbital diagrams of the PTDPPTFT4 repeat unit.

toward the DPP core, which promotes a strong interaction between the FT4 side of one chain and the DPP side of an adjacent chain. The existence of such donor−acceptor regions in DPP-based repeat units has been reported to promote selfassembly of the polymer and hence increase charge transport.6,39 The existence of intramolecular hydrogen bonds that enhance the chain coplanarity in PTDPPTFT4 has been validated by DFT based calculations, which have revealed the nonsymmetric distribution of electrons. These attributes, which contribute to the enhancement of the π−π stacking and hence the charge carrier mobility in PTDPPTFT4, are both absent in P2TDC17FT4 where no DPP is present.

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CONCLUSIONS A high performance polymeric thin film transistor semiconductor material has been synthesized. This polymer was characterized through spectroscopic and electrochemical methods to understand the band gap. The long linear sidechains rendered this polymer easily synthesized and purified for potential scaling-up, because of appropriate solubility in the synthetic workup steps. Variable temperature GPC character787



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(4) He, M.; Li, J.; Sorensen, M. L.; Zhang, F.; Hancock, R. R.; Fong, H. H.; Pozdin, V. A.; Smilgies, D.-M.; Malliaras, G. G. J. Am. Chem. Soc. 2009, 131, 11930−11938. (5) Lo, P.; Ding, J.; Hu, J.; Chan, Y.; Garner, S.; He, M.; Lin, J.; Li, X.; Sorensen, M.; Li, J. J. Soc. Inf. Disp., Int. Symp. 2011, 42 (Bk.1), 387−388. (6) Tsao, H. N.; Cho, D. M.; Park, I.; Hansen, M. R.; Mavrinskiy, A.; Yoon, D. Y.; Graf, R.; Pisula, W.; Spiess, H. W.; Müllen, K. J. Am. Chem. Soc. 2011, 133, 2605−2612. (7) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272−3275. (8) Chen, H.; Guo, Y.; Yu, G.; Zhao, Y.; Zhang, J.; Gao, D.; Liu, H.; Liu, Y. Adv. Mater. 2012, 24, 4618−4622. (9) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. Sci. Rep. 2012, 2, Article number: 754, DOI: 10.1038/srep00754. (10) Street, R. A. Adv. Mater. 2009, 21, 2007−2022. (11) He, M.; Gasper, S. M.; Zhang, F.; Sorensen, M. L.; Fong, H. H.; Pozdin, V. A.; Amassian, A.; Malliaras, G. G. J. Soc. Inf. Disp., Int. Disp. Res. Conf. 2008, 260−263. (12) Kronemeijer, A. J.; Gili, E.; Shahid, M.; Rivnay, J.; Salleo, A.; Heeney, M.; Sirringhaus, H. Adv. Mater. 2012, 24, 1558−1565. (13) Ha, J. S.; Kim, K. H.; Choi, D. H. J. Am. Chem. Soc. 2011, 133, 10364−10367. (14) Sun, B.; Hong, W.; Aziz, H.; Li, Y. J. Mater. Chem. 2012, 22, 18950−18955. (15) Sonar, P.; Singh, S. P.; Li, Y.; Soh, M. S.; Dodabalapur, A. Adv. Mater. 2010, 22, 5409−5413. (16) Chen, Z.; Lee, M. J.; Ashraf, R. S.; Gu, Y.; Albert-Seifried, S.; Nielsen, M. M.; Schroeder, B.; Anthopoulos, T. D.; Heeney, M.; McCulloch, I.; Sirringhaus, H. Adv. Mater. 2012, 24, 647−652. (17) Li, Y.; Sonar, P.; Singh, S. P.; Soh, M. S.; van Meurs, M.; Tan, J. J. Am. Chem. Soc. 2011, 133, 2198−2204. (18) Morrison, R. T.; Boyd, R. N. Alkanes. Organic Chemistry, 6th ed.; Prentice Hall: New York, 1992; Chapter 3. (19) Oh, J. H.; Lee, W.-Y.; Noe, T.; Chen, W.-C.; Könemann, M.; Bao, Z. J. Am. Chem. Soc. 2011, 133, 4204−4207. (20) Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. J. Am. Chem. Soc. 2011, 133, 20130−20133. (21) He, M.; Zhang, F. J. Org. Chem. 2007, 72, 442−451. (22) He, M.; Niu, W. Di-Tin Fused Thiophene Compounds and Polymers and Methods of Making. U.S. Patent 8,278,346, October 2, 2012. (23) Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q. J. Phys. Chem. C 2008, 112, 15543−15552. (24) Huo, L.; Hou, J.; Chen, H.-Y.; Zhang, S.; Jiang, Y.; Chen, T. L.; Yang, Y. Macromolecules 2009, 42, 6564−6571. (25) Yuen, J. D.; Fan, J.; Seifter, J.; Lim, B.; Hufschmid, R.; Heeger, A. J.; Wudl, F. J. Am. Chem. Soc. 2011, 133, 20799−20807. (26) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.; Cölle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; Hamilton, R.; Richter, L.; Salleo, A.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W. Adv. Mater. 2009, 21, 1091− 1109. (27) Nelson, T. L.; Young, T. M.; Liu, Y.; Mishra, S. P.; Belot, J. A.; Balliet, C. L.; Javier, A. E.; Kowalewski, T.; McCullough, R. D. Adv. Mater. 2010, 22, 4617−4621. (28) Li, Y.; Sonar, P.; Singh, S. P.; Zengb, W.; Soh, M. S. J. Mater. Chem. 2011, 21, 10829−10835. (29) Chen, K.-S.; Yip, H.-L.; Schlenker, C. W.; Ginger, D. S.; Jen, A. K.-Y. Org. Electronics 2012, 13, 2870−2878. (30) Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.; Locklin, J.; Bao, Z. J. Am. Chem. Soc. 2009, 131, 9396−9404. (31) Virkar, A.; Mannsfeld, S.; Oh, J. H.; Toney, M. F.; Tan, Y. H.; Liu, G.-y.; Scott, J. C.; Miller, R.; Bao, Z. Adv. Funct. Mater. 2009, 19, 1962−1970.

ization of this polymer indicates the formation of aggregates at temperatures below 200 °C. The molecular weight for nonaggregated polymer could only be obtained at an eluent temperature of 200 °C. The GPC characterization reported here for DPP based polymers indicates the importance for detailed studies to obtain molecular weight values representative of the free polymer chains. PTDPPTFT4 shows great solubility in a wide range of industrially friendly hydrocarbon solvents such as cyclooctane, xylene, toluene, and decahydronaphthalene. GIXD and device studies have demonstrated the film quality and electrical performance. Computational modeling has been used to help explain the structure property relationships contributing to high performance in organic thin film transistors. This new polymer performs well in all the major areas necessary to be suitable for commercial applications, including synthesis, processability, stability, and electrical performance.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, Differential Scanning Calorimetry (DSC) of PTDPPTFT4 and P2TDC17FT4, Thermal Gravimetric Analysis (TGA) of PTDPPTFT4 in air and in nitrogen, thin film device transistor making procedure and characterization results, GIXD patterns of PTDPPTFT4 after different annealing temperatures, synthetic procedures and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.H.F.), [email protected] (Z.B.), [email protected] (M.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by Corning Incorporated. Z.B. and S.M. acknowledge support by the Department of Energy, Laboratory Directed Research and Development funding, under contract DE-AC02-76SF00515. The authors wish to thank researchers at Konarka for electrochemical measurements made on PTDPPTFT4. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science user Facility operated for the U.S. Department of Energy Office of Science by Stanford University.

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ABBREVIATIONS P2TDC17FT4 represents: Poly[(3,7-diheptadecylthieno [3,2b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene-2,6-diyl)(2,2′-bithiophene-5,5′-diyl)]

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