High-Mobility Ambipolar Organic Thin-Film Transistor Processed From

Sep 5, 2016 - Keywords: ambipolar transistors; balanced charge carrier mobilities; difluorothiophene; diketopyrrolopyrrole; nonchlorinated solvent; po...
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High-Mobility Ambipolar Organic Thin-Film Transistor Processed From a Nonchlorinated Solvent Prashant Sonar,*,†,‡ Jingjing Chang,*,†,§,∥ Jae H. Kim,†,⊥ Kok-Haw Ong,† Eliot Gann,# Sergei Manzhos,g Jishan Wu,†,§ and Christopher R McNeillh †

Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*STAR), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634 ‡ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, Queensland-4001, Australia § Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 ∥ Wide Bandgap Semiconductor Technology Disciplines State Key Laboratory, School of Microelectronics, Xidian University, Xi’an 710071, China ⊥ Anglo-Chinese School, 121 Dover Road, Singapore 139650 # Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia g Department of Mechanical Engineering Faculty of Engineering, National University of Singapore Block EA #07-08, 9 Engineering Drive 1, Singapore 117576 h Department of Materials Science and Engineering, Monash University, Wellington Road, Clayton, Victoria 3800, Australia S Supporting Information *

ABSTRACT: Polymer semiconductor PDPPF-DFT, which combines furansubstituted diketopyrrolopyrrole (DPP) and a 3,4-difluorothiophene base, has been designed and synthesized. PDPPF-DFT polymer semiconductor thin film processed from nonchlorinated hexane is used as an active layer in thin-film transistors. As a result, balanced hole and electron mobilities of 0.26 and 0.12 cm2/ (V s) are achieved for PDPPF-DFT. This is the first report of using nonchlorinated hexane solvent for fabricating high-performance ambipolar thin-film transistor devices.

KEYWORDS: diketopyrrolopyrrole, difluorothiophene, polymer semiconductors, ambipolar transistors, nonchlorinated solvent, balanced charge carrier mobilities

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(highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of the polymer semiconductors via the design of appropriate chemical functionalities in the conjugated backbone. To fine-tune the energy levels of polymer semiconductors, a feasibly and commonly used strategy is to combine alternating donor−acceptor (D−A) conjugated moieties in the polymer backbone.13,14 The HOMO energy level of a polymer semiconductor is related to its ionization potential and the LUMO to the electron affinity; these are critical for the donor and acceptor moieties, respectively. Many of the high-performance ambipolar OTFTs reported recently are based on polymer semiconductors that contain thiophene

n the scientific community, the ambipolar organic thin-film transistors (OTFT) have gained significant attention in past few years because of their use in single-component OTFT devices for complementary metal oxide semiconductor (CMOS)-like circuits.1−7 Such circuits can significantly reduce the complexity of the patterning and fabrication processes. By using ambipolar OTFTs, we have successfully demonstrated high gain inverters and flexible memory devices with higher performance.8−10 In addition to the above applications, ambipolar OTFTs are also potential candidates for lightemitting transistor devices.11,12 Such light-emitting transistors are promising components for future lighting applications. For all the mentioned applications, it is extremely important for an ambipolar semiconductor material with high yet comparable hole and electron balanced charge carrier mobility. The balanced ambipolar charge transport characteristics can be achieved by modulating the energy levels of the HOMO © XXXX American Chemical Society

Received: July 3, 2016 Accepted: September 5, 2016

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DOI: 10.1021/acsami.6b08075 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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which may facilitate charge carrier transport. For ambipolar OTFT operation, DFB and DFT units can tune the HOMO and LUMO energies of the polymer via intermixing of frontier molecular orbitals, leading to optimal energy levels for hole/ electron conductance. The synthesis route to make the polymer semiconductor PDPPF-DFT is outlined in Scheme 1. The analysis of frontier

substituted diketopyrrolopyrrole copolymers, usually combined with strong acceptors.15−17 Furan, which is a five-membered heterocycle, is also a promising candidate; however, this block has so far not been effectively used for designing new ambipolar OTFTs. Furan is an important biomass precursor and can be considered a “green” electronic material.18,19 Furan-substituted diketopyrrolopyrrole (DPP) “3,6-Di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione” (DBF) is a favorable conjugated fused aromatic heterocyclic moiety for ambipolar OTFTs, and recently our group successfully demonstrated DBF-based copolymers for high-performance ambipolar OTFT devices.20,21 To date, almost all the high-performing OTFTs were fabricated using chlorinated solvents such as chloroform, chlorobenzene, and dichlorobenzene, which can cause significant environmental damage during manufacturing, use and disposal.22,23 For large-scale flexible OTFTs and roll-to-roll printable organic electronic devices, the use of toxic chlorinated solvents can be a serious issue because o fthe additional environmental costs at the mass-production stage. However, nonchlorinated solvents are still less used in organic electronic device processing, as most of the polymer semiconductors reported to date have poor solubility in such solvents. The replacement of the heterocylic thiophene moiety with furan of DPP core is a good molecular design approach to enhance solubility of such organic semiconductors in nonchlorinated solvents. Such tuning could allow environmentally benign processing and will also provide an opportunity to make biodegradable electronic materials and devices due to incorporation of “green” furan building blocks. Until now, there have been a limited number of approaches to fabricate devices using nonchlorinated solvents in the field of printable organic electronic devices.24,25 Thus, it is necessary to explore the possibility of using nonchlorinated solvents such as hexane in such device fabrication or printing technology. In this context, there is an emerging significance of developing both green electronic materials and green processing technologies. Environmentally friendly furan with DPP can become an ideal building block for constructing low-band-gap donor−acceptor semiconducting polymers. There are, however, few reports on this class of materials compared to its analogous thiophenesubstituted DPP. In this research work, we are reporting the molecular design, synthesis, and characterization of an innovative solutionprocessable alternating copolymer PDPPF-DFT using furansubstituted DPP (electron accepting DBF block) and novel electron-accepting 3,4-difluoro thiophene (DFT) moieties. We use this polymer as an active channel semiconductor in ambipolar OTFTs using hexane as the processing solvent. The substitution of two electron-withdrawing fluorine atoms on the thiophene makes DFT to be a promising novel electron accepting building block. Incorporation of such block with fused furan flanked DPP in the conjugated backbone of polymers could enhance π−π stacking through hydrogen bonding between electronegative fluorine atoms and hydrogen atoms on the neighboring furan DPP. Additionally, fluorination of the backbone of the polymer could diminish the HOMO energy level, resulting in improved stability and can promote the intramolecular interaction of S−F and H−F for better packing.26,27 Furthermore, higher thermal stability can be achieved by incorporating fluorine in the backbone of the conjugated chain. A strong donor−acceptor interaction between DBF and DFT may enhance the degree of planarity,

Scheme 1. (a) Synthesis of Donor−Acceptor PDPPF-DFT Copolymers and (b) Isosurfaces of HOMO and LUMO Orbitals of PDPPF-DFT Polymer Repeating Unita

a

Atom colors: C, brown; O, red; N, blue; S, yellow; H, pink; F, violet.

orbitals shows significant LUMO amplitude on the fluorine atoms in the fluorinated furan, reflecting the electron withdrawing nature of F (Scheme 1b). First, the compound 3,6-bis(5-bromo- furan-2-yl)-N,N′-bis(2-octyldodecyl)-1,4dioxo-pyrrolo[3,4-c]pyrrole (1) was easily synthesized by using furan flanked DPP core 3,6-di(furan-2-yl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione followed by alkylation and bromination reactions. Another monomer (3,4-difluorothiophene2,5-diyl)bis(trimethylstannane) (2) was synthesized via lithiation reaction using n-butyl lithium followed by addition of trimethyl tin chloride and the starting compound 3,4difluorothiophene using an earlier reported procedure.28,29 Reactions of compounds 1 and 2 via Stille coupling polymerization gave the polymer poly{3,6-difuran-2-yl-2,5di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt- 3,4-difluoro thiophene} (PDPPF-DFT) as a crude polymeric material (Scheme 1). PDPPF-DFT crude polymer was then purified by sequential Sohxlet extraction using methanol and acetone separately in order to remove impurities such as catalysts and oligomers. A good solubility of the polymer in the nonchlorinated hexane solvent is due to the long branched octyldodecyl chains substituted on the DPP core and use of short thiophene comonomer building blocks. Such molecular engineering (using long branched alkyl chain substitution and appropriate comonomer) strategy is helpful for enhancing solubility in nonchlorinated solvents such as hexane. The thermal properties of PDPPF-DFT were studied by the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques (see Supporting Information S3). The second heating scan of the DSC measurement revealed an endothermic peak at 202 °C, whereas upon cooling, an exothermic peak was revealed at 129 °C. The lower melting temperature of PDPPF-DFT polymer is attributed to the B

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was calculated to be around −5.40 eV. The obtained HOMO value of the polymer PDPPF-DFT (−5.40 eV) is lower than that of the earlier reported ambipolar polymer PDPPHD-T3 (−5.32 eV) because of electron withdrawing by two fluorine atoms substituted on the thiophene comonomer.30 The LUMO value could be obtained by calculating the difference between solid-state optical band gap and HOMO value. The LUMO value was found to be −4.03 eV. From both HOMO and LUMO values, it can be seen that the energy levels of PDPPFDFT are just appropriate for hole and electron injection if gold can be used as an electrode in OTFT devices. To explore the consequence of fluorine substitution on the energy level stabilization, frontier orbitals for fluorinated repeating unit and nonfluorinated repeating unit were computed with DFT (density functional theory). HOMO and LUMO for the fluorinated monomer are shown in Scheme 1 and are compared with nonfluorinated in Figure S6. From these theoretical HOMO−LUMO orbitals, it can be seen that following the fluorination of the thiophene units, both HOMO and LUMO are somewhat delocalized on the electronwithdrawing fluorine atom, which corresponds to the stabilization of these orbitals. The computed frontier orbital energies are listed in Table S1. Both HOMO and LUMO energies are stabilized by about 0.2 eV in comparison with the nonfluorinated analogues, reflecting the electron withdrawing nature of F. The band gap is little affected by fluorination. The extent of stabilization was similar in monomer and dimer calculations and in DFT (abridged side chains shown in Scheme 1) and DFTB (full side chains shown in Figure S5) and therefore likely holds for a polymer. The differences between the experimentally estimated HOMO and LUMO and those in Table S1 are attributable to the finite size of the model, aggregate state, and the approximations made. To confirm stacking behavior of polymer chains, we used two-dimensional X-ray diffractometry (2-D XRD) on polymer flakes. 2D-XRD analysis was performed on the PDPPF-DFT via X-ray parallel and perpendicular modes to the polymer films. Such measurement can provide important information about the molecular packing and solid state ordering. As presented in Figure 2a, when the polymer films are parallel to X-ray, the primary diffraction peak was measured at 2θ = 5.79°, which corresponds to the interspacing distance of 15.27 Å, whereas the wide-ranging peak positioned from 2θ = 16−24° demonstrates that PDPPF-DFT film is weak crystalline. The interlayer distance derived from the broad peak at 2θ = 20.89° is 4.24 Å. To get a clear understanding of nanostructuring and self-assembly using few tens of nanometer (40−50 nm) thin film of polymer directly deposited on Si/SiO2 substrates and orientation distribution of the semiconducting polymer chains, Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) measurements were performed. The GIWAXS results (Figure 2b) show an apparent lack of order in the as-cast film and only weak alkyl stacking and π-stacking observed in the annealed film. The GIWAXS results provide an alkyl stacking distance of 19 ± 1 Å, which is larger than that recorded from the bulk XRD measurement, suggesting a different packing geometry in thin film. The polymer crystallites that exist in the annealed film do not show a preference for edge-on or face-on with a value of Herman’s orientation parameter of S = −0.01 ± 0.01 for the annealed sample (S may vary from a value of S = 1.0 for perfectly edge-on stacking to which runs from perfectly face-on at S = −0.5 for perfect face-on stacking; a value of S = 0 shows no preferential orientation). To support this observation,

longer alkyl chain substituted on the DPP moiety. A decomposition temperature of 360 °C was determined by performing TGA under a nitrogen atmosphere, indicating that the polymer has high thermal stability. The optical properties of PDPPF-DFT polymer were studied by UV−vis absorption spectroscopy both in chloroform solution and thin film deposited on a glass substrate. The solution measurements show the maximum absorption peak (λmax) at 800 nm, with an almost identical λmax in the thin film spectrum (Figure 1a). The optical band gap of PDPPF-DFT

Figure 1. (a) UV−vis absorption spectra of PDPPF-DFT in solution (chloroform) and in solid state (thin film processed from chloroform solution). (b) Photoelectron spectroscopy (PESA) analysis of PDPPDFT thin films on glass in air.

was calculated from the solid-state UV−vis absorption spectrum by calculating absorption cutoff value. The absorption cutoff was found to be at ∼850 nm which gives an optical band gap of 1.45 eV. Theoretical absorption spectrum calculations were performed on the repeating unit monomer and dimer (see Figures S4 and S5) and the peak maximum of the dimer of 768 nm is close to the measured peak maximum shown in Figure 1. One expects the peak to shift slightly to the red for higher degree of polymerization (cf. monomer). To estimate the HOMO and LUMO energy levels of PDPPF-DFT, we first estimated the HOMO value from the photoelectron spectroscopy in air (PESA) measurements on the spin-coated thin film of PDPPF-DFT on glass. As shown in Figure 1b, the intersection of photoelectron yield ratio and UV photon energy gave the onset energy level value and the HOMO value C

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Figure 3. (a, b) Transfer and (c, d) output characteristic of a PDPPFDFT-based ambipolar OTFT on OTS-SiO2 substrate with 150 °C annealing. The transfer curves of hole and electron operation (channel length = 60 μm; channel width = 1 mm).

Figure 2. (a) 2D XRD diffractogram (inset: 2D XRD images) with the incident X-ray parallel to the PDPPF-DFT copolymer flakes, (b) 2D GIWAXS patterns of (left image) as cast and (right image) 250 °C annealed thin films of PDPPF-DFT.

An atomic force microscopy (AFM) study was performed in order to correlate the effect of thermal annealing on the device performance and to differentiate the morphological changes of the thin films of PDPPF-DFT. The height and phase AFM images of the polymer thin films are shown in Figure 4 and

PDPPF-DFT thin films deposited directly to the octadecyltrichlorosilane (OTS)-treated SiO2 (OTS-SiO2) substrates were also measured using conventional XRD (Figure S7b). The field-effect transistor characteristics using PDPPF-DFT as the active layer were evaluated. The transistor devices adopt a bottom-gate, top-contact configuration. The device fabrication is similar to the procedure reported previously.21 The PDPPF-DFT polymer thin film (∼40 nm) on OTS-SiO2 surface was obtained by spin-coating the hexane or chloroform solution (6 mg/mL). The OTFT devices showed ambipolar behavior and from the saturation regime of the transfer curves, the charge carrier mobilities could be obtained. Figure 3 and Figure S8 show the transfer and output characteristics, and Table S2) summarizes the device performance. Hole and electron mobilities of around 0.10 and 0.013 cm2/(V s) were achieved for 100 °C annealed thin film. With 150 °C annealing, the hole and electron mobility improves to 0.26 and 0.12 cm2/ (V s), which is a very balanced high hole/electron mobility for such a low annealing temperature. With 200 °C annealing, the hole mobility was further improved to 0.30 cm2/(V s) but electron mobility is reduced to 0.023 cm2/(V s). Figure 3 shows the transfer and output characteristics of PDPPF-DFTbased OTFTs annealed at 150 °C. The transfer curves clearly show typical ambipolar V shaped behavior. The transistor showed slightly bias stress effect in the saturation regime of output characteristics in the electron enhancement mode due to charge trapping from dielectric surface and/or bulk materials (impurities or solvent residues). The current on/off ratio (Ion/ Ioff) of ∼1 × 103 to 1 × 104 is calculated for all of the devices. In addition to the hexane fraction, we also used the chloroform soluble fraction for fabricating OTFT devices and a similar level of performance was obtained (see Table S2). This is the first time that such high-performance ambipolar OTFT devices comparable with conventional halogenated solvent processed devices have been fabricated using hexane.

Figure 4. (a−c) AFM height and (d−f) phase images of PDPPF-DFT thin films on OTS-SiO2 substrates annealed at 100 °C, 150 °C, and 200 °C.

Figure S9, and it is found that the polymer thin film with 100 °C annealing exhibits less organized thin film morphology due to its amorphous nature. This is consistent with the XRD results where no obvious diffraction peaks could be observed (Figure S7b). The thin film microstructure becomes slightly more organized and the surface roughness decreases from 1.7 to 0.9 nm when the annealing temperature increased. The annealing process further reduces the solvent residue related traps and hence improves the thin film mobility. When annealing temperature increases to 200 °C, the thin films D

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(2) Nielsen, C. B.; Turbiez, M.; McCulloch, I. Recent Advances in the Development of Semiconducting DPP-Containing Polymers for Transistor Applications. Adv. Mater. 2013, 25, 1859−1880. (3) Torricelli, F.; Ghittorelli, M.; Smits, E. C. P.; Roelofs, C. W. S.; Janssen, R. A. J.; Gelinck, G. H.; Kovács-Vajna, Z. M.; Cantatore, E. Ambipolar Organic Tri-Gate Transistor for Low-Power Complementary Electronics. Adv. Mater. 2016, 28, 284−290. (4) Sonar, P.; Chang, J.; Shi, Z.; Wu, J.; Li, J. Thiophene− tetrafluorophenyl−thiophene: A Promising Building Block for Ambipolar Organic Field Effect Transistors. J. Mater. Chem. C 2015, 3, 2080−2085. (5) Kanimozhi, C.; Yaacobi-Gross, N.; Chou, K. W.; Amassian, A.; Anthopoulos, T. D.; Patil, S. Diketopyrrolopyrrole-Diketopyrrolopyrrole-Based Conjugated Copolymer for High-Mobility Organic FieldEffect Transistors. J. Am. Chem. Soc. 2012, 134, 16532−16535. (6) Sonar, P.; Foong, T. R. B.; Singh, S. P.; Li, Y.; Dodabalapur, A. A Furan-Containing Conjugated Polymer for High Mobility Ambipolar Organic Thin Film Transistors. Chem. Commun. 2012, 48, 8383. (7) Chen, Z.; Lemke, H.; Albert-Seifried, S.; Caironi, M.; Nielsen, M. M.; Heeney, M.; Zhang, W.; McCulloch, I.; Sirringhaus, H. High Mobility Ambipolar Charge Transport in Polyselenophene Conjugated Polymers. Adv. Mater. 2010, 22, 2371−2375. (8) Gentili, D.; Sonar, P.; Liscio, F.; Cramer, T.; Ferlauto, L.; Leonardi, F.; Milita, S.; Dodabalapur, A.; Cavallini, M. Logic-Gate Devices Based on Printed Polymer Semiconducting Nanostripes. Nano Lett. 2013, 13, 3643−3647. (9) Zhou, Y.; Han, S.; Sonar, P.; Roy, V. A. L. Nonvolatile Multilevel Data Storage Memory Device from Controlled Ambipolar Charge Trapping Mechanism. Sci. Rep. 2013, 3, 2319. (10) Chen, H.; Guo, Y.; Mao, Z.; Yu, G.; Huang, J.; Zhao, Y.; Liu, Y. Naphthalenediimide-Based Copolymers Incorporating Vinyl-Linkages for High-Performance Ambipolar Field-Effect Transistors and Complementary-like Inverters under Air. Chem. Mater. 2013, 25, 3589−3596. (11) Bürgi, L.; Turbiez, M.; Pfeiffer, R.; Bienewald, F.; Kirner, H.-J.; Winnewisser, C. High-Mobility Ambipolar Near-Infrared LightEmitting Polymer Field-Effect Transistors. Adv. Mater. 2008, 20, 2217−2224. (12) Gwinner, M. C.; Kabra, D.; Roberts, M.; Brenner, T. J. K.; Wallikewitz, B. H.; McNeill, C. R.; Friend, R. H.; Sirringhaus, H. Highly Efficient Single-Layer Polymer Ambipolar Light-Emitting FieldEffect Transistors. Adv. Mater. 2012, 24, 2728−2734. (13) Guo, X.; Baumgarten, M.; Mullen, K. Designing Pi-Conjugated Polymers for Organic Electronics. Prog. Polym. Sci. 2013, 38, 1832− 1908. (14) Pron, A.; Leclerc, M. Imide/amide Based π-Conjugated Polymers for Organic Electronics. Prog. Polym. Sci. 2013, 38, 1815− 1831. (15) Li, Y.; Sonar, P.; Murphy, L.; Hong, W. High Mobility Diketopyrrolopyrrole (DPP)-Based Organic Semiconductor Materials for Organic Thin Film Transistors and Photovoltaics. Energy Environ. Sci. 2013, 6, 1684. (16) Mohebbi, A. R.; Yuen, J.; Fan, J.; Munoz, C.; Wang, M. F.; Shirazi, R. S.; Seifter, J.; Wudl, F. Emeraldicene as an Acceptor Moiety: Balanced-Mobility, Ambipolar, Organic Thin-Film Transistors. Adv. Mater. 2011, 23, 4644−4648. (17) Chang, J.; Lin, Z.; Li, J.; Lim, S. L.; Wang, F.; Li, G.; Zhang, J.; Wu, J. Enhanced Polymer Thin Film Transistor Performance by Carefully Controlling the Solution Self-Assembly and Film Alignment with Slot Die Coating. Adv. Electron. Mater. 2015, 1, 1500036. (18) Gidron, O.; Dadvand, A.; Sheynin, Y.; Bendikov, M.; Perepichka, D. F. Towards “green” electronic Materials. α-Oligofurans as Semiconductors. Chem. Commun. 2011, 47, 1976−1978. (19) Gidron, O.; Diskin-Posner, Y.; Bendikov, M. α-Oligofurans. J. Am. Chem. Soc. 2010, 132, 2148−2150. (20) Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.; Frechet, J. M. J. Incorporation of Furan into Low Band-Gap Polymers for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 15547−15549.

become discontinuous due to dewetting problem caused by the melting and reorganization of polymer chains on OTS-SiO2 dielectric, and this has been also observed and well supported during the DSC analysis of polymer. DSC data clearly indicate the phase transition occurring around 200 °C, and this may be the reason for the electron mobility decrease in the OTFT devices. In summary, with the incorporation of innovative 3,4difluoro thiopehene and furan-substituted DPP blocks, the resulting solution-processable polymer PDPPF-DFT has been synthesized via Stille coupling. Ambipolar OTFT using PDPPF-DFT as an active channel semiconductor has shown higher and balanced hole/electron mobility of 0.26 and 0.12 cm2/(V s), respectively. We have also demonstrated that by using a nonchlorinated hexane solvent, we can achieve comparable or higher hole/electron mobilities in ambipolar transistor devices. Such a green processing solvent and utilization of a green electronic building block furan is a promising approach to fabricate user-friendly printed electronic devices.

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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/acsami.6b08075. GPC elution curves, DSC and TGA thermograms, theoretical modeling details, XRD supporting data, OTFT and AFM data of PDPPF-DFT (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR) and the “Printable high performance semiconducting materials for OPVs and OTFTs” for financial support. P.S. is thankful to the CRC for Polymers at Queensland University of Technology (QUT) for equipment support. We are thankful to John Colwell (QUT) for the help in GPC measurement and Hong Duc Pham (QUT) for DSC and TGA measurement. We are also thankful to Mr. Poh Chong Lim (IMRE) for his help in 2D XRD of the polymer. C.R.M. similarly thanks the ARC for research support (DP130102616). J.W. acknowledges financial support from IMRE core funding (IMRE/13-1C0205). This research was undertaken in part on the SAXS/WAXS beamline at the Australian Synchrotron, Victoria, Australia.

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REFERENCES

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ACS Applied Materials & Interfaces (21) Sonar, P.; Chang, J.; Shi, Z.; Gann, E.; Li, J.; Wu, J.; McNeill, C. R. Hole Mobility of 3.56 cm2V−1s−1 Accomplished Using More Extended Dithienothiophene with Furan Flanked Diketopyrrolopyrrole Polymer. J. Mater. Chem. C 2015, 3, 9299−9305. (22) Kang, I.; Yun, H. J.; Chung, D. S.; Kwon, S. K.; Kim, Y. H. Record High Hole Mobility in Polymer Semiconductors via SideChain Engineering. J. Am. Chem. Soc. 2013, 135, 14896−14899. (23) Yi, Z.; Wang, S.; Liu, Y. Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors. Adv. Mater. 2015, 27, 3589−3606. (24) Yun, H.-J.; Lee, G. B.; Chung, D. S.; Kim, Y.-H.; Kwon, S.-K. Novel Diketopyrroloppyrrole Random Copolymers: High ChargeCarrier Mobility From Environmentally Benign Processing. Adv. Mater. 2014, 26, 6612−6616. (25) Chang, J.; Chi, C.; Zhang, J.; Wu, J. Controlled Growth of Large-Area High-Performance Small-Molecule Organic Single-Crystalline Transistors by Slot-Die Coating Using a Mixed Solvent System. Adv. Mater. 2013, 25, 6442−6447. (26) Kim, H. G.; Kang, B.; Ko, H.; Lee, J.; Shin, J.; Cho, K. Synthetic Tailoring of Solid-State Order in Diketopyrrolopyrrole-Based Copolymers via Intramolecular Noncovalent Interactions. Chem. Mater. 2015, 27, 829−838. (27) Shewmon, N. T.; Watkins, D. L.; Galindo, J. F.; Zerdan, R. B.; Chen, J.; Keum, J.; Roitberg, A. E.; Xue, J.; Castellano, R. K. Enhancement in Organic Photovoltaic Efficiency through the Synergistic Interplay of Molecular Donor Hydrogen Bonding and πStacking. Adv. Funct. Mater. 2015, 25, 5166−5177. (28) Homyak, P.; Liu, Y.; Liu, F.; Russel, T. P.; Coughlin, E. B. Systematic Variation of Fluorinated Diketopyrrolopyrrole Low Bandgap Conjugated Polymers: Synthesis by Direct Arylation Polymerization and Characterization and Performance in Organic Photovoltaics and Organic Field-Effect Transistors. Macromolecules 2015, 48, 6978−6986. (29) Park, S.; Cho, J.; Ko, M. J.; Chung, D. S.; Son, H. J. Synthesis and Charge Transport Properties of Conjugated Polymers Incorporating Difluorothiophene as a Building Block. Macromolecules 2015, 48, 3883−3889. (30) Li, Y.; Singh, S. P.; Sonar, P. A High Mobility P-Type DPPthieno[3,2-b]thiophene Copolymer for Organic Thin-Film Transistors. Adv. Mater. 2010, 22, 4862−4866.

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DOI: 10.1021/acsami.6b08075 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX