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Temperature-Driven Phase Transition of a Fused Dithienobenzothiadiazole−Tetrathiophene Based Semiconducting Copolymer Mingyuan Pei,† Jun Huang,‡ Mi Jang,† Ji-Hoon Kim,‡ Minjung Lee,† Junwu Chen,§ Do Hoon Hwang,*,‡ and Hoichang Yang*,† †

Department of Applied Organic Materials Engineering, Inha University, Incheon 402-751, Korea Department of Chemistry, Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea § Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: A high-molecular-weight donor−acceptor (D−A) copolymer, pDTfBT-4T, poly(5,8-di[3-octyldodecanthiopen-5-yl]dithieno[3′,2′:3,4:2″,3″:5,6]benzo[1,2-c][1,2,5]-thiadiazole-alt-2,2′bithiophene), including alternating dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiazole (DTfBT) and 3-octyldodecyltetrathiophene (4T) was synthesized by a Stille coupling polymerization. We found that pDTfBT-4T had a molecular number-average weight of 276 kg mol−1 and formed unexpectedly strong interchain aggregates in dilute solutions at room temperature, which was similar to those in as-spun thin solid films. pDTfBT-4T thin films were spun-cast from a warm dilute chlorobenzene solution on hydrophobic polymer-treated SiO2 dielectrics. Some were shortly annealed at various temperatures (T) for 10 min to improve π-overlapped structures as charge-carrier transport paths. The ordered domains of pDTfBT-4T in the annealed films could be tuned from onedimensional nanorods to two-dimensional nanosheets with an increasing in T, which also provided enhanced crystal orientation. Optimizing the conjugated structures of pDTfBT-4T in the annealed films, the polymer based OFETs yielded a hole mobility up to 1.45 cm2 V−1 s−1 as well as negligible hysteresis and excellent negative bias stability. entangled with each other, preventing a closer π-overlap between these chains. However, these semiconducting polymers possess excellent mechanical durability and thermal stability, which make them more useful as active channel materials in organic electronics, in comparison to small πconjugated molecules. One of the most powerful strategies to improve the intraand intermolecular interactions between semiconducting polymer chains is the design of semiconducting backbones that include alternated electron donor−acceptor (D−A) building blocks. The push−pull structure of the D−A alternating units combined with a low degree of torsional twisting between monomers leads to a polymer backbone with a very low degree of torsional twisting between segments, which provides polymer backbones with low energetic disorder. The resulting better coplanarity and smaller reorganization energies of the D−A conjugated backbones could induce closely π-overlapped conducting paths in the solid state.14

1. INTRODUCTION Organic field-effect transistors (OFETs) have attracted considerable research attention because of their potential applications in displays, radio-frequency identification (RFID) tags, and sensors as low-cost, flexible, and lightweight electronic components.1−5 To achieve high performance OFETs, it is important to control the intra- and intermolecular ordering of the semiconducting molecules that serve as charge carrier transport paths.6,7 Previous investigators have suggested that small molecules such as pentacene,8 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), 9 and 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT)10 could quickly self-assemble into highly ordered domains with long-range π-overlapped chains. This order is expected to provide better charge-carrier transport along electrodes in comparison to conventional semiconducting polymers with less-rigid backbones like regioregular poly(3-hexylthiophene) (P3HT),11 poly(3,3‴didodecylquarterthiophene) PQT-12,12 and poly(2,5-bis(3tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT).13 As the chain length increases, conjugated polymer backbones tend to form a twisted conformation in solutions and become © XXXX American Chemical Society

Received: November 16, 2015 Revised: December 31, 2015

A

DOI: 10.1021/acs.jpcc.5b11213 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Synthetic Scheme of the pDTfBT-4T Used in This Study

Figure 1. Typical characteristics of pDTfBT-4T used in this study: (a) UV−vis spectra in a CB solution and thin films. (b) CV curve of a thin film on an electrode in a 0.1 M TBABF4−acetonitrile solution. (c) TGA heating and (d) DSC heating and cooling curves obtained for pDTfBT-4T at a constant rate of 10 °C min−1.

(∼1.63 eV). However, the resulting OFET still showed a low mobility of 0.013 cm2 V−1 s−1.22 Dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiazole (DTfBT) was recently used as an A-block moiety, and it considerably improved the drawback of BT unit.23 As shown in Scheme 1, the fused ring in DTfBT extends the π-electron delocalization, providing improvement in both the intra- and intermolecular π-overlapped structures. Mei et al. demonstrated

Recently, many D−A conjugated copolymers have been designed and synthesized, yielding excellent electrical properties in organic electronics.15−17 Among these copolymers, several A-block derivatives have been reported, including 1,4diketopyrrolo[3,4-c]pyrrole (DPP),16,17 isoindigo,18,19 and benzothiadiazole (BT)20−22 derivatives. Chen and co-workers reported a BT-based conjugated polymer that exhibited a broad absorption ranging from 300 to 750 nm and a narrow band gap B

DOI: 10.1021/acs.jpcc.5b11213 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C that the DTfBT building block could provide excellent electrical properties in corresponding copolymers with a benzo[1,2-b:4,5-b′]dithiophene (BDT) derivative.24 Also, Huang et al. reported a conjugated DTfBT-3T copolymer including alternated DTfBT and solubilizing alkyl-substituted terthiophene (3T) blocks had short repeating units of approximately 32, but the spun-cast film of this polymer provided unexpected high field-effect mobility (μFET) values of 0.55 cm2 V−1 s−1 in an OFET.25 Here, we synthesized a high-molecular-weight DTfBT based copolymer including a relatively more flexible tetrathiophene (4T) with 3-octyldodecyl substituents, instead of 3T. The pDTfBT-4T had a molecular weight of 276 kg mol−1 and approximately 243 repeating units. pDTfBT-4T thin films were spun-cast on polymer-treated SiO2 dielectrics from a warm dilute chlorobenzene (CB) solution and were further annealed at various temperatures (T) for 10 min. An ordered phase transition behavior of the copolymer from one-dimensional (1D) nanorods to two-dimensional (2D) nanosheets was clearly observed in the annealed films with an increase in T. Optimizing the π-overlapped structures of the DTfBT-based copolymer in the solid films, the resulting OFETs showed a hole mobility up to 1.45 cm2 V−1 s−1 as well as negligible hysteresis and excellent negative bias stability.

atmosphere. Additionally, DSC (TA Instruments, Q20) was performed on a powder-like pDTfBT-4T sample with a scanning rate of 10 °C min−1. UV−vis absorption spectra of the copolymer in a dilute CB solution and on spun-cast films were measured using a UV−vis spectrometer (JASCO, JP/V-570). Cyclic voltammetry (CV) for the copolymer was performed using an electrochemical analyzer (CH Instruments) in an anhydrous acetonitrile solution containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as a supporting electrolyte at a scan rate of 50 mV s−1. A glassy carbon disk (∼0.05 cm2) coated with a thin polymer film, an Ag/AgNO3 electrode, and a platinum wire were used as the working, reference, and counter electrodes, respectively. All the film morphologies of pDTfBT-4T on the gPS-SiO2 surfaces were observed by atomic force microscopy (AFM, Bruker, Multimode 8). Current−voltage (I−V) measurements of pDTBT-4T based OFETs were conducted in a N2-purged glovebox at room temperature, using a Keithley 4200 SCS. Field-effect mobility (μFET) was calculated from drain current− gate voltage (ID−VG) transfer curves of the OFETs operating under the saturation region (at a drain voltage, VD = −20 V) using the following equation: ID = μFETCiW(2L)−1(VG − Vth)2. Here, Ci is the capacitance per unit area of the dielectric, and Vth is the threshold voltage. 2D grazing-incidence X-ray diffraction (GIXD) was conducted on all the pDTfBT-4T films at beamlines 3C, 6D, and 9A of the Pohang Acceleration Laboratory (PAL), Korea. The incident angle of the X-ray beam on the sample remained below 0.18°.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Poly(5,8-di[3octyldodecanthiopen-5-yl]dithieno[3′,2′:3,4:2″,3″:5,6]benzo[1,2-c][1,2,5]-thiadiazole-alt-2,2′-bithiophene), pDTfBT-4T, was synthesized by Stille coupling using Pd2(dba)3/P(o-toly)3 as catalyst in CB (Scheme 1; Figures S1, S2 and the Experimental Section in the Supporting Information). The crude copolymer was purified by successive washing with hot methanol, hexane, and acetone in a Soxhlet extractor. A 300 nm thick layer of thermally grown silicon dioxide (SiO2) on a highly n-doped Si substrate was used as a major gate dielectric. To modify the SiO2 dielectric surface, dimethylchlorosilane-terminated polystyrene (PS-Si(CH3)2Cl, number-average molecular weight, Mn = 8.0 kg mol−1, weightaverage molecular weight, Mw = 8.6 kg mol−1, Polymer Source Inc.) was dissolved in anhydrous toluene, and the solution was spun-cast onto SiO2/Si substrates in a N2-purged glovebox. Then, the resulting films were annealed at 100 °C for 60 min, followed by rinsing with toluene and sonicating in a toluene bath for 3 min to remove unreacted residue.10 pDTfBT-4T films were spun-cast onto the PS-grafted SiO2 (referred to as gPS-SiO2) dielectrics from a 5 mg mL−1 solution dissolved in warm CB (∼60 °C). Some were further annealed at various annealing temperatures (TAs) for 10 min, based on differential scanning calorimetry (DSC) analysis (Figure 1c). Finally, Au was thermally evaporated through a shadow mask to produce top-contact source/drain (S/D) electrodes in the OFET. The channel length (L) and width (W) were 100 and 1500 μm, respectively. 2.2. Characterization. Mn, Mw, and polydispersity index (PDI, Mw/Mn) of pDTfBT-4T were determined by gel permeation chromatography (GPC, calibrated with PS standards, Waters, M590) in hot CB (80 °C). The chemical compositions of precursors and pDTfBT-4T were obtained from 1H nuclear magnetic resonance (NMR, Varian, Mercury Plus 300 MHz spectrometer) analysis. The thermal stability of the copolymer was measured using thermogravimetric analysis (TGA, TA Instruments, Q50) operated under a N 2

3. RESULTS AND DISCUSSION The synthesized pDTfBT-4T had a Mn of 276 kg mol−1, but a broad PDI of 3.23 (Figure S3 and Table S1). Because of the low solubility of the high-Mn copolymer (below 5 mg in chloroform, CB, and o-dichlorobenzene at room temperature, RT), the copolymer in CB was kept at 60 °C before spincasting. Figure 1a represents UV−vis absorption spectra of the copolymer in a CB solution and the corresponding cast films before and after annealing at different TAs ranging from 150 to 300 °C. Detailed absorption parameters, such as absorption maxima (λmax), edge (λedge), and optical band gap (Egopt), are summarized in Table 1. A spun-cast pDTfBT-4T film exhibited Table 1. Optical and Electrochemical Properties of pDTfBT4T λmax (nm) polymer pDTfBT4T

solutiona

filmb

λedge (nm)

Egopt c (eV)

HOMOd (eV)

LUMOe (eV)

607

603

656

1.89

−5.10

−3.21

a Measured in a dilute CB solution. bMeasured on a quartz plate containing the corresponding spun-cast film. Calculations based on the absorption band edge of the copolymer films. cEg = 1240/λedge. d HOMO = −e(Eoxonset + 4.72) (eV). eLUMO = Egopt + HOMO (eV).

a 0−0 absorption peak at 592 nm and a well-resolved shoulder 0−1 peak at 553 nm. The UV−vis absorption intensities at 0−0 and 0−1 peaks for the annealed films, except for 300 °C annealed one with an irregular uniformity, tended to increase with an increase in TA, originating from the enhancement in crystallinity and ordering of the copolymer in the annealed C

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Figure 2. (a−e) AFM topographies of pDTfBT-4T films spun-cast on gPS-SiO2 surfaces before and after annealing at various TAs for 10 min: (a) asspun, (b) 150 °C, (c) 200 °C, (d) 250 °C, and (e) 300 °C. (f) Cross-sectional height profiles extracted from lines in (a)−(e).

Figure 3. (a−e) 2D GIXD patterns and (f) azimuthal angle scan profiles (at Q(100)) of spun-cast pDTfBT-4T films on gPS-SiO2 dielectrics before and after annealing at different TAs: (a) as-spun, (b) 150 °C, (c) 200 °C, (d) 250 °C, and (e) 300 °C.

films (will be discussed later). Interestingly, pDTfBT-4T in a dilute CB (at RT) showed similar absorption spectra to the spun-cast film. It suggests that the pDTfBT-4T chains in the RT solution already formed strongly π-conjugated aggregates. Similar results have been reported for benzo[1,2-c:4,5-c′]dithiophene-4,8-dione 26 and 5,6-difluorobenzothiadiazole (DFBT)27 based conjugated copolymers. However, this absorption behavior was not observed for DTfBT- and BT4T based copolymers with alkyl substitutions at the two thiophene (T) spacers.21,25,28 The UV−vis spectra of pDTfBT4T were mainly related to the enhanced coplanarity driven by the high-M n and π-conjugated backbone. The strong conjugation between the fused planar DTfBT and 4T was expected to improve the planarity of the copolymer backbones.

Density functional theory (DFT) calculation of a dimer system (D−A−D−A) with short methyl groups, as a simplified pDTfBT-4T, was conducted using B3LYP functional and 631G(d) basis sets. The highest occupied molecular orbital (HOMO) was predicted to be delocalized over the entire conjugated backbones, while the lowest unoccupied molecular orbital (LUMO) was mainly localized on an A unit (Figure S4). The minimum-energy conformation of the dimer suggested that the pDTfBT-4T chain adopted an extended conformation with a slightly tilted coplanar backbone (Figure S4). The optical band gap (Egopt) of pDTfBT-4T was estimated as 1.89 eV, based on the absorption edge (656 nm) of the film. HOMO and LUMO energy levels of a pDTfBT-4T film were calculated using Egopt, the CV curve (Figure 1b), and the D

DOI: 10.1021/acs.jpcc.5b11213 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) ID−VG transfer curve characteristics of pDTfBT-4T based OFETs before and after annealing at different TAs. (b) Typical output curves of the as-spun and 200 °C annealed devices.

following equation: EHOMO = −e(Eoxonset + 4.72) (eV), where Eoxonset is the onset oxidation potential versus Ag/Ag+.29 Based on an Eoxonset value of 0.38 V, the calculated HOMO and LUMO levels of the copolymer were −5.10 and −3.21 eV, respectively. pDTfBT-4T chains yielded good thermal stability (429.2 °C at a 5% weight loss) based on TGA measurements (Figure 1c), and it also showed a high melting temperature around 294.8 °C as well as clear crystallization behavior with a peak of 277.3 °C during a DSC cooling cycle (Figure 1d). The 25−30 nm thick pDTfBT-4T films were spun-cast on gPS-SiO2 dielectrics from a warm CB solution (60 °C). Some were further annealed at 150, 200, 250, and 300 °C for 10 min. Figure 2 shows typical AFM topographies of spun-cast pDTfBT-4T films before and after annealing at various TAs. AFM topography of an as-spun film showed randomly distributed and percolated aggregates, which included 1D short nanorods with lengths of 50−80 nm (Figure 2a). The nanorods in the as-spun film were merged together and continuously grew into 2D grains with increasing TA. Interestingly, the 200 °C-annealed films showed the percolated layer morphology that included small-sized 2D grains (Figure 2c). Furthermore, the 250 and 300 °C annealed films showed apparently layered morphologies, which comprised wellordered domains with a height of about 2 nm (Figure 2d,e). The TA-driven disorder−order phase transition in the copolymer films caused the lateral film disconnection and surface variation, resulting in changes in the surface roughness with an increase in TA (Figure 2f). The resulting morphologies of the semiconducting films were expected to affect the electrical properties in OFETs as will be discussed later.24 One way to achieve better charge carrier transport in solution-processed polymer-based FETs is to extend the ordering of π-conjugated chains preferentially along the S/D electrodes. Figure 3 represents typical 2D GIXD patterns of these pDTfBT-4T films on gPS-SiO2 dielectrics. Discernible intensities and orientation profiles of crystal reflections along the Qz (out-of-plane) and Qxy (in-plane) axes were observed. The crystallinities of the annealed films increased considerably with an increase in TA (1D out-of-plane X-ray diffraction in Figure S5a). We found that as-spun film had a relatively lower crystallinity and fewer oriented crystallites containing edge-on chains on the substrate, in comparison to the TA-annealed films.

In a 2D GIXD pattern, the preferential orientation of (010)edge‑on reflections along the Qxy axis depended on the portion of laterally π-overlapped aggregates (Figure S5b). After annealing the as-spun films, most of the chains had an edge-on conformation with respect to the surface, expecting a better conducting path in the OFET. As shown in Figures 3a−e, (h00) and (010)edge‑on reflections of ordered domains along the Qz and Qxy axes, respectively, were considerably improved with increasing TA. Particularly, the azimuthal scan profiles at Q(100) clearly revealed that the films annealed above 150 °C contained highly ordered domains (Figure 3f).30 Additionally, we found that the layer spacings between the (100) crystal planes, d(100), and between the (010) crystal planes, d(010) (referred to as the intermolecular π-overlap distance), in the pDTfBT-4T copolymer films changed slightly before and after annealing at different TAs. Values of d(100) in most of the treated films remained about 19.63 Å, as determined by Bragg’s law (d(hkl) = 2π/Q(hkl)) and Q(100) = 0.32 Å−1.31 The exception to this was the 300 °C annealed film, which had a d(100) value of 20.27 Å. Also, d(010) values increased slightly from 3.59 to 3.67 Å with an increase in TA. To determine the electrical properties of these pDTfBT-4T films, top-contacted Au electrode OFETs containing the semiconducting layers on gPS-SiO2 gate dielectrics were fabricated and tested. Figure 4a shows typical ID−VG transfer curves of pDTfBT-4T based OFETs operated in the saturation regime (VD = −20 V). These curves showed negligible gate voltage (VG)-sweep hysteresis. The electrical properties are summarized in Table 2. The fabricated OFETs had a typical ptype transistor behavior, as shown in ID−VD output curves (Figure 4b).32 The as-spun film yielded an average μFET value of 0.11 cm2 V−1 s−1 in the OFET as well as a threshold voltage Table 2. Electrical Properties of pDTfBT-4T Based OFETs before and after Annealing at Different TAs sample as-spun 150 °C 200 °C 250 °C 300 °C E

annealed annealed annealed annealed

TA (°C) 150 200 250 300

μFET (cm2 V−1 s−1)

Vth (V)

ION/IOFF

± ± ± ± ±

−3.1 −10.0 −11.9 −7.3 −8.8

>106 >106 ∼107 ∼107 >106

0.11 0.57 1.45 1.00 0.59

0.01 0.03 0.01 0.09 0.04

DOI: 10.1021/acs.jpcc.5b11213 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. (a) ID−VG transfer curves of 200 °C annealed pDTfBT-4T based OFETs under an applied gate bias stress of gate bias voltage = −20 V. (b) Variations in ΔVth and μFET depending on bias time.

(Vth) of −3.1 V and an on/off current ratio (Ion/Ioff) of rather than 106. After annealing at various TAs, the μFET and Ion/Ioff of the OFETs were drastically improved up to 1.45 cm2 V−1 s−1 and ∼107 for the 200 °C annealed sample. Unlike the nanorod aggregates in the as-spun film, 2D longrange π-overlapped ordering of the conjugated chains in the TAannealed films could be drastically improved along the ordered domains with a π-overlap distance of about 3.63 Å (Figure 2). After annealing above TA = 200 °C, the fast decay in μFET was mainly attributed to layer cracking in the pDTfBT-4T films. Particularly, the large difference in thermal expansion coefficient between the polymer and gate dielectric may have caused this severe cracking in the 250 and 300 °C annealed films, which showed μFET values of 1.00 and 0.59 cm2 V−1 s−1, respectively. The ordered structure-dependent μFET trend of the pDTfBT-4T based OFETs was attributed to the phase transition from nanorod aggregates to a percolated layer of less torsional D−A copolymers observed with increasing TA.14 Yuen et al. reported a similar trend for a BT-based copolymer system,17 and Jang et al. also demonstrated long-range πoverlapped layer-like morphologies in spun-cast films containing a DPP-based copolymer, yielding a high μFET of above 1.40 cm2 V−1 s−1.16 Overcome of bias-stress instability can be a significant challenge for D−A polymeric semiconductors. This instability is related to some electrical changes under an external bias stress, likely due to charge traps created through partial disorder in the structure of the π-conjugated thin films and the characteristics of the semiconductor/insulator interface.33 Figure 5 shows ID−VG transfer curves of pDTfBT-4T based OFETs continuously operated with an applied gate bias of −20 V for 12 h. Even after 12 h under a gate bias stress, the Vth of the OFETs remained stable within a variation of 1.20 V, and the mobility showed similar behavior (from 1.43 to 1.15 cm2 V−1 s−1). These results suggest that pDTfBT-4T possessed excellent electrical stability in comparison to other D−A copolymers. We found that the strong intermolecular interactions of the D−A copolymer provided a maximum hole mobility of up to 1.45 cm2 V−1 s−1 in pDTfBT-4T copolymer based OFETs. Negligible hysteresis and excellent negative bias stability were also observed. The rigidity of the DTfBT units enhanced the coplanarity of the polymer backbone, contributing to effective

quasi-one-dimensional charge transport along the backbone with occasional interchain hopping through π-stacking bridges.

4. CONCLUSIONS A D−A conjugated copolymer, pDTfBT-4T, that includes alternated dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiazole (DTfBT) and tetrathiophene (4T) with 3octyldodecyl substituents was synthesized by Stille polymerization. We found that the polymer, which had a rigid conjugated A unit to further facilitate overlap of π-orbitals along the polymer chains, formed unexpectedly strong interchain aggregates in CB solutions at RT. The absorption UV spectra in a RT solution were comparable to those of thin solid films of the same polymer. The polymer was dissolved in a warm CB solution (60 °C) to produce uniform spun-cast films as semiconducting channel layers in top-contact electrode OFETs. We demonstrated control of the conjugated domains of pDTfBT-4T by annealing at various temperatures. These domains changed from 1D nanorods to 2D grains in thermally treated films on polymer-treated SiO2 dielectrics. On the benefits of the DTfBT-driven coplanarity of the polymer backbone and strong intermolecular interaction of the D−A copolymer, pDTfBT-4T based OFETs yielded a hole mobility up to 1.45 cm2 V−1 s−1 as well as negligible hysteresis and excellent negative bias stability. The correlation between the molecular structure, phase transition, and electrical properties of the DTfBT-based copolymer may provide guidance for the molecular design of high performance OFETs with an excellent operation stability.

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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/acs.jpcc.5b11213. Synthesis, GPC, 1H NMR and DFT of pDTfBT-4T; 1D XRD plots of pDTfBT-4T films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(D.H.H.) E-mail [email protected], Ph +82-51-5102232. *(H.Y.) E-mail [email protected], Ph +82-32-860-7494. F

DOI: 10.1021/acs.jpcc.5b11213 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Author Contributions

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M.P. and J.H. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Center for Advanced Soft Electronics under the Global Frontier Research Program (2012M3A6A5055225), the Industrial Fundamental Technology Development Program (10051440, Development of fiber-based transistors for wearable integrated circuit device applications) funded by the Ministry of Trade, Industry and Energy (MOTIE), Korea, and INHA UNIVERSITY Research Grant. Also, this work was supported by New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Government Ministry of Trade, Industry & Energy (20143010011890).

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ABBREVIATIONS DTfBT, dithienobenzothiadiazole; 4T, tetrathiophene; TIPSpentacene, 6,13-bis(triisopropylsilylethynyl)pentacene; TESADT, 5,11-bis(triethylsilylethynyl)anthradithiophene; P3HT, poly(3-hexylthiophene); PQT-12, poly(3,3‴-didodecylquarterthiophene); pBTTT, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene); DPP, 1,4-diketopyrrolo[3,4-c]pyrrole; BT, benzothiadiazole; BDT, benzodithiophene; 3T, terthiophene; PS-Si(CH3)2Cl, dimethylchlorosilane-terminated polystyrene; gPS-SiO2, polystyrene-grafted SiO2; DFBT, 5,6difluorobenzothiadiazole; DFT, density functional theory; Eoxonset, onset oxidation potential; Rq, roughness.

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DOI: 10.1021/acs.jpcc.5b11213 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b11213 J. Phys. Chem. C XXXX, XXX, XXX−XXX