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Requirements for Forming Efficient 3-D Charge Transport Pathway in Diketopyrrolopyrrole-Based Copolymers: Film Morphology vs. Molecular Packing Gang-Young Lee, A-Reum Han, Taewan Kim, Hae Rang Lee, Joon Hak Oh, and Taiho Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00595 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on May 3, 2016

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ACS Applied Materials & Interfaces

Requirements for Forming Efficient 3-D Charge Transport Pathway in Diketopyrrolopyrrole-Based Copolymers: Film Morphology vs. Molecular Packing Gang-Young Lee,†,§ A-Reum Han,†,‡,§ Taewan Kim,† Hae Rang Lee,† Joon Hak Oh,†,* Taiho Park†,* †

Department of Chemical Engineering, Pohang University of Science and Technology, San31,

Nam-gu, Pohang, Gyoungbuk, 790-780, South Korea ‡

Department of Energy Engineering, School of Energy and Chemical Engineering, Low

Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea KEYWORDS: Charge Transport, Organic Field-Effect Transistors, Organic Electronics, Polymer Semiconductor, Microstructure

ACS Paragon Plus Environment

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ABSTRACT

To achieve extremely high planarity and processability simultaneously, we have newly designed and synthesized copolymers composed of donor units of 2,2'-(2,5-dialkoxy-1,4phenylene)dithieno[3,2-b]thiophene (TT-P-TT) and acceptor units of diketopyrrolopyrrole (DPP). These copolymers consist of a highly planar backbone due to intramolecular interaction. We have systematically investigated the effects of intermolecular interactions by controlling the side chain bulkiness on the polymer thin-film morphologies, packing structures, and charge transport. The thin-film microstructures of the copolymers are found to be critically dependent upon subtle changes in the intermolecular interactions, and charge transport dynamics of the copolymer based field-effect transistors (FETs) has been investigated by in-depth structureproperty relationship study. Although the size of the fibrillar structures increases as the bulkiness of the side chains in the copolymer increases, the copolymer with the smallest side chain shows remarkably high charge carrier mobility. Our findings reveal the requirement for forming efficient 3-D charge transport pathway and highlight the importance of the molecular packing and inter-domain connectivity, rather than the crystalline domain size. The results obtained herein demonstrate the importance of tailoring the side chain bulkiness, and provide new insights into the molecular design for high-performance polymer semiconductors.


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1. INTRODUCTION Conjugated polymers are highly suitable for low-cost fabrication of flexible and large-area electronic devices by solution processing on plastic substrates at low temperatures.1-3 They may also allow realization of ubiquitous plastic electronics including flexible displays, e-paper, radio frequency identification (RFID) tags, and large-area sensors.4-9 Recently, remarkable progress has been made in the performance of organic field-effect transistors (OFETs) based on donor– acceptor (D–A) polymers, with charge-carrier mobilities µ > 10 cm2 V−1 s−1.10-16 Rational design of the components of D−A is a prerequisite for developing high-performance polymer FETs, because it strongly affects the frontier orbital energy levels and the π-orbital overlap,17-19 and consequently affects the molecular coplanarity, packing geometries, and intra- or intermolecular interactions of the resulting polymers.20-22 Diketopyrrolopyrrole (DPP)10-13, 23, 24 and isoindigo (IIG)25-29 have been most widely used as acceptor units for high-mobility conjugated backbones. The electron-deficient and planar characteristics of DPP and IIG typically promote strong π−π interactions that can induce ordered solid-state packing. Compared with electron-deficient moieties, electron-donating groups are somewhat limited to simple thiophene or selenophene derivatives. Therefore, the development of new electron donating moieties that can accelerate efficient intra-molecular charge transfer and desirable polymer packing structures is of great importance for expanding the availability of D−A conjugated frameworks. As an alternative, side-chain engineering has recently emerged as a powerful strategy to boost the electrical performance of polymer semiconductors.10-12,

26, 27, 30, 31

Sophisticated structural

modifications of polymer side chains, such as alterations in length,10-12, 17, 26, 30, 32, 33 shape,34 position,35 bulkiness,27, 31 symmetry,36 density,37 and chirality,38 can enhance molecular ordering and µ. Particularly, unprecedentedly high-performance p-type,10,

12, 26, 27

n-type,39,

40

and


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ambipolar30, 31 semiconductors have been achieved by further reducing the π-planar distance by tuning branching position of the alkyl chains or introduction of siloxane hybrid side chains. To date, most research has used these molecular design strategies to facilitate charge transport. However, few reports have considered the effect of side-chain bulkiness on the microstructure and OFET performance of conjugated polymers. Side-chain bulkiness effects have been mostly explored in polymer bulk heterojunction (BHJ) photovoltaic cells.41-43

Here,

we

present

a

new

electron-donating

moiety,

2,2'-(2,5-dialkoxy-1,4-

phenylene)dithieno[3,2-b]thiophene (TT-P-TT), composed of thieno[3,2-b]thiophene and 2,5alkoxy benzene units, (Scheme 1). The 2,5-alkoxy benzene unit facilitates efficient intramolecular charge transfer owing to the strong electron-donating properties of the oxygen atoms at the ortho-positions of benzene. Thieno[3,2-b]thiophene unit extends the effective conjugation length and functions as a conjugated-spacer for reducing steric hindrance between DPP and 2,5-alkoxy benzene. TT-P-TT has two intramolecular interactions between sulfur of thieno[3,2-b]thiophene and oxygen of 2,5-alkoxy benzene, resulting in high planarity which can maximize intermolecular interactions. For these polymers, planar backbone conformations are expected to be induced by the strong S···O intramolecular interactions in both donor and acceptor moieties; if these interactions occur, they may enhance intermolecular interactions and efficient charge transport in polymer films. We have synthesized a novel series of DPP-based TT-P-TT polymers with various bulky side chains in the hydroxyl group of the TT-P-TT unit; i.e., 2-ethylhexyl (P1), 2-butyloctyl (P2), and 2-octyldodecyl groups (P3), and systematically investigated the effects of intermolecular interactions by the control of side chain bulkiness on charge transport (Figure 1 and Scheme 1). The thin-film microstructures were dramatically


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changed by tuning the side chain bulkiness. The charge transport dynamics of DPP-based TT-PTT polymers was thoroughly investigated by structure-property relationship study. Our findings about the effects of side-chain bulkiness in DPP-based polymers with new electron-donating moieties will guide development of ways to rationally design high-performance polymer semiconductors.

Figure 1. Schematic representation of the role of TT-P-TT in DPP based copolymer and side chain bulkiness effect on charge transport dynamics.

Scheme 1. Synthesis routes for TT-P-TT, P1, P2, and P3.


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2. RESULTS AND DISCUSSION 2.1. Synthesis Approach and Characterization To synthesize TT-P-TT (Scheme 1; Supporting Information) monomers were synthesized by several chemical reactions including alkylation, substitution reaction and stannylation. Various bulky side chains were introduced to systemically control intermolecular interactions. The copolymers described herein were prepared by a Stille cross-coupling polymerization method (Scheme 1). The copolymerization reaction was conducted using a microwave reactor. The reaction mixture was stirred for 5 min at 120 °C, 10 min at 150 °C, and 30 min at 170 °C, consecutively. The copolymers were purified by precipitation into methanol, the subjected to Soxhlet extraction with acetone, hexane, and chloroform consecutively to remove small residual molecules, oligomers, and low-molecular-weight (LMW) portions. After removal of the LMW compounds, the high-molecular-weight copolymers were extracted with chloroform. P1, P2, and P3 have different alkyl chains, i.e., 2-ethylhexyl, 2-butyloctyl, and 2-octyldodecyl chains, respectively. The overall polymer characterizations were summarized in Table 1. For the chloroform fraction of each polymer, gel-permeation chromatography (GPC) analysis using a polystyrene standard in chlorobenzene exhibited a number-averaged molecular weight Mn of 52, 86, and 66 kDa for P1, P2, and P3, respectively and polydispersity index (PDI) of ≈ 2.0 for all copolymers. These copolymers show good solubility in chloroform and chlorobenzene. Especially, P3 was fully dissolved even in hexane due to the high solubilizing power of the bulky side chain on TT-P-TT. Thermogravimetric analysis (TGA) of these polymers (scan rate 10 °C min−1 in N2 atmosphere), revealed decomposition temperatures ~400 °C (Table 1; Figure S1 in Supporting Information)


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Table 1. Summary of the Copolymer Characterizations

Polymer

Mn (kDa)

PDI

Td (oC) a

εabs, sol (dm3mol–1cm–1)b

Egopt (eV) c

EHOMO (eV)d

ELUMO (eV)e

P1

52

2.0

410

101,000

1.50

−5.20

−3.70

P2

86

2.0

397

106,000

1.49

−5.20

−3.71

P3

66

2.0

407

110,000

1.47

−5.20

−3.73

a)

Decomposition temperature (5% weight loss) determined by TGA under nitrogen; Maximum absorption coefficients in solution; c)Optical band gap estimated in the film state; d) EHOMO was estimated from Eonset of the first oxidation potential relative to ferrocene (Fc), Fc/Fc+ redox system the ionization potential (IP) value was −4.8 eV; e)ELUMO = EHOMO + Egopt.

b)

2.2. Theoretical Calculations The effects of intramolecular interactions between thieno[3,2-b]thiophene and 2,5-alkoxy benzene on the planarity of the main chain were also investigated by computational study. The quinoidal structures were estimated from the optimized electron density models of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states of the copolymers. The electronic structures were obtained from density functional theory (DFT) calculations at a B3LYP/6-311G* level. To demonstrate S···O interactions in the system, Model A and Model B were designed and their torsional potentials were compared as a function of the dihedral angle between benzene and thieno[3,2-b]thiophene moiety (Figure 2a). The interring torsional angle was varied from 0 to 180° in 10° increments, and at each increment a singlepoint energy calculation was performed. In the optimized geometry, the dihedral angle (55°) of Model A was much larger than that (22°) of Model B. This is indirect evidence for the occurrence of S···O intramolecular interactions.44 The optimized geometry, electron density distributions in the HOMO and LUMO, and dihedral angle were obtained for the of trimer state (Figure 2b). To simplify the calculations, alkoxy side chain on benzene and 2-octyldodecyl chain were replaced with isobutoxyl and methyl groups, respectively. The dihedral angle between


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benzene and thieno[3,2-b]thiophene was 7°. Electron density plots demonstrate that the HOMO orbital was well delocalized. The LUMO orbital was localized near DPP unit due to the efficient intramolecular charge transfer between TT-P-TT and DPP units.

Figure 2. (a) Torsional potentials for Model A and Model B. (b) Optimized geometries (left) and molecular orbitals (right) of trimers of polymers (B3LYP/6-311G*). 2.3. Optical and Electrochemical Properties The optical properties of the copolymers were investigated both in chlorobenzene solutions and films (Figure 3). All copolymers had two absorption peaks within the absorption window. The absorption edges of all copolymers in chlorobenzene solution were almost identical; this result means that the effective conjugation length did not vary in solution. The absorption peaks a short wavelength (i.e., 650 nm) were attributed to the strong intramolecular charge transfer between the electron donating unit and the electron accepting unit.45 Regardless of alkyl chain bulkiness, the effective conjugation length and intramolecular charge transfer were almost similar. Highly planar back bone induced by intramolecular interaction, leads to enhanced


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molecular aggregation in solution and similar absorption features in solution and the solid state.46-48 The optical band gap (Egopt) values of these copolymers as calculated from the absorption edges of films were 1.50 (P1), 1.43(P2), and 1.49 eV (P3), respectively. The difference of optical band gap arises from the different aggregation behavior of each copolymer.49 The absorption maxima of thin films of all the copolymers are slightly red-shifted, and the relative intensities of the 0−0 vibrational peaks are higher due to the enhanced π−π stacking.47,

50

P1 that has a 2-ethylhexyl chain exhibits weak π−π stacking interactions. In

contrast, P3 has a larger degree of π−π stacking interactions than P1 and P2. The 2-octyldodecyl alkyl chain on TT-P-TT in P3 strengthened the π−π stacking interactions between molecules compared with side chains in P1 and P2. Thus, the bulky alkyl chains can induce large motional freedom of the main backbone; this freedom enhances π−π stacking interactions.50, 51

Figure 3. UV-vis absorption spectra of the copolymers in chlorobenzene solution (a) and (b) asspun thin film normalized at 630 nm. Cyclic voltammetry (CV) analysis was performed to measure the molecular energy levels of the new copolymers (Table 1; Figure S2 in the Supporting Information). The HOMO levels were estimated from the onset potential during the oxidation sweep, and the LUMO levels were calculated from the difference between the HOMO level and the optical band gap. Regardless of


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side chain bulkiness, HOMO levels were all ~ −5.2 eV. This result suggests that the bulkiness of the side chain in the copolymers did not influence the electrochemical properties of copolymers.

2.4. Thin-Film Microstructure Analyses The effects of side-chain bulkiness on morphological and microstructural features were investigated using tapping-mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD) analyses. Thermally-optimized drop-cast polymer films were used in these characterizations (the optimum thermal annealing temperature (220 °C for P2; 260 °C for P1 and P3); vide infra). All of the polymers formed densely packed nanofibrillar networks with interconnected domains (Figure 4); this pattern indicates that intermolecular interactions between conjugated backbones were efficient. These results can be attributed to the highly planar backbone structures of these copolymers.


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Figure 4. AFM height (left) and phase (right) images of (a) P1, (b) P2, and (c) P3 films annealed at an optimal temperature. The molecular domain sizes were dependent on the bulkiness of alkyl side-chains. The largest domains were observed in P3; this observation implies that increase in the bulkiness of alkyl chains induces increase in the size of the fibril structures. In contrast, the small and dense needle type structure is observed in P1 having smallest sidechain. This suggests that the bulkiness of side chains has a crucial effect on the microstructure formation in thin polymer films. In different solvent systems (Figure S3), the trends in the thin film morphologies are almost identical to chlorobenzene solution system. The high boiling point and viscosity features of chlorobenzene help to form more well-defined fibrillar morphology.


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Similar trends were observed in the morphologies of the solution-sheared films, with relatively larger domains along the shearing direction than occurred in the drop-cast films (Figure S4 in the Supporting Information).

Figure 5. (a) 2D-GIXD images of the polymer thin films thermally annealed at an optimal temperature: P1 (left), P2 (middle), and P3 (right). Their 1D-GIXD profiles of (b) in-plane and (c) out-of-plane directions. (d) Pole figure for (100) diffraction of the polymer films. GIXD analyses and diffractogram profiles of the polymer films (Figure 5a−c; Figures S5, S6 in the Supporting Information) were used to identify the molecular packing behaviors and orientations of the films. Despite their identical backbone structures, these films have very different molecular orientations. P1 adopted edge-on orientations relative to the substrate, and showed well-defined lamellar peaks up to the fourth order, whereas P3 adopted face-on orientations as deduced from the strong peak at qz= 0.317 Å−1 and no lamellar peaks along the out-of-plane direction. Additionally, P3 exhibited a remarkably strong diffraction peak at qz=


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1.683 Å−1, which corresponds to a π−π stacking distance of ~3.7 Å, which is consistent with the results of the UV-vis absorption spectra analyses that the intensity of the 0−0 vibrational peak is higher in P3 films than in the other films. P1 formed a relatively weak π−π stacking with edgeon orientations. To quantitatively evaluate the molecular orientations in these polymer films, pole figure analysis was performed for the (100) diffraction (where χ is defined as the semicircular angle between the crystallite orientation and the surface normal, and the semicircles were determined from (100) peaks at qz axis). The distributions of the edge-on and face-on orientations were calculated in terms of the integrated area obtained from −45° ≤ χ ≤ 45° and −90° ≤ χ ≤ 90°, respectively. Judging from the area ratio of the two peaks, P1 and P2 films, 95% of the film thickness consisted of molecules in the edge-on orientation, but slightly widespread distribution was observed in P2 (Figure 5d). In P3 films, the face-on distribution was dominant with an area ratio of ≈ 3:7 (29% vs. 71%). Face-on crystallites gradually developed as the degree of bulkiness of side chains increased. These results indicate that side-chain engineering induced packing transitions from edge-on packing (P1 films), to bimodal distribution (P2 films), to face-on packing (P3 films). In contrast, the lamellar distances increased in proportion to the bulkiness of side chains, and corresponded to (100) distances of 18.37 (P1 films), 19.23 (P2 films), and 19.81 Å (P3 films). These results reveal that as the bulkiness of side chains increases, the density of packing of perpendicularly oriented lamellar structures decreases, and intermolecular lamellar interaction weakens and freely interacts with substrates; the latter trend facilitates kinetically favored out-ofplane π-stack packing with formation of a metastable state.21 Outstanding electrical properties are expected for P1 films in the parallel charge transport system of FET devices.


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Figure 6. I–V curves of optimized OFETs based on solution-sheared copolymer films. Transfer (top) and output characteristics (bottom) for (a) P1, (b) P2, and (c) P3 films at hole-enhancement operation. 2.5. Fabrication of Solution-Processed FETs and I−V Characterizations To elucidate the charge transport properties of these materials, bottom-gate top-contact FETs were fabricated on OTS-modified SiO2/Si substrates (the details in the Supporting Information). The polymer thin films were prepared by solution-shearing a chlorobenzene solution (2 mg mL−1) on a preheated substrate to make highly-crystalline thin films. The optimum thermal annealing temperature (220 °C for P2; 260 °C for P1 and P3) was deduced after electrical characterization of FETs based on films that had been annealed at various temperatures. These polymers exhibited unipolar p-type field-effect behaviors, mainly due to the smaller injection barriers for holes with respect to the gold contacts. All the devices exhibited positively-shifted


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turn-on voltages (Figure 6); this response indicates that the polymers were p-doped, probably due to unintentional doping by residual impurities. Table 2. OFET performance of solution-sheared copolymer films as a function of annealing temperaturea

Polymer

P1

P2

P3

T (°C)

µmaxb (cm V−1 s−1)

µavgc (cm V−1 s−1)

Ion/Ioff

N/A

0.46

0.28 (±0.08)d

106 – 107

180

1.20

0.95 (±0.17)

104 – 105

220

1.55

1.26 (±0.19)

105 – 106

260

2.27

1.86 (±0.16)

105 – 106

300

0.36

0.32 (±0.02)

104 – 105

N/A

0.23

0.16 (±0.03)

106 – 107

180

0.54

0.44 (±0.04)

105 – 106

220

1.14

0.90 (±0.07)

105 – 106

260

0.97

0.81 (±0.15)

105 – 106

300

0.46

2

2

105 – 106

0.31 (±0.08)

N/A

5.81×10

−3

−3

−4

4.28×10 (±5.4×10 )

105 – 106

180

2.22×10−2

1.86×10−2 (±2.2×10−3)

105 – 106

220

3.69×10−2

2.70×10−2 (±3.9×10−3)

105 – 106

260

7.03×10−2

5.19×10−2 (±1.2×10−2)

104 – 105

300

4.63×10−3

4.24×10−3 (±2.9×10−4)

104 – 105

a

The performances of more than 15 devices were measured in nitrogen atmosphere. bThe maximum and caverage mobility of the FET devices (L = 50 μm and W = 1000 μm). dThe standard deviation. The optimized polymer films after the thermal annealing and solution-shearing processing showed significantly enhanced FET performance (Table 2), compared with other polymer thin films prepared using spin-coating and drop-casting methods (Figure S7 and S8, Table S1 and S2 in the Supporting Information). The optimized P1, P2, and P3 films exhibited efficient p-channel operations with high hole mobilities of up to 2.27, 1.14, and 0.07 cm2 V−1 s−1, respectively. The electrical performance was conspicuously decreased as the bulkiness of the side chains increased, as expected from the results of the GIXD analyses. P1 had the highest µ, possibly


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because of its edge-on orientation in crystal domains with respect to the substrates. The relatively poor performance of P3 was presumably due to the formation of π−π stacking with only face-on orientations without well-ordered edge-on lamellar packing. In general, 3-D charge transport, which improves FET performance, requires both well-ordered edge-on orientations and face-on π−π stacking.11, 30, 52 Therefore, the absence of well-ordered edge-on lamellar packing in P3 may have reduced the efficiency with which it formed 3-D conduction pathways between source and drain electrodes.

Figure 7. Temperature effects on the electrical characteristics of the polymer FETs. (a) Temperature dependence of hole mobilities measured at various temperatures (161−380 K) with VDS = −100 V under high vacuum condition (1×10−5 Torr). (b) Arrhenius plot of the temperature dependence of the hole mobilities. (c) Activation energies extracted from the linear fits to the Arrhenius plot: 35.11, 50.70, and 65.38 meV for P1, P2, and P3, respectively. (d) Schematic diagram of the correlation between activation energy and inter-grain distance of P1 and P3.


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Furthermore, temperature-dependent charge transport behaviors depending on the bulkiness of side chains were investigated for the optimized P1, P2, and P3 films, at temperature 161 ≤ T ≤ 380 K in high vacuum (1×10−5 Torr). The measurements at each T were conducted after allowing the devices to stabilize 1 h. The hole mobilities of all the polymers increased with increasing temperature; this trend indicates that these polymer FETs follow a thermally activated charge hopping transport model (Figure 7a−c, Figure S9 in the Supporting information).53 The activation energy (Ea) for charge hopping transport was extracted from Arrhenius plots of temperature-dependent average mobilities, μ∝exp(−Ea/kBT), and were 35.11, 50.70, and 65.38 meV for the hole transport in P1, P2, and P3, respectively. P1 had the lowest Ea, possibly because it has shallower traps than the other polymers; this low Ea may also explain why P1 has the highest µ. This tendency is also closely related to film microstructures; the denser and smaller fibril structures in P1 films versus the less ordered and lager fibrillar domains in P3 films (Figure 4). Therefore, the low activation energy of P1 might result from the relatively shorter intergrain distance leading to a small charge hopping barrier compared with that of P3 (Figure 7d). These results imply that, in addition to the formation of larger fibrillar structures, the interdomain connectivity and molecular packing are also critical for achieving efficient charge transport in polymer films.

3. CONCLUSION We designed and synthesized a novel series of DPP-based TT-P-TT polymers with various bulky side chains and systematically investigated the effects of intermolecular interactions and side chain bulkiness on charge transport. Almost identical electrochemical properties were observed regardless of the alkyl chain bulkiness, whereas the degree of π−π stacking was


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enhanced with increasing side chain bulkiness. Moreover, significantly different tendencies between film morphologies and molecular packing orientations were observed, depending on the side-chain bulkiness. Although P3 with the bulkiest side chains formed the larger fibrillar structures, it could not form efficient 3-D charge transport pathways due to the lack of wellordered edge-on lamellar packing. Furthermore, P1 had a much shorter inter-grain distance than P3, resulting in the smaller charge hopping barrier for efficient charge transport. These results imply the significance of forming the efficient charge transport pathways as well as the beneficial film morphologies for charge transport in devices. P1 with the smallest side chain showed remarkably high carrier mobilities up to 2.27 cm2 V−1 s –1. We believe that these findings provide new insights into the molecular design of high-performance copolymer semiconductors by side chain engineering.

ASSOCIATED CONTENT Supporting Information. General synthesis procedures, experimental section, and additional figures (TGA data, cyclic voltammetry plots, additional AFM and GIXD images, and OFET data). This material is available free for charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Author Contributions


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‡These authors contributed equally. ACKNOWLEDGMENT This work was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program (Grant no.: 2012M3A6A5055225 and 2013M3A6A5073175) through the National Research Foundation of Korea (NRF) by MSIP, Korea.


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Table of Contents Graphic


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