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Influence of Simultaneous Tuning of Molecular Weights and Alkyl Substituents of Poly(thienoisoindigo-altnaphthalene)s on Morphology and Change Transport Properties Hye Jin Cho, Seok-Ju Kang, Sang Myeon Lee, Minkyu Jeong, Gyoungsik Kim, Yong-Young Noh, and Changduk Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07856 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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ACS Applied Materials & Interfaces

Influence of Simultaneous Tuning of Molecular Weights and Alkyl Substituents of Poly(thienoisoindigo-alt-naphthalene)s on Morphology and Change Transport Properties Hye Jin Cho,†,§ Seok-Ju Kang,‡,§ Sang Myeon Lee,† Mingyu Jeong,† Gyoungsik Kim,†,ǁ YongYoung Noh,‡,* and Changduk Yang†,* †

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

Dimensional Carbon Materials Center, Perovtronics Research Center, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea ‡

Department of Energy and Materials Engineering, Dongguk University, 30 Pildong-ro 1-gil,

Jung-gu, Seoul 04620, Republic of Korea ǁ

Current address: Photo-Electronic Hybrids Research Center, Korea Institute of Science and

Technology, Seoul 02792, Republic of Korea §These authors contributed equally to this work.

Keywords: Alkyl side substituents, Charge transport properties, Field-effect transistors, Molecular weights, Thienoisoindigo 1 ACS Paragon Plus Environment


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Abstract

To simultaneously assess the impact of molecular weight (Mn) and alkyl substituent variations of polymers on the structural and optoelectronic properties; herein, we conduct a systematic study of a series of poly(thienoisoindigo-alt-naphthalene) (PTIIG-Np)-based polymers containing different alkyl substituents (2-hexyldecyl (HD), 2-octyldodecyl (OD), and 2-decyltetradecyl (DT) chains) and Mns (low (L) and high (H)). All the polymers produce almost identical energy levels, whereas their optical spectra show a clear dependence on Mns and the alkyl substituents. Interestingly, increasing the alkyl substituent sizes of the polymers steadily increases the lamellar d-spacings (d100), ultimately leading to a densely-packed lamellar structure for PTIIGHD-Np. In addition, both H-PTIIGOD-Np and H-PTIIGDT-Np exhibit larger π-stacking crystallites than the corresponding low-Mn polymers, while for PTIIGHD-Np, their size increases in the low-Mn batch. Ultimately, L-PTIIGHD-Np shows the best hole mobility of 1.87 cm2 V-1 s-1 in top gate and bottom-contact organic field-effect transistors (OFETs) with a poly(methyl methacrylate), which is nearly one order of magnitude higher than other polymers tested in this study. Our results demonstrate that the simultaneous Mn and alkyl substituent engineering of the polymers can optimize their film morphology to produce high-performance OFETs.

1. Introduction π-Conjugated polymer-based organic field-effect transistors (OFETs) have received significant attention from both the academic and industrial communities, owing to their solution processability, tunability in structural modification, potential for large area device fabrication of low-cost, flexible and large-area electronic applications.1-6 Benefiting from new molecular designs and device fabrication improvements, OFETs have made considerable 2 ACS Paragon Plus Environment

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progress in the past decade, resulting in excellent carrier mobility approaching or surpassing that of amorphous silicon (1 cm2 V-1 s-1).7-10 In the design of semiconducting polymers, significant effort is focused on molecular packing and organization via the polymer backbones engineering, promoting the crystalline structures that in turn can facilitate the good charge carrier transport.11-17 Therefore, the correlation between the backbone structures and OFET performances has been well established.4-5,18-23 On the other hand, some recent studies have begun to observe that the molecular weight and the alkyl substituent of a given polymer platform have a profound influence on molecular ordering and microstructure, and additionally on optoelectronic and charge transport characteristics, thus making it one of the key parameters governing the device performance. 15, 24-27

For example, it was found that not only increasing the molecular weight but also

decreasing the distance of the branching point of branched alkyl chains to backbones and their bulkiness and are linked to improved charge transport arising from better intergrain connectivity.2,28 However, in most cases, the effects of molecular weight and alkyl substituent variations on polymer properties and device performances have been studied independently.15, 26, 29-31

To the best of our knowledge, there is only a few reports on a structure-property

relationship regarding the simultaneous tuning of molecular weight and alkyl substituents,15,32 which has to be further investigated. Just

reverently,

we

have

reported

an

easily

accessible

top-performing

poly(thienoisoindigo-alt-naphthalene) (PTIIG-Np) in OFETs.12,33 In this study, as part of our ongoing research effort of the PTIIG-Np backbone, we investigate how the molecular weight and alkyl substituent variations influence photophysical and electrochemical properties, film morphology, and charge carrier transport in PTIIG-Np-based OFETs. A series of PTIIG-Npbased polymers is synthesized, with alkyl substituents of 2-hexyldecyl (HD), 2-octyldodecyl (OD), and 2-decyltetradecyl (DT), where the control of each polymer Mns is accomplished by 3 ACS Paragon Plus Environment


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modulating stoichiometric balance between the two monomers and the polymerization time. 34-36

We find that shortening the alkyl substituent with proper tuning of Mns is critical for

optimizing the OFET performance. As a result, the best mobility of 1.87 cm2 V-1 s-1 is achieved for low-Mn PTIIGHD-Np (L-PTIIGHD-Np). The results obtained in this work are meaningful to understanding the influence of molecular weight and alkyl substituent variations in each polymer system, which can essentially contribute to the OFET advancements.

2. Results and Discussion 2.1. Synthesis and Characterization: Three branched alkyl amines (2-hexyldecyl-1-, 2octyldodecyl-1-, and 2-decyltetradecyl-1- amines) were first prepared from Gabriel synthesis,37-38 followed by Ullmann coupling with 3-bromothiophene to afford the corresponding alkyl-thiophen-3-amine compounds. The TIIG-based key monomers were synthesized according to the previous reported procedures in three laboratory steps (intramolecular Friedel-Crafts cyclization with oxalyl chloride, dimerization with Lawesson’s reagent, and bromination with N-bromosuccinimide (NBS)) (See Figure S1-S4).33,39-42 A series of TIIG-based polymers were synthesized via typical Pd-catalyzed Suzuki coupling (Pd2(dba)3/P(o-tolyl)3)/K3PO4 system) using 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)naphthalene co-monomer (Scheme 1). The general polymerization reaction was conducted in toluene at 90oC under inert atmosphere and the molecular weight controls can be achieved by varying the stoichiometric balance (r = A0/B0) between the two monomers and reaction time, where A0 and B0 are mole quantities of dibromide and diboronic ester monomers, respectively. The samples with high molecular weights (H-PTIIGHD-Np, H- PTIIGOD-Np, and HPTIIGDT-Np) were prepared using the exact stoichiometric balance (r = 1.00) and with 4 ACS Paragon Plus Environment

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sufficient reaction times to go to complete polymerization (over 5 days), while the stoichiometric imbalance (r = 0.77) with a 3-h reaction time gave the corresponding samples with low molecular weights (L-PTIIGHD-Np, L-PTIIGOD-Np, and L-PTIIGDT-Np), respectively. Note that for the low-Mn polymers, the actual Mn values determined by GPC are significantly greater than the corresponding those calculated from Carothers equation using r = 0.77,27,36 likely reflecting the aggregated forms composed of several polymer chains during the room-temperature GPC measurement.43 All the polymers were obtained as dark green solids after careful purification of precipitation into methanol and subsequent Soxhlet extraction with acetone, hexane, and chloroform. The detailed synthesis and characterization of all the polymers are provided in the Experimental section and their structures are identified by 1H NMR measurements. Their number-average molecular weight (Mn) and polydispersity index (PDI) values were determined by gel permeation chromatography (GPC) using a polystyrene standard in a tetrahydrofuran (THF) eluting solvent (see Figure S5-S10) and the relevant data are summarized in Table 1. All the polymers are soluble in common organic solvents, such as chloroform, chlorobenzene (CB), and o-dichlorobenzene (o-DCB).

Scheme 1. Synthesis of PTIIG-Np polymers. 5 ACS Paragon Plus Environment


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2.2. Optical and Electrochemical Properties and Theoretical Calculation: The UV-Vis absorption spectra of the polymers are measured both in chloroform solution and as thin films. All the polymers displayed dual characteristic bands in the absorption spectra, wherein the absorption peak at ~425 nm can be ascribed to the π-π* transitions of the polymers backbone, while the absorption peak at ~810 nm can be attributed to the intramolecular charge transfer (ICT) effects from the donor to acceptor units.30 With respect to the absorption characteristics in the solution, the lower energy bands of all the polymers slightly broadened with a less pronounced vibrionic structure in the thin films, indicating that strong intermolecular interaction existed in the film.

Figure 1. Normalized UV vis-absorption spectra of PTIIG-Np polymers with low (L-) molecular weight in CHCl3 solution (left) and film (right) (a) and with high (H-) molecular weight in CHCl3 solution (left) and film (right) (b). 6 ACS Paragon Plus Environment

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In addition, Figures 1 and S11 clearly indicate that the absorption spectra are almost insensitive to the variations in alkyl side chains and Mn values of the polymers. For example, all the polymers showed similar absorption maxima (λmax) and absorption onsets values in both solution and thin films. The optical bandgaps (Egopt) of all the polymers, calculated from the film absorption onsets, are nearly identical within 1.32 − 1.34 eV (Table 1).

Figure 2. Cyclic voltamograms of PTIIG-Np polymers with different alkyl substituent (-HD, -OD, -DT) and low (L-) and high (H-) molecular weight. The frontier molecular orbital energies (EHOMO and ELUMO) were determined using cyclic voltammetry (CV) in nitrogen atmosphere (Figure 2 and Table 1). The EHOMO and ELUMO values of all the polymers lie from -5.36 to -5.32 eV and from -3.92 to -3.90 eV, respectively, reflecting manipulation of the alkyl substituents and Mn of PTIIG-Np polymers has negligible effect on their energy levels.44 In addition, to elucidate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels after optimizing the 7 ACS Paragon Plus Environment


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geometry of compound name using the same method, the computational calculation of the molecular geometries over the PTIIG-Np-based polymers with different alkyl substituents (HD, -OD, -DT) were performed using the Gaussian 09 package with the nonlocal hybrid Becke three-parameter Lee-Yang-Parr (B3LYP) function and the 6-31G* basis set (see Figure S12-S15) For all cases, the electron densities of the HOMO and LUMO are both well delocalized over the conjugated repeat units and high co-planar backbone structures with similar HOMO/LUMO values are observed. Table 1. Optoelectrical and electrochemical properties of PTIIG-Np polymers. copolymer

Mn (kDa)a

PDI

λmaxsol (nm)

λmaxfilm (nm)

∆Egopt (eV)b

EHOMO (eV)

ELUMO (eV)

∆Egelec (eV)c

L-PTIIGHD-Np

24.6

2.05

798

813

1.34

- 5.35

- 3.90

1.45

H-PTIIGHD-Np

61.3

2.11

813

819

1.34

- 5.32

- 3.90

1.41

L-PTIIGOD-Np

34.3

2.81

796

797

1.32

- 5.35

- 3.92

1.43

H-PTIIGOD-Np

63.8

3.19

799

810

1.34

- 5.36

- 3.91

1.45

L-PTIIGDT-Np

50.6

2.57

799

810

1.33

- 5.33

- 3.90

1.46

H-PTIIGDT-Np

108.7

3.79

808

819

1.32

- 5.35

- 3.91

1.44

a

Estimated from the gel permeation chromatography (GPC) against polystyrene standard in

tetrahydrofuran (THF) at 40°C. bOptical energy bandgap estimated from the absorption onset of the thin films. ∆Egopt = 1240/λonsetfilm. c∆Egelec = LUMO-HOMO.

2.3. Thin-Film Microstructure Analysis: To elucidate the effects of the alkyl substituents and Mns on the film morphological structures (molecular packing/crystallinity) of the polymers, atomic force microscopy (AFM) and grazing incident X-ray diffraction (GIXD) were employed, where the polymer films were prepared under identical conditions by spin-coating from 1,2,4-trichlorobenzene and thermally annealed at 300 oC.33,45 As shown in Figure 3 and 8 ACS Paragon Plus Environment

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Figure S16, the AFM images of all the annealed films show fine interconnecting grains. The high-Mn batches (e.g., both PTIIGOD-Np and PTIIGDT-Np) exhibited a more defined fibrillike polymer microstructure than the corresponding low-Mn batch with a nodule-like feature, which is likely the result of the strong intermolecular π-π interactions, similar to other high performance OFET.46-47 In sharp contrast, such a fibrillar network is formed for L-PTIIGHDNp rather than H-PTIIGHD-Np. Thus, at this point in time, a correlation between Mns, surface morphologies, and charge transport properties for this family of polymers cannot be easily traced; for example, increasing Mns triggered a rise in OFET mobility for PTIIGODNp, while adversely affecting that of PTIIGHD-Np and PTIIGDT-Np (vide infra). One can conclude that all three polymers have different morphology and OFET performance trends with Mns.

Figure 3. AFM height images of the PTIIG-Np polymers depending on the length of alkyl substituents (-HD, -OD, -DT) and low (L-) and high (H-) molecular weight, where inner scale bar is 500 nm. 9 ACS Paragon Plus Environment


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As illustrated in Figure 4, all the polymer films appear to have dual textures of edge-on and face-on crystallites, as judged by the presence of the long-range ordered (h00) and strong π-π (010) stacking peaks in both out-of-plane (qz axis) and in-plane (qxy axis) orientations. Interestingly, the lamellar d-spacings (d100) increase gradually with longer alkyl substitutes, namely PTIIGHD-Np (~21 Å) < PTIIGOD-Np (~24 Å) < PTIIGDT-Np (~27 Å), though they are almost insensitive to the variation of the Mns (see Figure 5a). These results are consistent with not only the theoretical lengths of alkyl side chains obtained by DFT calculations (see Figure S15), but also the previously reported trends resulted from the sequential extension of their lengths on a given conjugated core.30 Consequently, this indicates that the shorter HD chains are closely interdigitated with the other side chains in the adjacent layers. Note also that compared with the other samples, the PTIIGHD-Np films showed slightly more-intense and more-distinct diffraction peaks.

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Figure 4. Grazing incidence X-ray diffraction (GIXD) patterns of pristine PTIIG-Np polymer thin films obtained with a 2D-image plates, where critical angle is 0.112° (a). In-plane (left) and out-of-plane line cut profiles (right) of the pristine PTIIG-Np polymer thin films, each of which is characterized as low (L-) and high (H-) molecular weight (b). In general, the π-stacking distances (d010) decrease as the Mn increased for many conjugated polymers,48 which can greatly affect the interpolymer chain carrier transport.43,49-53 However, 11 ACS Paragon Plus Environment


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we observed similar π-stacking distances of ~3.7 Å for all the polymer samples, implying that other factors, such as the crystallites size and rotation, and bimodal orientation distribution could contribute to the OFET mobility differences above, which resulted in the inconsistency between OFET performances and π-stacking distances. Therefore, we calculated the (010) crystal coherence length (CCL010) along qz using Debye Scherrer’s equation,48 where Gaussian fitting is used to obtain full widths at halfmaximum (FWHM) values (Figure 5a and Table S1). No systematic trends are found in the CCL010 values as a function of the alkyl substituents and Mn variations. For example, the CCL010 values are virtually comparable for all the polymers (~27 − ~33 Å), except for the HPTIIGDT-Np, a significantly increased CCL010 value (~49 Å) is observed. This result suggests that the π-stacking crystallites size in TIIG-Np based polymer is rather independent of Mn and alkyl substituent variations. The pole figures of the (100) reflection were also used to quantitatively compare the relative degree of bimodal orientation of the polymer films.50,54-55 Figure 5b compares the intensities of azimuthal angle (χ) of 45–135° (Az) and χ of 0–45° and 135–180° (Axy), attributed to the edge-on crystallites and face-on crystallites, respectively. Thereby, the ratios of Axy to Az (Axy/Az) signify the relative face-on and edge-on crystallite populations. Although distinguishing trends with varying alkyl substituents were not observed, low-Mn batches have clearly higher ratio values than the corresponding high-Mns cases; that is, the population of the face-on crystallites decreases with Mns values. (see Table S2) Taken together with overall AFM and GIXD data, one can conclude that compared with other samples, L-PTIIGHD-Np has the aligned nanofibrillar intercalating network with denselyinterdigitated alkyl chains, which is mainly responsible for the best mobility that we observed with the OFET measurements.


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Figure 5. Values of d-spacing and coherence length of PTIIG-Np polymers depending on different alkyl substituents (-HD, -OD, -DT) and low (L-) and high (H-) molecular weight (a). Pole figures extracted from (100) diffraction of PTIIG-Np polymer thin films on the SiO2/Si substrates, indicating that Axy and Az are region of face-on and edge-on crystallites, respectively (αi = 0.12°, critical angle), where from the integrated intensity of the azimuthal angle (χ) of 45–135° (Az) and χ of 0–45° and 135–180° (Axy), attributed to the edge-on crystallites and face-on crystallites, respectively (b).

2.4. Performance of OFETs: Top gate and bottom-contact (TG-BC)-based OFET characteristics of the polymers as a channel semiconductor were investigated using a poly(methyl

methacrylate)

(PMMA)

gate

dielectric.33

Using

the

conventional

photolithography method, source and drain contact electrodes (12 nm/3 nm = Au/Ni) were patterned on glass substrates by thermal evaporation deposition. The channel length and channel width are 20 µm and 1 mm, respectively. The enhanced mobility of the PTIIG-Np polymers upon thermal annealing was reported in our previous work,33 and therefore all the OFET devices were subsequently annealed at the optimized temperature of 300 oC. Device



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fabrication and characterization were performed in a nitrogen gas filled glovebox. The details of the transistor fabrication process are described in the Experimental section.

Figure 6. Summary and comparison of OFET characteristics. Transfer curves (a) and output curves (b) from PTIIG-Np polymers with low (L-) or high (H-) molecular weights varying the alkyl substituents (-HD, -OD, -DT).


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Table 2. Summary of PTIIG-Np polymer OFET performances with different alkyl substituents and low (L-) or high (H-) molecular weights

copolymers

L-PTIIGHD-Np

H-PTIIGHD-Np

L-PTIIGOD-Np

Mn (kDa)

µhole

VT

(cm2 V-1 s-1) a,b

(V)

1.31

-55.3 ~

(1.87)

--64.2

1.17

-59.2 ~

(1.64)

-65.0

6.35ⅹ10-2

-55.2 ~

24.6

61.3

34.3 (1.11ⅹ10 )

-61.2

3.46ⅹ10-1

-55.0 ~

-1

H-PTIIGOD-Np

63.8

H-PTIIGDT-Np

588 ~ 1357

217 ~ 413

347 ~ 520

VT

(cm2 V-1 s-1) a,c

(V)

1.61ⅹ10-2 68.9 ~ 73.7 -2

(3.44ⅹ10 ) 1.611ⅹ10-2 70.3 ~ 72.5 (1.89ⅹ10-2) 2.58ⅹ10-3 69.4 ~ 72.7 -3

(4.02ⅹ10 ) 1.356ⅹ10-2 74.7 ~ 77.2

(6.06ⅹ10 ) 1.54ⅹ10-1

-45.6 ~

(3.56ⅹ10-1)

-60.5

(4.78ⅹ10-2)

7.48ⅹ10-2

-48.0 ~

1.14ⅹ10-2

50.6

-2

(1.49ⅹ10 ) 2.66ⅹ10-2

36 ~ 84

108.7

61.4 ~ 66.1

49 ~ 92 -1

(1.08ⅹ10 ) a

160 ~ 411

µelectron

-63.5

-1

L-PTIIGDT-Np

Ion/Ioff

-56.1

61.5 ~ 65.6 -2

(1.63ⅹ10 )

Average values of hole or electron mobilities are indicated with over ten devices, and

maximum values are in parenthesis. bMeasured at VDS = -100 V. cMeasured at VDS = +100 V.

All OFET parameters (e.g., hole (µhole) and electron (µelectron) mobilities, threshold voltage (VT), and on/off current ratio (Ion/Ioff) of the annealed polymer films are summarized in Table 2 and Figure S17, and their transfer and output characteristics are shown in Figure 6.56-60 All the polymer films showed hole dominant p-channel characteristics with good draincurrent modulation. Although all the polymers have moderate µelectron values in the range of 15 ACS Paragon Plus Environment


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2.57 × 10-3 to 4.77 × 10-2 cm2 V-1 s-1; clearly, PTIIGHD-Np polymers exhibit higher µhole values over 1.0 cm2 V-1 s-1 than those of other polymers. By increasing the Mns of both PTIIGHD-Np and PTIIGDT-Np, the µhole values slightly decrease, while H-PTIIGOD-Np exhibits relatively higher mobility than L-PTIIGOD-Np.15,61 Ultimately, the highest µhole value is 1.87 cm2 V-1 s-1, seen in the L-PTIIGHD OFET, which is about one order of magnitude higher than those of other samples. Note also that relatively well-balanced ambipolar transport properties are essentially realized in both H-PTIIGOD-Np (µhole = 6.06 × 10-1 cm2 V-1 s-1 and µelectron = 1.49 × 10-2 cm2 V-1 s-1) and L-PTIIGDT-Np (µhole = 3.56 × 10-1 cm2 V-1 s-1 and µelectron = 4.78 × 10-2 cm2 V-1 s-1). As mentioned above in the CV results, all the polymers showed almost identical energy levels. Therefore, the differences in the OFET performances of all polymers might have arisen from changes in their intermolecular interactions and crystallinity rather than from electronic structure.

3. Conclusion In summary, we synthesized a series of PTIIG-Np-based polymers and simultaneously investigated the influence of their Mn and alkyl substituent variations on the photophysics, film morphology, and OFET performances. The optical properties, such as band shape and absorption shiftiness are clearly affected by varying Mns and alkyl substituent chains of the polymers, whereas the frontier energy levels (EHOMO and ELUMO) remain almost unchanged. The lamellar d-spacings (d100) of the polymers increased from PTIIGHD-Np to PTIIGOD-Np, and to PTIIGDT-Np with similar π-stacking distances (~3.7 Å), as the length of alkyl substituents increase. In addition, the CCL values of both PTIIGOD-Np to PTIIGDT-Np exhibited a clear increasing trend in high-Mn samples, except for PTIIGHD-Np where a relatively larger CCL value was observed in its low-Mn case. Consequently, L-PTIIGHD-Np 16 ACS Paragon Plus Environment

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formed a more compacted lamellar structure with large π-stacking crystallites, which led to the highest µhole of 1.87 cm2 V-1 s-1, outperforming the other polymer-based OFETs. Our findings suggest that relatively high mobilities of the polymers can be achieved with optimized Mn and alkyl substituents together.

ASSOCIATED CONTENT Supporting Information. Experimental details; GPC data; UV plots; DFT calculations; AFM phase image data; GIXD data with the crystallographic parameters of six PTIIG-Np-base polymers. This material is available free of charge on the ACS Publications website at DOI: xx.xxxx /acsami.xxxxxxx.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ACKNOWLEDGMENTS H. J. Cho and S.-J. Kang contributed equally to this work. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2015R1A2A1A10053397) and the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT, Future Planning as Global Frontier Project (CASE2013M3A6A5073175) and the Center for Advanced Soft-Electronics (2013M3A6A5073183) funded by the Ministry of Science, ICT & Future Planning. The GIXD experiment at the 17 ACS Paragon Plus Environment


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PLS-II 6D UNIST-PAL beamline was supported in part by MEST, POSTECH and UNISTUCRF.


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