Linear Conjugated Polymer Backbones Improve Alignment in

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Linear Conjugated Polymer Backbones Improve Alignment in Nanogroove-Assisted Organic Field-Effect Transistors Ming Wang, Michael J. Ford, Cheng Zhou, Martin Seifrid, Thuc-Quyen Nguyen, and Guillermo C. Bazan J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Journal of the American Chemical Society

Linear Conjugated Polymer Backbones Improve Alignment in Nanogroove-Assisted Organic Field-Effect Transistors Ming Wang,#,†,‡,∇ Michael J. Ford,#,†,‡,§ Cheng Zhou,† Martin Seifrid,†,∇ Thuc-Quyen Nguyen,†,‡,∇ and Guillermo C. Bazan*,†,‡,∇,§ Center for Polymers and Organic Solids, ‡Mitsubishi Chemical Center for Advanced Materials, ∇Department of Chemistry & Biochemistry, §Materials Department, University of California, Santa Barbara, California 93106, United States †

KEYWORDS donor-acceptor conjugated polymer, fluoro substitution, organic field-effect transistor, nanogroove substrate, anisotropic charge transport ABSTRACT: Three cyclopentadithiophene-difluorophenylene copolymers (named PhF2,3, PhF2,5 and PhF2,6), which differ by the arrangement of fluorines on the phenylene structural unit, were designed and synthesized for the fabrication of organic field-effect transistors (OFETs). Single crystal structures of model compounds representative of the backbone and density functional theory (DFT) were used to estimate the backbone shape for each copolymer. The different substitution arrangements impact the backbone secondary structure through different non-bonding F···H interactions. PhF2,5 and PhF2,6 assumed more linear backbones relative to PhF2,3, which in turn impacts self-assembly and polymer chain alignment on nanogrooved (NG) substrates. A larger improvement of charge carrier mobility for the more linear backbones was achieved when using NG substrates. Among the three polymers, PhF2,6 achieved the highest average fieldeffect hole mobility (5.1 cm2V-1s-1). As evidenced by grazing incidence wide-angle X-ray scattering (GIWAXS), thin films of PF2,5 and PF2,6 exhibited a higher degree of anisotropic alignment, relative to the more curved PF2,3 counterpart, consistent with the higher hole mobilities. This work gives insight into how non-bonding interactions can influence charge carrier mobility through changes in secondary structure and suggests that polymers with more linear shapes might be preferred for achieving greater levels of alignment within the confines of a NG environment.

INTRODUCTION Polymer semiconductors for OFET applications are under intense study because their solution-processability enables fabrication of low-cost, large-area, light, and flexible devices.1 High mobility (µ) polymer semiconductors are often comprised of an electron rich unit (donor, D) and electron deficient unit (acceptor, A) in an alternating fashion.2 It has been suggested that the D-A feature improves close packing of adjacent polymer chains, thereby enhancing interchain charge hopping.2b,3 However, other reports have indicated that interchain D-A interactions do not influence polymer packing for all chemical structures; the structural unit lengths and side-chains also affect the conformation of adjacent polymer chains.4 From these latter findings, it was hypothesized that employing strongly electron-withdrawing A units might not be required to achieve optimal interchain association. Moreover, electron injection from commonly-used gold electrodes has been observed for some OFETs utilizing D-A polymers, and has been correlated with variations in current-voltage characteristics under bias stress and ultimately ambiguity in the analysis of OFET characteristics.5,6 To achieve high mobility, large on/off ratios, and stable OFET operation for a variety of device architectures,

new chemical design strategies for conjugated polymers are therefore being developed . One such design strategy outlined in a recent report described the unipolar p-type polymer PhF2,5 (see Scheme 1); PhF2,5 incorporates cyclopentadithiophene (CDT) as the D unit and 2,5-difluorophenylene (2,5-DFPh) as a weak A unit. PhF2,5 OFETs achieved a µ of 0.9 cm2V1s-1 without complications arising from electron injection.7 The electron withdrawing property of 2,5-DFPh is less pronounced, relative to A units such as 2,1,3benzothiadiazole and its derivatives, resulting in a higherlying LUMO level that provides a sufficient energetic barrier for minimizing electron injection from gold. Consequently, D-A polymers using weak A units comprise a promising strategy for future materials design, granting good performance as well as a versatile accessibility of chemical structure modifications relative to donor type, e.g., polythiophene, alternatives.8

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Scheme 1. Chemical structures of PCDTPT, PCDTFBT, PCDTBT, DFPh-based polymers, and small molecules with three types of difluoro substitutions, used as model compounds. (Red dashed lines indicate F···H noncovalent interactions). To further improve µ, processing conditions can be modified to obtain well-ordered film morphologies that maximize electronic coupling between subunits.9 In particular, both polymer9c and small molecule9h orientation can be induced upon confinement due to surface grooves on the substrates. Previous studies revealed that the D-A copolymers PCDTPT,10 PCDTBT2g and PCDTFBT5b (see Scheme 1) can be aligned using dielectric substrates containing nanogrooves (NG) to improve carrier transport. Literature precedent exists, that illustrates how achieving a well-ordered organization requires matching processing conditions to the right chemical structure.11 However, how variations in molecular structure impact the ability of polymer chains to self-assemble within the constrained NG environment under kinetically-constrained casting conditions remains poorly understood. For example, the molecular shape (i.e., curved vs. linear) impacts polymer film organization and ultimately modulates charge transport characteristics for polythiophene derivatives.12a Takimiya et al. reported that, among four differently-shaped polymers, a pseudo-straight polymer poly(2,7-bis(3-alkylthiophene-2-yl)-naphtho[1,2-b:5,6b’]dithiophene exhibited the highest degree of thin-film order and therefore obtained the best OFET performance.12b Since PhF2,5 exhibited similar hole mobilities relative to D-A polymers such as PCDTPT on planar substrates, under both spin-coating and blade-coating conditions,7 it was of interest to examine how changes in molecular and backbone secondary structures would impact self-assembly and electrical performance on NG-modified dielectrics. Thus, we introduced subtle variations in the molecular structure of PhF2,5 and examined structural order and OFET characteristics.

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formance for PhF2,5 relative to non- and monofluorinated analogs, changes to fluorine positions may influence the secondary structure of the chains and thus the self-assembly from solution of PhF2,3 and PhF2,6.13 The different arrangements of F···H non-covalent interactions in model compounds (denoted as CF2,5, CF2,3 and CF2,6 in Scheme 1) lead to different preferred molecular conformations, which we use to estimate lowest energy polymer conformations. Note that the binding energy of F···H (0.94 kcal/mol) is greater than that of F···S (0.44 kcal/mol).13a Questions relating to how these structural variations alter the physical properties, backbone shapes, and the ability to direct organizations by NG substrate, as well as the influence on charge transport in OFETs, were investigated by optical, electronic, and morphological characterizations. RESULTS AND DISCUSSION Synthesis of model compounds and polymer semiconductors Three small molecules (Scheme 2, CF2,3, CF2,5 and CF2,6) were synthesized as structural models of the target polymers. The common starting material 2trimethylstanyl-4,4-dimethyl-4H-cyclopenta[2,1-b:3,4b']dithiophene (C1) was synthesized from cyclopenta[2,1b:3,4-b']dithiophene via standard alkylation and stannylation according to previously reported methods.14 Relevant core precursors (2,3-difluoro-1,4-diiodobenzene, 2,5difluoro-1,4-dibromobenzene and 3,5-difluoro-4bromoiodobenzene) were obtained from commercial sources. The three regioisomers were synthesized via Stille coupling using catalytic Pd(PPh3)4 in toluene solution under microwave heating at 160 oC. These regioisomers were purified via chromatography and single crystals suitable for X-ray diffraction studies were obtained from a chloroform/methanol co-solvent system under slow evaporation.

Scheme 2. Synthesis and molecular structures of CF2,3, CF2,5 and CF2,6.

Two regioisomers of PhF2,5, namely PhF2,3 and PhF2,6 (Scheme 1), were designed and compared. PhF2,3, PhF2,5 and PhF2,6 consisted of a similar CDT-DFPh backbone design, but the arrangements of the two fluorine atoms in the phenylene fragment are different. These structures were designed with the perspective that since F···H noncovalent interactions improved ordering and OFET per-


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Scheme 3. Synthesis and molecular structures of PhF2,3, PhF2,5 and PhF2,6. Three polymers (Scheme 3, PhF2,3, PhF2,5, and PhF2,6) were then synthesized. PhF2,5 was synthesized via a previously reported method7 using compound M1 and 2,5-

difluoro-1,4-dibromobenzene under microwave-assisted Stille polycondensation using Pd(PPh3)4 in o-xylene at 200 oC for 40 minutes. Under these conditions, we obtained a product with an average molecular weight (Mn) of 28 kDa and a polydispersity index (Đ) of 2.1, as measured by gel permeation chromatography (GPC) at 150 oC using 1,2,4trichlorobenzene as the eluent and polystyrene standards. We applied these polymerization conditions for the synthesis of PhF2,3 using M1 and commercially available 2,3difluoro-1,4 diiodobenzene, but the molecular weight was considered too low (Mn = 15 kDa, Đ = 2.2) for subsequent characterization. To achieve a comparable Mn to that of PhF2,5, we first synthesized compound M2, which contains two DFPh units and a CDT unit. M1 and M2 were subsequently polymerized to obtain the target PhF2,3 with Mn = 34 kDa and Đ = 2.0. PhF2,6 is a regioregular polymer with two adjacent 2,6-difluoro-1,4-diphenylene units symmetrically located at both sides of a CDT unit. For the preparation of PhF2,6, we first

Figure 1. Molecular structures as determined by single crystal X-ray diffraction studies of: (a) CF2,3; (b) CF2,5; (c) CF2,6. Note that the green atom is F in CF2,3 and CF2,5. For CF2,6, green atoms indicate F or H since there is no preferential orientation of the phenylene unit in the lattice. Proposed tetramer and polymer backbone shape from combined single crystal and DFT data: (d) PhF2,3; (e) PhF2,5; (f) PhF2,6. Blue dashed lines are guides to the eye.

synthesized the symmetric monomer M3 by carrying out a reaction of M1 and 2 eq of 3,5-difluoro-4bromoiodobenzene. Ph2,6 with Mn = 33 kDa and Đ 2.3 can be obtained by using M3 and M1 under a similar polymerization condition to that for the preparation of PhF2,5. Complete details on the synthesis and characterization are provided in the Supporting Information (SI). Note that molecular weight variations between PhF2,3, PhF2,5 and PhF2,6 are small, which should minimize any molecular weight impact on differences in morphology and OFET performance.4a,15 Structural considerations by single crystal X-ray diffraction studies and DFT calculations

The molecular structures of model compounds were determined from single crystal X-ray diffraction studies of CF2,3, CF2,5 and CF2,6 (Figure 1). The most favorable conformations in the lattice between the CDT unit and adjacent DFPh units, as introduced in Scheme 1, were quantified by torsional angles between the CDT and DFPh units and packing distances between molecules (Table 1). The torsional angles of the three model compounds are similar, at ~16° - 17°, as are the adjacent packing distances (~ 3.7 - 3.8 Å). How F···H interactions influenced the molecular geometry was then examined. As shown in Figure 1a, for the CF2,3 molecule, F···H interactions between the DFPh core and the CDT units influence the molecular structure so that the methyl groups of the two CDT units point toward the same direction (pseudo cis). For the CF2,5 molecule, as shown in Figure 1b, two



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F···H interactions between the DFPh core and the two side CDT units cause the methyl groups of the two CDT units to point in opposite directions (pseudo trans). For the CF2,6 molecule (Figure 1c) the F and H are not distinguishable in the single crystal; however, the molecular shape still displays two CDT units in a pseudo trans conformation, similar to what is observed for CF2,5. The difference in cis vs. trans conformations of the CDT arrangements in CF2,3 vs. CF2,5 and CF2,6 was anticipated to induce different curvatures in the backbones of the polymeric analogs, as discussed below.16 From the single crystal diffraction studies, one obtains insight into how the different F···H non-bonding interactions in CF2,5, CF2,3 and CF2,6 lead to variations in molecular shapes. Such interactions can be extrapolated to extended molecules (i.e., oligomers to polymers). As shown in Figure 1d, for the PhF2,3 tetramer, it could be expected that the pseudo cis structural preference leads to a curvature of the tetramer backbone. Theoretical calculations using DFT methods at the CAM-B3LYP/631G(d,p) level of theory also indicate that the tetramer of PhF2,3 adopts an all-cis conformation as the lowest energy geometry (see Figure S17, SI). The calculated conformations of the PhF2,5 and PhF2,6 tetramer models shown in Figure 1e and 1f are consistent with the pseudo trans structures of CF2,5 and CF2,6. A more linear secondary structure for PhF2,5 and PhF2,6, relative to PhF2,3, was predicted. These differences in polymer conformation will be used to rationalize subsequent studies on relevant physical properties and OFET performance.

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ence suggests that the PhF2,5 aggregates are more resistant to break up relative to PhF2,3 and PhF2,6.17 Absorption spectra of polymer thin films were measured next (Figure 2c). One immediately observes similarities to the features observed in solution at room temperature. Specifically, PhF2,3 displays a main peak at 528 nm, together with a shoulder at 562 nm. The absorption of PhF2,6 is red-shifted relative to PhF2,3, with a main peak at 541 nm and a shoulder at 580 nm. PhF2,5 exhibits the most red-shifted absorption, with an peak at 547 nm and a shoulder at 592 nm. Optical gaps (Eg) were calculated from the film absorption onset. Thus, PhF2,5 has an Eg of 1.94 eV. PhF2,6 has a similar Eg of 1.96 eV while PhF2,3 has the largest Eg of 2.05 eV. Overall, the similarities of the absorption in thin films to solutions at room temperature imply considerable pre-aggregation in solution.

Table 1. Torsional angles and molecule distances of CF2,3, CF2,5 and CF2,6 from their single crystal structures. CF2,3

CF2,5

CF2,6

Torsional angle (°)

16.4

17.1

17.0

Packing distance (Å)

3.83

3.70

3.77

Optical absorption spectroscopy, thermal transitions and orbital energy levels The impact of molecular structure on optical properties was studied by UV-vis absorption spectroscopy. Polymer solutions (0.01 mg/mL in chlorobenzene) at room temperature were measured first (Figure 2a). PhF2,3 exhibits an intramolecular charge transfer (ICT) transition peak at 527 nm with an obvious shoulder at ~560 nm. PhF2,6 displays red-shifted absorption relative to PhF2,3, with the main transition peak located at 540 nm and a weak shoulder at ~580 nm. Finally, PhF2,5 exhibits the most red-shifted absorption among the three polymers, with a maximum at 552 nm and a shoulder at 590 nm. All these absorption shoulders indicate that the polymers have a tendency to aggregate in dilute solutions.17 The solutions were then heated at 70 oC and absorbance spectra were measured (Figure 2b). Interestingly, at higher temperature the shoulders in the bands of both PhF2,3 and PhF2,6 are relatively weak, while an obvious absorption shoulder remains for PhF2,5 at approximately 581 nm. This differ-

Figure 2. (a) Solution UV-vis absorption; (b) Solution UV-vis absorption at 70 °C; (c) Thin films UV-vis absorption; (d) DSC measurements. Thermal transitions were probed by differential scanning calorimetry (DSC). From Figure 2d, the three polymers display melting (Tm) and crystallization points (Tc), which are Tm = 299 °C, Tc = 269 °C for PhF2,6, Tm = 312 °C, Tc = 275 °C for PhF2,3 and Tm = 337 °C, Tc = 315 °C for PhF2,5 respectively. Interestingly, the melting and crystallization trends correlate with what was observed in the UV absorption. Specifically, the PhF2,5 crystalline domains are the most thermally stable (i.e., higher Tm), which is in accord with the largest resistance to aggregate break up in solution. Orbital energy levels were estimated by cyclic voltammetry (CV, Figure S15).18 The oxidation onset in the CV measurement was used to estimate the ionization potential, which in turn has been commonly used to estimate the highest occupied molecular orbital (HOMO) energy. These measurements showed indistinguishable differences within experimental errors: PhF2,3 and PhF2,6 (5.30 eV), PhF2,5 (-5.25 eV). LUMO levels were estimated by adding thin film optical Eg to HOMO values: ELUMO = 3.25 eV for PhF2,3, ELUMO = -3.31 eV for PhF2,5, and ELUMO = -3.34 eV for PhF2,6. The lack of significant differences in


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LUMO levels (< 0.1 eV, within the experimental error) would suggest that charge injection barriers at a given metal electrode would be similar for the three polymers.5,7 OFET fabrication and characterization Bottom-contact, bottom-gate OFETs were fabricated with the following top-to-bottom architecture: polymer/(Au/Ni)/SAM/SiO2/doped Si. The silicon dioxide substrate was passivated by a self-assembled monolayer of decyltrichlorosilane. Thin films were prepared via blade coating from 5 mg/mL solutions in chlorobenzene. The solutions and substrates were preheated at 70 °C before casting at a speed of 0.1 mm/s. Films were subsequently thermally annealed at 200 °C for ca. 10 minutes. Devices were tested under nitrogen and the average mobilities were calculated from six devices, complete details are provided in the SI. Output curves from devices with planar SiO2 substrates are shown in Figure 3a. Drain currents (Id) from the output traces saturate when the drain voltage (Vd) is greater than -60 V at the measured gate voltages (Vg), and all the devices show unipolar p-type transport characteristics. As shown in Figure 3b, transfer curves were collected from the first scan of current-voltage characteristics at a Vd of 80 V to obtain mobilities in the saturation region.5 The mobilities were calculated from the slope of Id1/2 vs. Vg curves.6b Table 2 provides highest and average µ, together with the corresponding average on/off ratios and threshold voltages (Vt). Characteristics for individual devices are provided in the SI. From Table 2 one observes that the resulting average µ values are similar for the three polymers: 0.85 ± 0.24 cm2V-1s-1 for PhF2,3, 0.63 ± 0.26 cm2V-1s-1 for PhF2,5 and 0.69 ± 0.24 cm2V-1s-1 for PhF2,6. On/off

ratios are on the order of 106. These observations indicate that the device performances among the three polymers are indistinguishable within experimental error when using a planar SiO2 dielectric. We also evaluated the effects of bias stress by scanning each device 20 times. All polymers show stable on/off current in the I-V characteristics (see Figure S19, SI), which indicate no adverse effects by electron injection from the gold electrode into the semiconductor layer5 and highlights the relevance of designing high-lying LUMO levels of polymer semiconductors for unipolar p-type OFETs. Table 2. OFET parameters (hole mobilities, on/off ratios and Vt values) calculated from the saturation regime (Vd = -80 V). Averages and standard deviations were calculated from at least 6 devices. The highest mobility for each condition is indicated in parentheses. polymer PhF2,3

PhF2,5

PhF2,6

NG

µ (cm2V-1s-1)

On/off

Vt (V)

No

0.85 ± 0.24 (1.1)

9.8 × 105

1.6

Parallel

2.5 ± 0.2 (2.8)

2.5 × 106

-4.4

No

0.63 ± 0.26 (1.0)

1.4 × 106

7.1

Parallel

4.0 ± 0.2 (4.2)

3.3 × 106

-7.2

No

0.69 ± 0.24 (1.1)

1.3 × 106

4.2

Parallel

5.1 ± 0.4 (5.7)

3.9 × 106

-6.7

Figure 3. Output curves and transfer curves of planar OFET devices. (a) Output curves measured at various Vg values; (b) planar OFETs are shown with log-scale Id (blue) as well as Id1/2 (red, ). (c) OFETs with transport parallel to the NG directions are also shown with log-scale Id (blue) as well as Id1/2 (red).



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We next examined the performance of devices that were prepared by blade coating atop NG substrates. The NGs were fabricated by uniaxial rubbing of a planar SiO2 substrate using a diamond lapping film.9c,10 The resulting NG features were about 100-200 nm wide and 1-5 nm deep. The current-voltage characteristics were measured (Figure 3c) and µ, along with the corresponding average on/off ratios and Vt values, were calculated (Table 2). When the NGs are parallel to the charge transport direction, PhF2,3 devices exhibit an average µ of 2.5 ± 0.2 cm2V1s-1, PhF2,5 devices had an average µ of 4.0 ± 0.2 cm2V-1s-1, and PhF2,6 devices obtained an average µ of 5.1 ± 0.4 cm2V-1s-1. The highest µ among all devices was obtained with PhF2,6 (5.7 cm2V-1s-1). In addition, all devices show high on/off ratios at the order of 106.

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To probe the influence of the dielectric nanostructure on the film morphology, we examined the thin film organization by GIWAXS.19 Polymer films were blade-coated using the same solution temperature of 70 oC, but at a faster coating speed of 1 mm/s to produce thin, uniform films appropriate for thin film scattering. These conditions produced thinner films that allowed us to probe predominately the bottom of the semiconductor layer, which is expected to be more sensitive to NG alignment10a and is more relevant for charge transport.20 Films were measured with incident X-rays parallel and perpendicular to the NGs by rotating the sample by 90°.

We note that for conjugated polymers, charge transport in OFETs relies both on intrachain (along the conjugated backbone) and interchain (facilitated by π-π orbital overlap) transport pathways.1b Without NG alignment, the polymer long axis orientation is mostly random, and thus charge transport is isotropic with respect to channel orientation. Alignment by NGs orient the polymer chains near the dielectric surface with the backbone parallel to the direction of the NGs. Therefore, intrachain charge transport was enhanced, and μ improves relative to OFETs without NGs. With the background in the preceding paragraph, we examined the average µ improvement due to NG alignment. As expected, µ values of all parallel devices were substantially enhanced relative to the non-NG devices, which gives evidence of alignment. For polymer PhF2,3, the µ improved 190%, while the enhancements of PhF2,5 and PhF2,6 were 530% and 640%, respectively. Since the three polymers showed similar performance on planar substrates, the greater increases in µ suggest that the alignment by the NGs for PhF2,5 and PhF2,6 films is better than that of PhF2,3 film. When the transport direction was perpendicular to the NGs, the polymer backbone was oriented perpendicular and charge hopping was the predominant transport mechanism. If the degree of alignment for PhF2,3 was worse than for PhF2,5 and PhF2,6, we expected that µ of OFETs aligned perpendicular to the NGs would be higher for PhF2,3 relative to PhF2,5 and PhF2,6. However, for all three polymers, OFETs oriented perpendicular to the NGs exhibit similar µ (0.1 cm2V-1s-1); see Table S8-S10 in the SI. In addition, OFETs oriented perpendicular to NGs exhibited µ that was less than the µ obtained with planar OFETs. Planar OFETs had µ of ca. 0.6-0.8 cm2V-1s-1 while perpendicular NG OFETs had µ ca. 0.1 cm2V-1s-1. The decrease in μ in the perpendicular devices relative to the planar devices led us to posit another possible reason for decreased performance in perpendicular devices: some NGs may become topographic barriers in the bottom layer that separate polymer chains (or bundles of chains). Therefore, the charge transport pathway is much less efficient than those available on a planar substrate. Film organization

Figure 4. 2-D GIWAXS patterns of polymer thin films under difference conditions: w/o NGs: (a) PhF2,3 (b) PhF2,5 (c) PhF2,6; Parallel to NGs: (d) PhF2,3 (e) PhF2,5 (f) PhF2,6; Perpendicular to NGs: (g) PhF2,3 (h) PhF2,5 (i) PhF2,6. Intensity scale is of arbitrary units, scaling from blue to green to yellow to brown to white (lowest to highest intensity.)

2-D GIWAXS profiles were measured for films deposited on a planar substrate (Figure 4a-4c). For PhF2,3, as shown in Figure 4a, one observes a peak at qz = 0.25 Å-1 in the nominally out-of-plane direction, corresponding to a distance of 2.5 nm, which is a typical lamella stacking distance of the hexadecyl side chain. In the in-plane direction, there is a peak at qxy = 1.75 Å-1, corresponding to a distance of 3.6 Å, which is typical of π-π stacking. The alkyl chain stacking peak extends to the in-plane direction and the π-π stacking peak weakly extended toward the out-of-plane direction, which indicate a predominately edge-on orientation with some degree of face-on orientation. For PhF2,5 (Figure 4b) and PhF2,6 (Figure 4c) films, similar lamella and π-π stacking features are observed, also at qz = 0.25 Å-1 and qxy = 1.75 Å-1. PhF2,5 and PhF2,6 films exhibit more anisotropic scattering relative to PhF2,3, with predominately edge-on orientations.


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GIWAXS was measured for thin films on NG substrates with the incident X-rays parallel to the NG direction (Figure 4d-4f). For PhF2,3, as shown in Figure 4d, the scattering pattern is almost identical to that in Figure 4a, with similar lamella and π-π stacking distances as well as mixed edge-on and face-on orientations. For PhF2,5 (Figure 4e) the scattering peaks remain at similar locations to those measured on substrates without NGs (Figure 4b); however, the π-π stacking diffraction peak (qxy = 1.75 Å-1) was sharper and less isotropic. For the PhF2,6 film (Figure 4f), a similar scattering pattern to films measured on substrate without NGs (Figure 4c) is observed. These measurements suggest that the NGs do not significantly alter the lamella or π-π stacking position or orientation. By rotating the sample 90o, GIWAXS was measured for X-rays perpendicular to the NG direction. For PhF2,3 film, as shown in Figure 4g, the π-π stacking diffraction peak (qxy = 1.75 Å-1) is present but is significantly weaker than that in the without (Figure 4a) and parallel to (Figure 4d) NG measurements. For PhF2,5 (Figure 4h) and PhF2,6 (Figure 4i), π-π stacking peaks (qxy = 1.75 Å-1) practically disappear in the perpendicular diffraction measurement.

Anisotropic alignment was compared using parallel vs. perpendicular π-π stacking peaks. As shown in Figure 5a, in the PhF2,3 film on the NG substrate, both parallel and perpendicular diffraction measurements show π-π stacking diffraction peaks, which indicates that polymer backbones are assembled in both parallel and perpendicular directions relative to the NGs, as shown in Figure 5d. However, the π-π stacking peak intensity in Figure 5a is significantly greater for the parallel measurement than that for the perpendicular measurement. Note that both parallel and perpendicular scattering images were collected from a film of uniform thickness with identical X-ray irradiation conditions. Thus, differences in intensity are more reasonably attributed to more PhF2,3 backbone alignment. This is consistent with OFET transport characteristics used to evaluate polymer alignment on NG substrates. As shown in Figure 5b and 5c, PhF2,5 and PhF2,6 π-π stacking scattering signals are relatively strong under the parallel condition and nearly negligible in the perpendicular condition, which indicates that their polymer backbones are almost strictly parallel to the NGs, as shown in Figure 5e and 5f. Alignment, as evaluated by GIWAXS, is better for

Figure 5. In-plane GIWAXS line-cut profiles: (a) PhF2,3; (b) PhF2,5; (c) PhF2,6. Schematic morphologies: (d) PhF2,3; (e) PhF2,5; (f) PhF2,6.

PhF2,5 and PhF2,6 relative to PhF2,3, consistent with OFET measurements. Considering the geometrical fitting, the curved backbone of PhF2,3 appears to have a greater difficulty in aligning within the confines of the linear NG environments. Morphological features of PhF2,5 and PhF2,6 measured by GIWAXS were similar; however, the OFET mobilities upon NG substrates had some differences. Relative peak intensity changes of parallel vs. perpendicular π-π stacking as well as π-π stacking distances were similar in the two polymers. We also calculated crystalline correlation lengths (CCLs) along the π-π stacking direction of parallel device films, which may have influenced their inter-chain charge hopping. Both films had a similar value of ~ 6.2 nm, see Table S1 in the SI. For aligned PhF2,5 and PhF2,6 films, transport in OFETs may have differed due to differences in intra-chain transport or in the amorphous region, since GIWAXS mostly probes the crystalline domains. A

number of recent publications have discussed the ideal morphology (considering amorphous and crystalline regions) for efficient charge transport in polymer semiconductor films,8e,15b,21 but there is no straightforward method to quantify morphology differences within the amorphous regions. Among other factors, though we tried to synthesize similar molecular weight values to minimize the molecular weight influence, the Mn of PhF2,6 (33 kDa) was slightly larger than that of PhF2,5 (28 kDa), which may also play a role in charge transport differences.15 CONCLUSION In conclusion, we synthesized three difluorophenylene polymers and examined their differences in chemical structure, focusing particularly on how chemical connectivity impacts the µ in NG aligned OFETs. The study shows that the different difluoro substitution orientations affect the self-assembling behavior on the NG substrate, since the F···H interactions dominate the conformation of



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adjacent building blocks and thereby the overall backbone secondary structure. Planar substrate device characterization indicates that structural variations bear negligible impact on OFET characteristics. However, the impact of chemical structure is more influential on devices containing substrates with NG features. The two more linear polymers, namely PhF2,5 and PhF2,6, gained a larger improvement of µ, relative to the more curved PhF2,3. Examination of their morphologies by GIWAXS experiments shows a greater level of anisotropic alignment along the NG direction for the two linear backbones, consistent with their higher mobilities. These results suggest that a linear backbone shape may be preferred as a design element for conjugated polymer OFETs that utilize alignment on NG substrates as an element for morphological control.

ASSOCIATED CONTENT Supporting Information. Experimental details for the synthesis and characterization of monomers and polymers, cyclic voltammetry measurements, theoretical calculations, OFET device measurements are available. This material is available free of charge via the internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected]

Author Contributions #Ming

Wang and Michael J. Ford contributed equally.

ACKNOWLEDGMENT We acknowledge financial support from the National Science Foundation under grant DMR 1411240 and the Mitsubishi Chemical Center for Advanced Materials (MC-CAM). We also acknowledge support from the Center for Scientific Computing (NSF DMR-1121053 and CNS-0960316). GIWAXS measurements were done at the Advanced Light Source on beamline 7.3.3.22 The Advanced Light Source is supported by Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Dr. Guang Wu’s help on the single crystal analysis.

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Linear Conjugated Polymer Backbones Improve Alignment in

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