A Role of Side-Chain Regiochemistry of Thienylene−Vinylene−Thienylene (TVT) in the Transistor Performance of Isomeric PolymersSo-Huei Kang,† Hae Rang Lee,‡ Gitish K. Dutta,∥ Junghoon Lee,† Joon Hak Oh,*,‡

and Changduk Yang*,†

†Department of Energy Engineering, School of Energy and Chemical Engineering, Perovtronics Research Center, Low DimensionalCarbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919,South Korea‡Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Pohang,Gyeongbuk 37673, South Korea∥Department of Chemistry, National Institutes of Technology (NIT) Meghalaya, Bijni Complex, Laitumkhrah, Shillong, Meghalaya793003, India

*S Supporting Information

ABSTRACT: The π-extended (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene(TVT)-based polymers are an interesting class of semiconducting polymersbecause of their excellent mobilities and unique film microstructures. Despitethese properties, the effect of the side-chain regiochemistry of TVT skeletonson the intrinsic properties of these polymers remains unclear. To investigatethis, in this study, hexyl-substituted TVT subunits with a “tail in (TI)” or “tailout (TO)” regiosymmetrical arrangement were first introduced into diketo-pyrrolopyrrole (DPP)-based copolymer main chains to afford “isomeric”polymers PI and PO, respectively. By combining optical spectroscopy, atomicforce microscopy (AFM), and grazing incidence X-ray diffraction (GIXD) data,we quantitatively characterized the aggregation, crystallization, and backboneorientation of both polymer films, which were then correlated to the charge-carrier mobilities. The PI film exhibited a bimodal packing motif comprising amixture of edge-on and face-on orientations, which was beneficial for three-dimensional (3D) charge transport and resulted in ahole mobility 2-fold larger than that in the PO film (μh = 1.69 cm2 V−1 s−1). This comparative study substantiates the importantrole of the regiochemistry of TVT in developing high-performance semiconducting polymers.

■ INTRODUCTION

Organic field-effect transistors (OFETs) based on π-conjugatedpolymers have attracted substantial scientific interest in thequest for “plastic electronics” such as flexible and low-cost E-papers, smart cards, radio-frequency identification tags, anddisplays. In the past decade, extensive studies in materialsdesign have boosted the OFET mobilities, exceeding those ofamorphous silicon.1−11 It has long been believed that anoutstanding OFET performance originates from highly orderedstructures with optimally oriented and π-stacked domains.2,4,5 Aπ-extended (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene (TVT)moiety has proven to be a promising building block forfacilitating significant long-range ordered structures into thepolymer backbone because of the rotational freedom betweenconsecutive thiophenes and the strong π−π interactions exertedby the thiophene units.2 Currently, several TVT-basedsemiconducting polymers optimized for either p-channel or n-channel OFET operation exhibit the highest charge-carriermobilities reported to date. The representative materials in thiscategory include donor−acceptor copolymers, incorporating

diketopyrrolopyrrole (DPP)−TVT,2,12 isoindigo (IIG)−TVT,13 or naphthalene diimide (NDI)−TVT moieties.14−16

On the other hand, several recent studies have indicated thata change of side chains (e.g., tuning the chain length and type,polarity, and branching point) on the polymer backbones has asignificant impact on molecular packing and thin-filmmorphology and therefore on the device perform-ance.2,5,12,17−22 Despite these various advances in side-chainengineering, no attention has yet been given to the effects ofside-chain regiochemistry for TVT-based frameworks.23−27 Itshould be of great interest to determine whether the side-chainregiochemistry in TVT-based polymers has a strong influenceon the physical, morphological, and electronic properties, as hasbeen observed in regioregular poly(3-hexylthiophene) (rr-P3HT), often referred to as the fruit fly of semiconductingpolymers.28,29

Received: November 12, 2016Revised: January 6, 2017Published: January 24, 2017

Article

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© 2017 American Chemical Society 884 DOI: 10.1021/acs.macromol.6b02447Macromolecules 2017, 50, 884−890

To this end, we designed hexyl-substituted TVT subunitswith a “tail in (TI)” or “tail out (TO)” regiosymmetricalarrangement, wherein TI and TO refer to the hexyl chainconfiguration on the thiophene moieties relative to the vinylenelinkages, as shown in Figure 1. We chose DPP with siloxaneside chains as a counterpart comonomer because of itssuccessful utilization in conjugated polymers as well as excellentcharge-carrier mobility and ambient stability, as demonstratedby our previous studies.5,30,31 In this study, two new TVT-based“isomeric” polymers, PI and PO, were synthesized viacopolymerization of each hexyl-substituted TVT monomericunit with DPP. A comparison of the properties of the two PIand PO copolymers allows us to systematically investigate theinfluence of TVT regiochemistry on relevant optical andelectrochemical properties, molecular ordering and orientation,and macroscopic charge-carrier transport characteristics.Notably, we found that the relative crystallinity of the lamellarpacking and the crystallite orientation distribution can begreatly affected by the regiochemistry of the TVT blocks alongthe polymer backbones, thereby resulting in a marked effect onOFET performances. For example, the solution-processedOFETs of PI show a high hole mobility of up to 1.69 cm2

V−1 s−1, which is approximately twice that of PO under theoptimized conditions. Our systematic study demonstrates that

this distinct difference in hole mobility originates from the largedifference in their intergranular connectivity and molecularpacking motifs, which highlights the importance of the side-chain regiochemistry in TVT-based polymers.

■ RESULTS AND DISCUSSION

Synthesis and Characterization. The synthetic routes ofall intermediates and TVT-based polymers (PI and PO) areshown in Figure 1. First, the compounds 2-formyl-3-hexyl-thiophene (1) and 2-formyl-4-hexylthiophene (4) weresynthesized using different formylation methods to obtainposition selectivity according to the procedure reported in theliterature.32,33 A McMurry-type reduction coupling wasperformed using the respective aldehydes and a low-valenttitanium reagent (TiCl4/Zn) to afford the key intermediates(E)-1,2-bis(3-hexylthiophen-2-yl)ethene (2) and (E)-1,2-bis(4-hexylthiophen-2-yl)ethene (5), respectively. Finally, each TVT-based compound was converted into the corresponding bis-stannylated monomer (3 and 6) via lithiation and subsequenttreatment with trimethyl tin chloride. The palladium-catalyzedStille copolymerization of monomers 3 and 6 with adibrominated monomer, 3,6-di(2-bromothien-5-yl)-2,5-bis[6-(1,1,1,3,5,5,5,-heptamethyltrisiloxan-3-yl)butyl]pyrrolo[3,4-c]-pyrrole-1,4-dione, afforded the target copolymers PI and PO,

Figure 1. Synthetic routes of PI and PO.

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respectively. Both the copolymers are readily soluble incommon organic solvents, including chloroform, chloroben-zene, and dichlorobenzene, at room temperature. The syntheticdetails and complete characterizations of these copolymers areprovided in the Experimental Section of the SupportingInformation. The number-average molecular weights (Mns) ofPI and PO were determined to be 10.3 and 10.1 kDa, withpolydispersity indices (PDI) of 3.76 and 2.54, respectively, asestimated via a high-temperature gel permeation chromatog-raphy (HT-GPC) at 120 °C with 1,2,4-trichlorobenzene as aneluent. The calculated degree of polymerization for bothpolymers was approximately 8. Note that the similar Mn andPDI values can minimize possible interference from Mn andPDI variations.Theoretical Calculations and Optical and Electro-

chemical Properties. To predict the polymer structuralconformations and π-electronic structures, a density functionaltheory (DFT) calculation for the dimeric units of PI and POwas performed using a Gaussian B3LYP/6-31G base with theGaussian 09 program. The estimated frontier molecular orbitalsand the optimized geometry of the dimers are shown in Figure2. For both copolymers, the highest occupied molecular orbital

(HOMO) and the lowest unoccupied molecular orbital(LUMO) were well-delocalized over the polymer backbones,and the calculated HOMO and LUMO levels of each isomericdimer were −4.59 and −2.86 eV for PI and −4.54 and −2.85eV for PO, respectively.Another interesting feature in the side view of the structures

is that the backbone coplanarity correlates with theregiopositioning of the side chains in the TVT subunit (Figure3). For example, the intratorsion angles (θ1) between twothiophene units within the TVT subunits for PI and PO are23.3° and 1.0°, respectively, and both the structures exhibitsimilar intermonomeric torsion angles (θ2) between the DPPand TVT blocks. Additionally, the neighboring oligothiophenerings within the PI backbone cross toward the left and right ofeach other, whereas for PO, they are mutually twisted in thesame direction, thereby implying a certain coil-like polymerconformation. In addition, using the DFT calculation, we alsoobtained the energy levels and molecular geometry of the dimerof the unsubstituted parent polymer consisting of DPP andunsubstituted TVT. As shown in Figures S7 and S8, theunsubstituted parent polymer showed deeper-lying HOMO/LUMO levels with better planar conjugated backbonecompared to both the substituted copolymers.Figure 4 shows the optical absorption spectra and cyclic

voltammograms of PI and PO, and the relevant data aresummarized in Table 1. Both the copolymers present dual-absorption bands covering the whole visible to near-infraredrange; a less intense π−π* band was observed in the shorter-wavelength region, and a strong intramolecular charge-transfer(ICT) band was observed in the longer-wavelength region.The absorption profiles of PI were red-shifted in both

solution and the solid state relative to those of PO; this in turnresulted in a smaller optical band gap (Eg

opt) being estimatedfrom the absorption onset in PI films (1.30 eV) in comparisonto PO (1.37 eV). Nevertheless, upon going from solution tothe solid state, the main absorption peak of PI was slightly blue-shifted, whereas for PO, a large red-shift occurred towardlonger wavelengths, together with pronounced vibronicshoulders at approximately 802 nm. We propose that thisbehavior involves different types of aggregates that are formedby these two different regiosymmetrical copolymers. Forexample, the PI film adopts a cofacial structure with H-aggregation, whereas in the PO film, an end-to-end formationleads to J-aggregation. This implies that PI is favorable for

Figure 2. Optimized geometries of the dimers by DFT calculation andthe charge density isosurfaces for HOMO and LUMO levels.

Figure 3. Side views of the optimized geometries of the dimers.

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interchain interactions, while PO predominantly involvesintrachain interactions. Additionally, considering the fact thatthe ratio of the intensity of the 0−0 vibrational transitionrelative to the 0−1 transition increases as a result of a decreasein the Coulombic coupling,19 this can also support the theorythat PI has stronger interchain coupling than PO.The HOMO and LUMO energy levels were determined via

cyclic voltammetry (CV) (Figure 4b). It can be clearly observedthat PI shows deeper-lying HOMO and LUMO levels than PO(EHOMO/ELUMO = −5.2/−3.5 and −5.1/−3.3 for PI and PO,respectively); the same trend was observed in the computa-tional study mentioned above. This indicates that a conforma-tional change in the alkyl side chains in TVT has a strong effecton the energy levels of the resulting polymers, as indicated byprevious observations of rr-P3HT chemistry.34

Thin-Film Microstructural Analyses. The effects of side-chain regiochemistry on the microstructure of TVT-basedisomeric copolymer films were investigated using tapping-mode

atomic force microscopy (AFM) and grazing incidence X-raydiffraction (GIXD) analyses. The copolymer films were drop-cast from a 3 mg mL−1 solution onto an n-octadecyl-trimethoxysilane (OTS)-modified substrate. Figure 5 showsthe representative AFM images of solution-processed copoly-mer films. Both the films exhibited small granular structureswithout thermal treatment. A relatively low surface roughnessof ∼1.3 nm was obtained for PI, whereas PO exhibited aconsiderably agglomerated surface with a high roughness of∼4.9 nm. After thermal annealing at 220 °C, the PI filmexhibited highly developed fibrillary networks with an averagethickness of 63 nm resulting from the reorganization ofpolymeric chains, which would be beneficial for efficient chargetransport in OFETs. However, the PO film lacked theinterconnected nanofibrillar networks and still exhibited highroughness, with a significant height difference of at least 20 nmafter thermal treatment. This result implies a difficulty incharge-carrier transport resulting from poor intergranularconnectivity in PO. As predicted from the DFT calculationresults, these significant crystalline morphological differencescan be attributed to the significantly disturbed intermolecularinteractions caused by the distortion along the backbone of PO;the thermal energy required to change the moleculararrangement in this case will be much higher than that requiredin the case of PI. Furthermore, the large discrepancy in thesurface energies (∼10.3 mJ m−2) of the PO polymer film andthe OTS-modified substrate might result in the formation ofstructures more agglomerated than that between the PIpolymer film and its OTS-modified substrate (∼7.6 mJ m−2,Table S1).The optimal two-dimensional (2D) GIXD images and the

corresponding diffractogram profiles of the PI and PO films areshown in Figure 6 (see Figure S9 for as-cast films). PI exhibitedhighly ordered (h00) Bragg peaks in both in-plane and out-of-plane directions, whereas PO exhibited peaks only in the qzdirection. In both the as-cast and annealed films, the PI filmsshowed shorter lamellar layer distances than the PO films (seeTable S2) presumably because of the better π-backboneplanarity of PI, which would facilitate well-ordered lamellarstacking. The enhanced lamellar stacking behavior can beestimated from the smaller crystalline distortion parametervalue, a criterion of stacking orderliness: 3.55% for PI and3.92% for PO, as shown in Figure S10.35 Note that PI exhibiteda strong π−π interaction in the out-of-plane direction, indicatedby an outstanding peak at qz ∼ 1.7 Å−1. This fact can becombined with the bimodal crystalline nature of PI to validatethe adoption of a three-dimensional (3D) charge conductionchannel with enhanced charge transport in PI films.5,36,37 Incontrast, PO did not exhibit a discernible (010) peak in theout-of-plane direction and only showed a (010) peak in the in-plane direction, thereby indicating the formation of aconduction pathway only through the in-plane direction (2Dcharge transport). Our morphological study revealed that PIpolymer films with beneficial granular microstructures andbimodal molecular packing have more favorable micro-structures for charge transport than PO polymer films.

Electrical Characterization and the Performance ofOFETs. The electrical performance of bottom-gate/top-contactOFETs was measured to elucidate the impact of side-chainregiochemistry on charge-transport properties. The experimen-tal details of device fabrication and measurements are describedin the Experimental Section of the Supporting Information.The optimal post-treatment conditions were determined by

Figure 4. UV−vis−NIR absorption spectra of PI and PO (a) in dilutechloroform solution and (b) as thin films on a quartz plate. (c) Cyclicvoltammograms of the polymer films in n-Bu4NPF6/CH3CN solution(scan rate: 100 mV s−1).

Table 1. Photophysical and Electrochemical Properties of PIand PO As Derived from the UV−Vis−NIR AbsorptionSpectra and Cyclic Voltammetry Curves

λmaxsol λmax

film EHOMOa ELUMO

a Egoptb

polymer [nm] [nm] [eV] [eV] [eV]

PI 795 772 −5.2 −3.5 1.30PO 703 729, 802 −5.1 −3.3 1.37

aThin films in n-Bu4NPF6/CH3CN versus ferrocene/ferrocenium at100 mV s−1. HOMO and LUMO estimated from the onset oxidationand reduction potentials, respectively, assuming the absolute energylevel of ferrocene/ferrocenium to be 4.8 eV below vacuum bCalculatedfrom the absorption band edge of the polymer film, Eg

opt = 1240/λedgefilm .

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testing the electrical performance of OFETs based on as-prepared films and those annealed at various temperatures(Figure S11 and Table S3). Interestingly, both copolymers

exhibited unipolar p-type field-effect behavior because theHOMO levels of both polymers are relatively well-matched tothe work function (∼5.1 eV) of Au electrodes. As-cast PI and

Figure 5. AFM height (top) and phase (bottom) images of drop-cast (a, b) PI and (c, d) PO films (a, c) before and (b, d) after thermal treatment.The scale bar is 500 nm.

Figure 6. 2D-GIXD images of drop-cast (a) PI and (b) PO films after thermal treatment at 220 °C. The corresponding 1D-GIXD profiles of (c) in-plane and (d) out-of-plane GIXD patterns. Pole figures for (e, f) lamellar and (g, h) π−π stacking of (e, g) PI and (f, h) PO (where χ is defined asthe semicircular angle between the crystallite orientation and the surface normal).

Figure 7. I−V characteristics of the optimized FETs obtained from drop-cast thin films of (a, b) PI and (c, d) PO. (a, c) Transfer curves obtained athole-enhancement operation with VDS = −100 V and (b, d) output characteristics (L = 50 μm and W = 1000 μm).

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PO films exhibited maximum hole mobilities (μh,max) of 1.28and 0.46 cm2 V−1 s−1, respectively. As the annealingtemperature increased, the charge-carrier mobilities increasedand reached the maximum values of 1.69 cm2 V−1 s−1 for PIand 0.90 cm2 V−1 s−1 for PO at an optimized annealingtemperature of 220 °C. The corresponding transfer and outputcharacteristics under the optimized conditions are depicted inFigure 7. The enhanced charge transport in PI with a tail-inside-chain conformation can be attributed to the well-delocalized HOMOs and LUMOs along the relatively planarconjugated backbone and the effective intermolecular inter-action with strong interchain coupling. Moreover, comparedwith the PO film, which had a poor intergranular connectivityand unimodal molecular packing, the PI film exhibited highlyinterconnected fibrillary networks and bimodal molecularpacking that could form 3D charge-conduction channels; thisalso accounts for the facilitated charge transport in PI.

■ CONCLUSION

In summary, we prepared two new TVT-based isomericpolymers (PI and PO) by tailoring the side-chain regiochem-istry on the TVT subunit. By analyzing and comparing theoptical, structural, and charge-transport data for the resultingpolymers, we found that different regiosymmetrical arrange-ments in the main backbone can play a pivotal role in governingthe intrinsic properties of the polymers. For example, the PIpolymer film has a strong tendency to form H-aggregates andbimodal molecular packing for 3D charge transport, whereas J-aggregation of the PO film induces a predominantly unimodalmolecular packing motif. In addition, we found that side-chainregiochemistry significantly influences the grain morphologyand intergranular connectivity. By controlling the side-chainconformation, we observed a large difference in OFET mobilitybetween PI (μh = 1.69 cm2 V−1 s−1) and PO (μh = 0.90 cm2

V−1 s−1) under the optimized conditions. This studydemonstrates that incorporation of a TVT−TI subunit intothe polymer backbone is an effective way to achieve highbaseline mobility; this furthers our understanding of how thefilm microstructure and charge-transport characteristics areinfluenced by the side-chain regiochemistry of TVT.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.6b02447.

Detailed synthetic procedures and fabrication method,additional figures (NMR spectra of new materials, surfaceenergy analyses, GIXD data and profiles, mobilitydistributions based on various temperature, transfer andoutput characteristics for TVT-based isomeric polymers(PI and PO)), and summary of crystallographicparameters (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: joonhoh@postech.ac.kr (J.H.O.).*E-mail: yang@unist.ac.kr (C.Y.).

ORCIDChangduk Yang: 0000-0001-7452-4681

Author ContributionsS.-H.K. and H.R.L. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Research Foundationof Korea (NRF) funded by the Ministry of Education(2015R1A2A1A10053397, 2014K1A3A1A19066591,2014R1A2A2A01007467) and Center for Advanced SoftElectronics under the Global Frontier Research Program(Grant 2013M3A6A5073175) of the Ministry of Science,ICT & Future Planning, Korea. The GIXD experiment at thePLS-II 3C SAXS I beamline was supported in part by MEST,POSTECH.

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Macromolecules Article

DOI: 10.1021/acs.macromol.6b02447Macromolecules 2017, 50, 884−890

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