Donating Building Block for Hig

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Fluorinated Head-to-Head Dialkoxybithiophene: A New Electron-Donating Building Block for High-Performance Polymer Semiconductors Jun Huang, Han Guo, Mohammad Afsar Uddin, Jianwei Yu, Han Young Woo,* and Xugang Guo* cells (OSCs) and organic thin-film transistors (OTFTs).[6–13] Among various semiconducting polymers, the head-to-tail linkage-containing regioregular poly(3hexylthiophene) (rr-P3HT) has been extensively studied, showing highly crystalline film morphology.[14] Well-ordered rr-P3HT hence facilitates charge carrier transport and shows a hole mobility (μh) up to 0.1 cm2 V−1 s−1 in OTFTs. Whereas the regiorandom P3HT (rra-P3HT) containing 20% head-to-head linkage exhibits a blue-shifted absorption, amorphous film morphology, and hence inferior OTFT performance with a μh of 10−4 cm2 V−1 s−1.[15] When P3HT was applied in OSCs, the charge carrier transport and collection highly depend on the self-assembly behavior of polymer chains, which shows a close relation with the degree of head-tohead linkages in a polymer backbone.[16] Therefore, head-to-head linkages are typically avoided in the design of organic semiconductors for achieving promising OTFT and OSC performance. Poly(3,4-ethylenedioxythiophene) (PEDOT), one of the most used conducting polymers, shows remarkable conductivity (up to ≈4100 S cm−1) in doped state by polystyrene sulfonic acid.[17,18] The substantial conductivity is partially attributed to its highly planar backbone, which is achieved via the intramolecular nonconvalent (thienyl) S⋯(ethylenedioxy)O Coulombic interactions with conformational lock.[19,20] Based on the same motif, we first reported a head-to-head building block, 3,3′-dialkoxy-bithiophene (BTOR, Figure 1a) and synthesized

New building blocks with good solubility and optimized optoelectrical property are critical for materials development in organic electronics. Herein, a new head-to-head linkage containing a donor unit, 4,4′-difluoro-3,3′-dialkoxy-2,2′bithiophene (BTfOR), is synthesized. The dialkoxy chains afford good materials solubility and also planar backbone via noncovalent (thienyl)S⋯(alkoxy) O interactions. Compared to the reported 3,3′-dialkoxy-2,2′-bithiophene (BTOR), F addition leads to BTfOR with lower-lying frontier molecular orbitals and can further promote polymer packing via additional F⋯S or F⋯H interactions. BTfOR can be readily stannylated to afford tin monomer with high purity and excellent reactivity toward Stille polymerization. As a proof of concept for materials design, BTfOR-based homopolymer (PBTfOR) is synthesized, showing high molecular weight and strong aggregation. Moreover, the HOMO (−4.98 eV) of PBTfOR is greatly lower than that (−4.54 eV) of nonfluorinated counterpart PBTOR, which is attributed to the addition of F atoms. When incorporated into thin-film transistors, PBTfOR exhibits a remarkable hole mobility of 0.57 cm2 V−1 s−1, showing an exceptional example of high-mobility head-to-head polythiophene. This study demonstrates that introduction of F atoms can lead to BTfOR with optimized physicochemical properties, and the new BTfOR should find promising use for constructing donor–acceptor copolymers for high-performance electronic devices.

1. Introduction Organic and polymeric semiconductors have attracted considerable attention due to their solution-based processability, which enables production of electronic devices via various printing or coating techniques.[1–5] In the last decade, great progress has been achieved in the development of high-performance organic electronic materials and devices, that is, organic solar

Dr. J. Huang, Dr. H. Guo, J. Yu, Prof. X. Guo Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed Organic Electronics South University of Science and Technology of China No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.201700519.

DOI: 10.1002/aelm.201700519

Adv. Electron. Mater. 2017, 1700519

Dr. J. Huang Department of Chemistry Wuhan University Wuhan, Hubei 430072, China Dr. M. A. Uddin, Prof. H. Y. Woo Research Institute for Natural Sciences Department of Chemistry Korea University Seoul 136-713, South Korea E-mail: [email protected]

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Figure 1. Chemical structures and frontier molecular orbital energy diagrams of a) 3,3′-dialkoxy-2,2′-bithiophene (BTOR), b) 4,4′-dialkoxy5,5′-bithiazole (BTzOR), c) 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR), d) 3,3′-dialkoxy-4,4′-dicyano-2,2′-bithiophene (BTCNOR), and e) 4,4′-difluoro-3,3′-dialkoxy-2,2′-bithiophene (BTfOR). The energy levels are calculated based on density functional theory.

a donor–acceptor (D–A) copolymer with phthalimide. The resulting polymer shows a remarkably high μh of 0.2 cm2 V−1 s−1 in OTFTs.[21] It was found that inserting oxygen atom into head-to-head dialkyl bithiophene leads to BTOR with planar backbone, and the alkoxy chains on the 3 and 3′ positions can also afford good materials solubility. However, the strong electron-donating alkoxy chains result in elevated highest occupied molecular orbital (HOMO), which deteriorates the OTFT device stability of the resulting polymers. To lower HOMO level, a novel head-to-head bithiazole (BTzOR, Figure 1b) based on the electron-deficient thiazole core and a monoalkoxy-functionalized head-to-head bithiophene (TRTOR, Figure 1c) were also developed to construct D–A copolymers for OTFTs and OSCs.[22–24] The BTzOR- and TRTOR-based polymers show suppressed HOMOs as well as improved device performance compared to BTOR-based analogues. In spite of their improved electrical properties, the HOMOs of BTzOR- and TRTORbased polymers are still not sufficiently low, revealed by electrochemical characterization and the open-circuit voltage (Voc) measured in OSCs. To further optimize frontier molecular orbitals (FMOs) of head-to-head containing building blocks, we designed and synthesized a cyano-functionalized dialkoxy bithiophene (BTCNOR, Figure 1d). The strong electron-withdrawing cyano group leads to greatly decreased HOMO (−5.60 eV) for the BTCNOR-benzodithiophene copolymers, thus a remarkable high Voc up to 1.0 V in OSCs was achieved.[25] However, due to its high electron-deficiency, the stannylation of BTCNOR failed to yield pure monomer, and the Stille coupling-based polycondensation was unsuccessful. As a result, the brominated BTCNOR monomers were prepared instead and polymerized with electron-rich stannylated co-monomers,


such as benzodithiophene. For OSC application, strong acceptor-weak donor motif is often utilized in materials design to maximize OSC performance.[26,27] However, stannylated BTCNOR only enables construction of weak donor (or weak acceptor)–weak donor copolymers, preventing further optimization of polymer optoelectrical properties. In addition, the cyano group imposes a certain degree of steric hindrance on neighboring arenes, detrimental to polymer backbone planarity and film crystallinity, which hence limits the applications of BTCNOR for constructing high-performance polymers in organic electronics. Fluorination of polymer backbone is a highly promising strategy to optimize the optoelectrical properties of polymer semiconductors by decreasing the FMO levels and improving film crystallinity via strong intra- and interchain Coulombic interactions (such as dipole–dipole, hydrogen bonding, and F⋯S).[28–33] Heeney and co-workers reported the fluorinated poly(3-alkylthiophene), which displays a lower-lying HOMO and stronger interchain aggregation in solution, compared to nonfluorinated counterpart. OTFTs fabricated by a zone-casting technique leads to an average μh of 0.70 cm2 V−1 s−1, which is increased by fivefold compared to the nonfluorinated analogue polymer.[34] Fluorine has the highest Pauling electronegativity of 4.0 and small van der Waals radius (r = 1.35 Å), which effectively lowers the FMOs without increasing steric hindrance in polymer chains.[35] Therefore, it is expected that modifying the electron-rich BTOR with electron-withdrawing F atom should result in a weaker electron-donating unit, 4,4′-difluoro-3,3′-dialkoxy2,2′-bithiophene (BTfOR, Figure 1e), with improved optoelectrical property. We report here the design and synthesis of a novel headto-head building block (BTfOR) based on alkoxy-substituted bithiophene modified by F atom on 4 and 4′ positions. Singlecrystal analysis shows direct evidence of intramolecular noncovalent S⋯O interaction, leading to self-planarization of the molecular/polymeric backbone. More importantly, stannylation of BTfOR yields the tin monomer with good purity (>99%) as well as high chemical reactivity in the Stille coupling-based polymerization, which should enable the access of various BTfOR-based D–A semiconducting copolymers. To understand the basic properties of this new head-to-head building block, a BTfOR-based homopolymer PBTfOR was synthesized. The HOMO of PBTfOR (−4.98 eV) was measured to be decreased by 0.44 eV compared to that of nonfluorinated polymer analogue (−4.54 eV). In solution, PBTfOR shows strong aggregation, likely originating from intramolecular noncovalent S⋯O interactions. Using an off-center spin-coating method, OTFTs incorporating PBTfOR active layer show a high μh of 0.57 cm2 V−1 s−1, which is among the highest values for polymer semiconductors containing only thiophene in backbone.[34,36–38] This study showcases an exceptional example of high mobility head-to-head type polythiophene. Hence, the new BTfOR is a promising electrondonating building block and it is expectable that incorporating BTfOR into D–A copolymers should generate various semiconductors with good solubility, optimized optoelectrical properties, and promising device performance in OTFTs and OSCs.

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Scheme 1. Synthetic route to 4,4′-difluoro-3,3′-dodecyloxy-2,2′-bithiophene monomer and correponding homopolymer PBTfOR, and nonfluorinated counterpart PBTOR. Reagents and conditions: (i) NaOH, ethanol, 80 °C; (ii) Br2, acetic acid; (iii) C12H25Br, K2CO3, DMF; (iv) NaOH, ethanol, 100 °C; (v) copper chromate, quinoline, 200 °C; (vi) LDA, TMSCl, THF, −78 °C; (vii) LDA, CuCl2, THF, −78 °C; (viii) t-BuLi, NFSI, THF, −78 °C; (ix) Br2, CHCl3, 70 °C; (x) n-BuLi, SnMe3Cl, THF; (xi) Pd2(dba)3, toluene, microwave; (xii) NBS, CHCl3.

2. Results and Discussion 2.1. Materials Synthesis Fluorination is an effective approach to modify the properties of organic semiconductors, however it is not easy to design facile synthetic routes.[34,39–41] On the basis of our previous work,[23] the direct introduction of F atom onto 3,3′-bis(dodecyloxy)-2,2′-bithiophene (BTOR) was tried. 5 and 5′ positions of BTOR were first protected using trimethylsilyl groups and the compound was directly subjected to a typical fluorination method (Scheme S1, Supporting Information).[34,42] However, the reaction does not proceed to give the product. Thus, a new synthetic route to BTfOR was designed as described in Scheme 1, which enables successful synthesis of the brominated and stannylated monomers. Methyl 3-hydroxy thiophene-2-carboxylate (1) was prepared according to the reported procedure in literatures, then followed by bromination to afford methyl 4-bromo-3-hydroxythiophene-2-carboxylate (2).[43–45] The etherification of compound 2 was readily realized in the presence of 1-bromododecane and potassium carbonate to give methyl 4-bromo-3-(dodecyloxy)thiophene-2-carboxylate (3). 3-Bromo-4-(dodecyloxy)thiophene (5) was then obtained by decarboxylation of 4-bromo-3-(dodecyloxy)thiophene-2-carboxylic acid (4), which resulted from saponification of ester 3 under microwave irradiation. From 3 to 5, the reactions show excellent efficacy with an overall yield of 97%. Due to the higher acidity of the proton near Br atom, compared to that of the proton at 5-position next to the dodecyloxy chain, trimethylsilane (TMS)


was then introduced selectively to the 2-position of compound 5 as the protecting group. Lithiation of (3-bromo-4-(dodecyloxy) thiophen-2-yl)trimethylsilane (6) with LDA followed by CuCl2mediated coupling afforded 4,4′-dibromo-3,3′-bis(dodecyloxy)[2,2′-bithiophene]-5,5′-diyl)bis(trimethylsilane) (7) in a good yield (72%). Then Br-Li exchange followed by reaction with (PhSO2)2NF (NFSI) proceeded smoothly to afford the key compound, 3,3′-bis(dodecyloxy)-4,4′-difluoro-[2,2′-bithiophene]5,5′-diyl)bis(trimethylsilane) (8) in 35% yield. Treatment of compound 8 with Br2 yielded the brominated monomer 9 in 69% yield. The structure of compound 9 is confirmed by singlecrystal X-ray diffraction measurement, and the detailed crystal data are included in Section 2.3. Compound 9 was readily lithiated and quenched with trimethyltin chloride to afford a distannylated monomer 10 in a decent yield (95%) with a high purity (>99%) after recrystallization, confirmed by high-performance liquid chromatography and NMR spectra (Figure S1, Supporting Information). As a proof of concept for materials design, the BTfOR-based homopolymer PBTfOR was first synthesized using the dibrominated 9 and the distannylated 10 as monomers under the typical Stille coupling-based polycondensation (Scheme 1). After purification via Soxhlet extraction, the high-temperature gel permeation chromatography (GPC) measurement reveals that PBTfOR has a number-average molecular weight (Mn) of 34.5 kDa with a polydispersity index (Đ) of 2.5 (Figure S2, Supporting Information). In spite of long solubilizing chain on each thiophene unit, PBTfOR can only be dissolved in hot chlorobenzene, dichlorobenzene, and trichlorobenzene (TCB), due

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Figure 2. Normalized UV–vis absorption spectra of PBTfOR and PBTOR a) in trichlorobenzene solutions (1 × 10−5 m) and b) in films.

to the strong aggregation induced by the intramolecular S⋯O interaction.[34,37,46] For better understanding the structure–property correlations of the new BTfOR-based polymer, the known BTOR-based homopolymer PBTOR without F atoms was also synthesized for comparison (Scheme 1).

2.2. Optical, Electrochemical, and Thermal Properties UV–vis absorption spectra of polymers PBTfOR and PBTOR in solution and film are shown in Figure 2. Compared to PBTOR, the introduction of F leads to blue-shifted absorption in both solution and thin film for PBTfOR. The TCB solution of PBTfOR at room temperature exhibits an absorption maximum (λmax) at 568 nm with structured features, indicative of interchain aggregation in solution. Upon heating the solution from 60 to 100 °C, the λmax is slightly blue-shifted (Figure S3, Supporting Information), suggesting that PBTfOR maintains significant aggregation even at elevated temperatures,[34,37,47] which is attributed to the planar backbone with strong intermolecular interaction. In film state, the λmax of PBTfOR is similar to that of polymer solution at room temperature. The optical

bandgap (Egopt) of PBTfOR derived from the film absorption onset is 1.82 eV, greatly larger than that (1.55 eV, Figure S4, Supporting Information) of PBTOR, which is attributed to the lower-lying HOMO of PBTfOR (vide infra). Raman spectroscopy was also employed to investigate the conformation of PBTfOR backbone. The density functional theory (DFT)-based calculated Raman spectra and experimentally determined Raman spectra of PBTfOR film are shown in Figure 3a. The calculated Raman peaks are located in the range of 1400–1600 cm−1. The measured Raman spectra of polymer films are in good consistency with the calculated result and display the same features at different annealing temperatures, where the 1424 cm−1 peak is assigned to the main CC collective stretch and the peak at 1525 cm−1 corresponds to the CC collective stretch. The high intensity of the CC mode compared to CC mode indicates highly planar polymer backbone conformation resulting from intramolecular noncovalent S⋯O interaction and the introduction of F atom, which could lead to F⋯S or F⋯H interactions.[34,48,49] Therefore, it is reasonable to expect that high charge carrier mobility could be realized in OTFT device using PBTfOR as the active layer.[38,50]

Figure 3. a) Calculated and experimentally measured Raman specrta of PBTfOR. b) Cyclic voltammograms of polymer films of PBTfOR and PBTOR.


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Figure 4. Single-crystal structure of the brominated BTfOR. a) S⋯O distance and dihedral angle; capped stick representation of b) π-stacked tetramer, and c) intermolecular stacking in solid state.

The electrochemical properties of PBTfOR and the nonfluorinated PBTOR films were characterized using cyclic voltammetry (CV) in 0.1 m (n-Bu)4N·PF6 acetonitrile solution relative to the ferrocene/ferrocenium (Fe/Fe+) redox couple as the standard. The CV curves are shown in Figure 3b. The HOMO levels are determined from the onset of oxidation peak by assuming a formal potential of −4.8 eV for the Fe/Fe+.[51] The HOMO of PBTfOR was calculated to be −4.98 eV, which is substantially lower compared to that of nonfluorinated analogue PBTOR (−4.54 eV), reflecting the electron-withdrawing effect of F atoms. The lower-lying HOMO of PBTfOR shows a good agreement with DFT calculation (Figure 1),[52] in which both HOMO and LUMO could be effectively lowered by ≈0.4 eV by introduction of F atom into 3,3′-dimethoxy-2,2′-bithiophene. It should be pointed out that the HOMO (−4.98 eV) of homopolymer PBTfOR is not sufficiently low to suppress OTFT off-currents and increase OTFT device air stability, which is mainly attributed to the p-type nature of the polymer backbone consisting of all electron-rich thiophene moieties. It is expected that copolymerizing BTfOR with acceptor co-units will lead to the resulting D–A copolymers with lower-lying HOMOs. The PBTfOR thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA data reveal that the decomposition onset with a 5% weight loss (Td) under N2 is 326 °C (Figure S5, Supporting Information), probably due to the breakup of CO bond. The thermal stability of PBTfOR should be sufficient for OTFT fabrication and optimization. DSC measurement shows no distinctive thermal transitions in the temperature range of 30–320 °C (Figure S6, Supporting Information).[53,54]

is 2.86 Å, which is considerably shorter than the sum (3.35 Å) of van der Waals radius of S and O atoms, indicative of significant noncovalent S⋯O Coulombic interactions. The crystal packing structure of the compound 9 shows a slipped face-toface stacking motif with a close interfacial distance of 3.47 Å (Figure 4b,c). The results indicate that BTfOR is a promising building block for enabling polymers with planar backbone and close interchain stacking distance.[56] To gain more information on backbone planarity and electronic structures of polymer PBTfOR, theoretical calculation was performed on the trimer of polymer repeating unit using the DFT method.[52] As shown in Figure 5a,b, each thiophene unit adopts an anticonformation with respect to adjacent thiophenes and the polymer features a planar backbone, examplified by the dihedral angles of 179.58° and 179.60° between the neighboring thiophenes. The backbone planarization is mainly attributed to the intramolecular noncovalent interactions.[34] The electron density in HOMO and LUMO orbitals is highly delocalized along the backbone (Figure 5c,d). The molecular backbone coplanarity and delocalized FMOs should facilitate intrachain charge carrier transport.[57]

2.4. Organic Thin-Film Transistor Performance Top-gate/bottom-contact (TGBC) OTFTs were fabricated to characterize the charge transport properties of the new semiconducting polymer PBTfOR, together with PBTOR for comparison.[58] The source/drain electrodes (3 nm Cr and 30 nm Au) were patterned on glass substrates prior to device fabrication using standard photolithography process. After

2.3. Single-Crystal Structure and Molecular Theoretical Modeling Single crystals of building blocks can provide insights into backbone conformation, intramolecular noncovalent interactions, and intermolecular packing structure of the resulting polymers.[25,55] Thus, the single crystal of the compound 9 was grown by diffusing ethanol into its dichloromethane solution and X-ray diffraction data were collected (Table S1, Supporting Information). The single-crystal X-ray diffraction data (Figure 4) reveal that two thiophenes adopt transoid orientation with highly planar backbone conformation. The distance between S and O atoms


Figure 5. a) Front view, b) side view, c) HOMO spatial distribution, and d) LUMO spatial distribution of trimer of BTfOR calculated by DFT method at B3LYP/6-31G (d, p) level.

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0.11 cm2 V−1 s−1 (Figure S8, Supporting Information), when the coating flow is perpendicular to the source-to-drain electrode direction. The charge transport anisotropy ratio is ≈5 by taking the mobility ratio in the parallel and perpendicular directions, which is higher than that of 1.7 obtained from the UV–vis absorption spectra (Figure S9, Supporting Information), probably due to higher degree of polymer chain alignment near the surface than in bulk.

substrate cleaning, the polymer semiconductors were spincoated from corresponding solutions in a N2-filled glovebox and then annealed on hotplate at various temperatures. Subsequently, 400 nm CYTOP dielectric layer was spin-coated from the diluted solution (CTL-809M:CT-SOLV180 = 2:1, v/v, Asahi Glass Co., Ltd.), then annealed at 100 °C for 30 min. Finally, 50 nm Al gate electrode was thermally evaporated through shadow mask under high vacuum to complete device fabrication (channel length: 20, 50, or 100 µm, channel width: 5 mm). To fully optimize the device performance, we systematically investigated the effects of processing solvents and film annealing temperatures. It was found that using TCB as the casting solvent afforded improved OTFT performance for PBTfOR, likely due to its good solubilizing capability. Furthermore, the OTFT performance highly depends on the annealing conditions of the semiconductor films, thermal annealing at elevated temperatures generally leads to improved device performance. As shown in Table 1, the as-cast OTFTs based on PBTfOT active layer show typical p-type transport characteristics with an average μh of 0.098 cm2 V−1 s−1 and a current modulation ratio (Ion/Ioff) of 103, which is similar to the performance of P3HT (Figure S7, Table S2, Supporting Information). Under optimal annealing temperature of 200 °C, the average and maximum saturation μhs are increased to 0.16 and 0.24 cm2 V−1 s−1 with Ion/Ioff ’s of 103, respectively (Figure 6a,d). In contrast, the OTFTs based on the nonfluorinated analogue polymer PBTOR under the optimal fabrication condition exihibit much lower average and maximum μh of 5.4 × 10−3 and 7.4 × 10−3 cm2 V−1 s−1 together with two order lower Ion/Ioff ratio of 101 (Figure 6c,f). The performance variation between PBTfOR and PBTOR demonstrates the importance of fluorination of polymer backbone in boosting the charge carrier mobility and improving the device Ion/Ioff ratios in OTFTs. We further optimized the performance of PBTfOT OTFT devices by switching to off-center spin-coating method for casting the polymer semiconductor layer, which has been shown to be able to induce a certain degree of polymer chain alignment in the flow direction.[36,50] Under the same annealing condition at 200 °C, the average and maximum saturation μh values were further enhanced to 0.50 and 0.57 cm2 V−1 s−1 with Ion/Ioffs of 103 (Figure 6b,e), respectively, for OTFTs where the coating flow is parallel to the source-to-drain electrode direction. However, the device performance becomes lower with an average saturation mobility of

2.5. Polymer Film Morphologies and Correlations to OTFT Performance To build the correlations between film morphology and charge carrier mobility in OTFTs, atomic force microscopy (AFM) was used to characterize the surface morphology of PBTfOR films. The polymer films were prepared by on-center or off-center spin-coating on glass substrates under the same conditions for the optimized OTFT device fabrication. The topography and phase images are shown in Figure 7. Compared to ascasted film, the 200 °C annealed film by on-center spin-casting (Figure 7b,e) displays smaller size grains with slightly reduced root-mean-square (RMS) surface roughness of 6.25 nm, indicating better organization of polymer backbone. The off-center spin-coated film annealed at 200 °C reveals even smoother film with RMS roughness of 5.11 nm as well as the formation of domains with good alignment, which can provide more efficient transport channel with reduced grain boundaries for charge carrier transport.[50,59] The film morphology variation is in a good agreement with OTFT performance. The polymer chain packing and orientation of PBTfOR films prepared by various casting methods were further characterized by 2D grazing incidence wide-angle X-ray scattering (2D GIWAXS). Figure 8 shows the 2D GIWAXS patterns of the PBTfOR films: on-center as-cast and thermally annealed at 200 °C, and off-center thermally annealed, respectively. All three films show well-ordered structures, showing interlamellar scattering up to (300) in both in-plane (IP) and out-of-plane (OOP) directions. The diffraction patterns indicate the high crystallinity of the head-to-head polymer PBTfOR. A similar d-spacing of 0.29 nm was extracted for all the films in both IP and OOP directions with lamellar (100) diffraction at 0.22 Å−1. Three PBTfOR films casted differently display the (010) peaks

Table 1. Summary of device performance characteristics of PBTfOR and PBTOR-based top-gate/bottom-contact OTFTs. Semiconductors

SC method

Flow direction/Tanneal [°C]

μh,lina) [cm2 V−1 s−1]

μh,sata) [cm2 V−1 s−1)

Vtb) [V]

Ion/Ioff

PBTfOR

On-center

As-cast

0.082 (0.067)

0.13 (0.098)

−18

103

120

0.091 (0.079)

0.11 (0.089)

−16

103

160

0.19 (0.13)

0.20 (0.13)

−15

103

200

0.18 (0.13)

0.24 (0.16)

−12

103

240

0.13 (0.11)

0.15 (0.14)

−16

103

Parallel/200

0.51 (0.43)

0.57 (0.50)

−13

103

Orthogonal/200

0.12 (0.090)

0.13 (0.11)

−12

103

10−3

10−3

−21

101

Off-center

PBTOR a)Maximum

On-center

200

6.1 ×

(5.2 ×

10−3)

7.4 ×

(5.4 ×

10−3)

mobility from at least five devices (average values shown in parentheses); b)Average threshold values.


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Figure 6. I–V characteristics of top-gate/bottom-contact OTFT devices with a,d) on-center spin-coated PBTfOR semiconductor layer; b,e) off-center spin-coated PBTfOR semiconductor layer, coating flow parallel to the source/drain direction; c,f) on-center spin-coated PBTOR semiconductor layer. The scanning gate voltage in output plots (a, b, c) is 0 to −80 V with −10 V interval.

Figure 7. a–c) Tapping mode atomic force microscopy topography and d–f) phase images (5 × 5 µm) of polymer PBTfOR: a,d) pristine film on glass; b,e) film on glass annealed at 200 °C; c,f) film from off-center spin-coating on glass annealed at 200 °C.


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Figure 8. 2D-GIWAXS patterns of PBTfOR films: a) on-center spin-coated film without thermal annealing, b) on-center spin-coated film with thermal annealing at 200 °C, c) off-center spin-coated film with thermal annealing at 200 °C, d) in-plane (IP) and out-of-plane (OOP) line-cut profiles, and e) IP and OOP line-cut profiles of off-center spin-coated, thermally annealed film with respect to the radial direction parallel and perpendicular to the X-ray direction.

in both OOP and IP directions with d-spacing of 3.6 and 3.9 Å, respectively, suggesting a bimodal packing structure with mixed edge-on and face-on orientations, which is beneficial to 3D charge carrier transport.[60] A close π–π stacking distance of 3.6 Å was measured in the OOP direction. By using the Scherrer equation,[61] the crystal coherence length (CCL) was also deduced from the full width at half maximum (FWHM) of the IP and OOP lamellar (100) peaks and the results are summarized in Table S3 (Supporting Information). The thermally annealed on-center spin-coated film shows a slightly increased CCL values of 277 and 124 Å from the IP and OOP (100) peaks, respectively, compared to as-cast one with CCL values of 270 and 104 Å from the IP and OOP (100) peaks, respectively. Interestingly, the off-center spin-coated film (thermal annealed) exhibits a remarkably improved CCL (231 Å) in the OOP direction, which indicates a further enhanced chain ordering in the edge-on fashion, benefitting the carrier transport in the OTFT architecture. Next, we have measured anisotropy in the chain alignment at the center of off-center spin-coated films by recording the GIWAXS signals two times with 0° and 90° rotation of the film (Figure 8e). For two measurements with 0° and 90° rotation, the off-center spin-coated films show different


intensity in the OOP lamellar peaks but no difference in the IP peaks, confirming an anisotropic ordering in the in-plane chain alignment in the off-center spin-coated films, which is consistent with charge carrier mobility variation.

3. Conclusion In summary, a novel head-to-head bithiophene building block, 4,4′-difluoro-3,3′-dialkoxy-2,2′-bithiophene, was designed and synthesized. In comparison to various alkoxy-functionalized bithiophenes, the incorporation of F atoms into BTOR enables the new building block BTfOR as a “weaker donor” with optimized physicochemical properties. Moreover, unlike the bithiophene functionalized with strong electron-withdrawing groups, the BTfOR can be readily stannylated and the resulting tin monomer shows high purity and excellent reactivity toward the following Stille polymerization, making it possible to develop various D–A type copolymers for opto electronic device applications. For understanding fundamental properties of BTfOR unit, the BTfOR-based homopolymer PBTfOR was prepared, which displays a −0.44 eV lower-lying

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HOMO compared to that of nonfluorinated polymer counterpart, showing distinctive advantages of incorporating F atoms. The F atoms can also promote intrachain noncovalent F⋯S or F⋯H interactions, which improve polymer chain planarity and interchain packing. In OTFTs, PBTfOR shows a remarkably high μh of 0.57 cm2 V−1 s−1, considering head-to-head polythiophene configuration, while the nonfluorinated BTORbased polymer shows a inferior μh of 7.4 × 10−3 cm2 V−1 s−1 with two order smaller current on/off ratios. Our study offers a promising materials design strategy based on head-to-head linkage bithiophene with suppressed HOMOs and high crystalline interchain ordering for high-performance organic semiconductors.

4. Experimental Section DFT Calculation: Theoretical calculations in this work were performed using DFT at B3LYP/6-31G (d, p) level. The side chains were replaced by methyl groups for simplifying calculation. Single-Crystal Analysis: Single-crystal X-ray diffraction data were collected at 178 K on a Bruker D8 Venture diffractometer with CuKα radiation (λ = 1.54178 Å). The Apex3 program package was used for cell refinement and data reduction. The structure was solved by intrinsic phasing method using SHELXT and refined by full-matrix least-squares on |F2| algorithm (SHELXL)4 using Olex2 program. Device Fabrication and Characterization: The source and drain electrodes (3 nm Cr and 30 nm Au) were prepatterned on borosilicate glass prior to device fabrication using standard photolithography process. For device fabrication, the patterned glass substrates were cleaned by sonication in acetone and isopropanol for 10 min each, followed by UV–ozone treatment for 30 min. The cleaned substrates were then transferred into N2-filled glove box (O2, H2O concentration