Drastic Effects of Fluorination on Backbone Conformation of Head-to

Apr 11, 2018 - This study shows that the backbone conformation of head-to-head type 3,3′-dialkyl-2,2′-bithiophene can be tuned via fluorination of...
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Letter Cite This: ACS Macro Lett. 2018, 7, 519−524

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Drastic Effects of Fluorination on Backbone Conformation of Headto-Head Bithiophene-Based Polymer Semiconductors Qiaogan Liao,†,‡ Yulun Wang,†,‡ Mohammad Afsar Uddin,§ Jianhua Chen,† Han Guo,† Shengbin Shi,† Yang Wang,† Han Young Woo,*,§ and Xugang Guo*,† †

Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed Organic Electronics, South University of Science and Technology of China (SUSTC), No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China § Department of Chemistry, Korea University, Seoul 136-713, South Korea S Supporting Information *

ABSTRACT: This study shows that the backbone conformation of head-to-head type 3,3′-dialkyl-2,2′-bithiophene can be tuned via fluorination of the neighboring benzothiadiazole (BTz). Without fluorination, the polymer backbone is highly twisted, whereas difluorination of BTz produced a coplanar backbone. Monofluorination of BTz yielded a tunable polymer backbone conformation depending on the film annealing temperature. In organic thin-film transistors, the polymer with the head-to-head linkages showed a remarkable hole mobility of >0.5 cm2 V−1 s−1 upon attaining a planar backbone. Thus, the head-to-head linkage does not necessarily lead to backbone nonplanarity, and achieving planar conformation of 3,3′-dialkyl-2,2′bithiophene has profound implications in materials design for organic semiconducting devices, yielding good solubility, reduced materials synthetic steps, and improved opto-electrical properties.

T

thiophene derivatives) should afford BTR-based polymers with a reduced thiophene number15−18 and, hence, a lower-lying highest occupied molecular orbital (HOMO) level, which should enhance open-circuit voltage (V oc ) and power conversion efficiency (PCE) in OSCs as well as air-stability of OTFTs.17,18 It was found that backbone conformation strongly depends on both intramolecular and intermolecular noncovalent Coulombic interactions.10,19 Hence, it is expected that such interactions may overcome the thermodynamic torsional barrier to enforce BTR-based polymers to adopt a planar conformation. Herein, using a series of synthesized benzothiadiazole (BTz)-BTR copolymers (Figure 1a), we demonstrate that fluorinating BTz moiety results in a tunable conformation of the neighboring head-to-head bithiophene. Hence, the BTR-based polymers show distinct opto-electrical properties, film morphologies, and device performance in OTFTs. The results indicate that head-to-head bithiophene does not necessarily have to be nonplanar, and this study offers a new approach for conformation control in organic semiconductors. Four BTR-based polymers, BTz-BTC12, fBTz-BTC12, ffBTz-BTC12, and ffBTz-BTCEH (Figure 1a), in which the BTz was fluorinated at different degrees (0 to 2F atoms) and bithiophene contains various side chains, were prepared under Stille coupling-based polycondensation (SI). All polymers

he backbone conformation of organic and polymer semiconductors plays a critical role on their opto-electrical properties and film morphologies and, therefore, exerts a profound impact on the performance of organic thin-film transistors (OTFTs) and organic solar cells (OSCs).1,2 Due to the steric repulsion between the alkyl chains on the 3 and 3′ positions, head-to-head linkage-containing 3,3′-dialkyl-2,2′bithiophene (BTR) is avoided in the design of organic semiconductors, as this linkage leads to a twisted backbone, amorphous film morphology, and limited charge transport.1,3−5 In order to achieve a planar backbone and promote threedimensional interchain packing, several material design strategies have been developed, including controlling regioregularity,5 adding unsubstituted π-spacers,1,6,7 and incorporating conformation locks via both covalent bonds8,9 and noncovalent Coulombic interactions.10−13 These strategies have enabled the development of a great number of organic semiconductors that display promising device performance. However, these semiconductors suffer from some distinctive drawbacks, such as reduced solubility, low crystallinity, and suboptimal energy levels of the frontier molecular orbitals (FMOs).11,14 If a planar backbone conformation can be achieved for bithiophene with the head-to-head linkage, the incorporation of this unit should lead to semiconductors with distinctive advantages for applications in organic electronics. The alkyl chains on thiophene should afford good solution processability. Compared to polymers based on the π-spacer insertion strategy, the elimination of π-spacers (typically electron-rich © XXXX American Chemical Society

Received: January 12, 2018 Accepted: April 6, 2018

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DOI: 10.1021/acsmacrolett.8b00032 ACS Macro Lett. 2018, 7, 519−524

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ACS Macro Letters

elevated temperature, we designed and synthesized a small molecule model compound, s-ffBTz-BTC6 (SI), for such study. The spectrum (Figure S13) shows two distinct NOE signals, which indicate that the noncovalent interaction is likely to attributed to the F···H attraction. On the basis of absorption onsets of as-cast films (Figure 1b), the optical bandgaps (Egopt) of BTz-BTC12, fBTz-BTC12, ffBTz-BTC12, and ffBTz-BTCEH are 1.98, 1.63, 1.60, and 2.07 eV, respectively. The smallest bandgap of ffBTz-BTC12 clearly indicates that difluorination can greatly enhance the conjugation of head-to-head bithiophene-based polymers, resulting in a planar backbone and narrow bandgap. In order to achieve good solubility and attain a planar backbone simultaneously, tetrathiophene, with an unsubstituted bithiophene π-spacer, was copolymerized with benzothiadiazole to afford polymer PffBT4T-2OD,2 which showed strong aggregation with a planar backbone and remarkable photovoltaic performance in OSC devices. Comparison of the absorption of ffBTz-BTC12 and PffBT4T-2OD (Figure S16) shows a pronounced red-shift of the absorption profile for ffBTz-BTC12 film with a smaller Egopt. This observation clearly indicates that the head-to-head bithiophene-containing polymer ffBTz-BTC12 has a higher degree of conjugation than the well-known PffBT4T-2OD and, hence, should feature comparable backbone planarity, if not higher. The temperature-dependent absorption spectra of polymer solutions (Figure 2) show that increasing the temperature from

Figure 1. (a) Chemical structures of 3,3′-dialkyl-2,2′-bithiophene (BTR)-based polymers with different F numbers on benzothiadiazole and various side chains on bithiophene; (b) Polymer UV−vis absorption spectra in solution (1 × 10−5 M in o-DCB) and film state; (c) Polymer DSC thermograms.

exhibited good solubility in common organic solvents at elevated temperatures and had number-average molecular weights (Mns) in the range of 16−29 kDa (Table S1). It is informative to compare the polymer solubility. Among them, nonfluorinated BTz-BTC12 and the branched 2-ethylhexyl containing ffBTz-BTCEH were readily soluble, with a solubility >30 mg mL−1 in o-dichlorobenzene (o-DCB) at room temperature. Their excellent solubility is attributed to their twisted polymer backbone (vide infra) induced by the head-tohead linkage. Compared to BTz-BTC12, monofluorinated fBTz-BTC12 resulted in greatly reduced solubility of about 0.1 mg mL−1 and difluorinated ffBTz-BTC12 was almost insoluble at room temperature. The reduced solubility of fBTzBTC12 and ffBTz-BTC12 is attributed to the increased polymer backbone planarity and strong aggregation of polymer chains. Such backbone planarity and reduced solubility in the head-to-head bithiophene-based polymers is unexpected, which should offer new insights for the materials design. The absorption spectra of the four polymers in solution and in film state are shown in Figure 1b. Except difluorinated ffBTzBTC12, all other polymers had similar absorption profiles with a comparable absorption maximum (λmax) in the range of 510− 528 nm and lacking of structured absorption features, indicative of their twisted backbone in solution.2,20 In a film state, the absorption of all polymers showed a small bathochromic shift relative to solution state, which indicates that they adopted a similar backbone conformation in both states. However, compared to other polymers, ffBTz-BTC12 exhibited a structured absorption with a λmax at 705 nm accompanied by a distinctive shoulder, indicative of strong polymer aggregation with ordered structures in solution.21 This suggests that the bithiophene can attain a planar backbone conformation with strong interchain packing by the fluorination of the neighboring BTz. The underlying mechanism is not completely clear at the present stage, which is likely attributed to the intramolecular noncovalent H···F and S···F interaction and increased intermolecular Coulombic attraction in fluorinated polymers.4,22,23 Such interactions may overcome the thermodynamic barrier to enforce polymers to adopt a planar chain conformation. In order to attain the clue of the nonbonding interaction, the 19F−1H nuclear overhauser effect (NOE) spectrum was collected. Due to the strong polymer aggregation at room temperature and the twisted backbone conformation at

Figure 2. UV−vis absorption spectra of the BTR-based polymer solutions (1 × 10−5 M in o-DCB) as a function of solution temperature: (a) BTz-BTC12; (b) fBTz-BTC12; (c) ffBTz-BTC12; (d) ffBTz-BTCEH.

30 to 100 °C induced a minor blue-shift (435 °C (Figure S18 and Table S1). It was found that thermal annealing produced distinct effects on the absorption of the polymer films. After annealing at 250 °C for 15 min, the nonfluorinated BTz-BTC12 and difluorinated ffBTz-BTCEH containing the branched 2-ethylhexyl chain showed nearly identical absorption profiles (Figure 3), which

Figure 3. UV−vis absorption spectra of as-cast and thermally annealed films (250 °C for 15 min): (a) BTz-BTC12; (b) fBTz-BTC12; (c) ffBTz-BTC12; (d) BTz-BTCEH. Inset shows the colors of polymer films. Among them, fBTz-BTC12 shows a drastic shift of λmax, accompanied by a distinctive color change after thermal annealing.

Figure 4. 2D-GIWAXS images of the as-cast and thermally annealed films of the BTR-based polymers at 250 °C.

°C. According to the GIWAXS data, both as-cast BTz-BTC12 and ffBTz-BTCEH films show an amorphous morphology and thermal treatments did not improve polymer chain ordering. However, monofluorinated fBTz-BTC12 exhibits a drastic change in the film morphology. The as-cast film shows a weak hump shape (100) scattering at q = 0.28 Å−1, suggesting a low film crystallinity with a randomly oriented nanostructure. Upon annealing, well resolved OOP lamellar scattering peaks progressing up to (300) were observed together with a strong IP (010) scattering at q = 1.65 Å−1 (d-spacing of 0.38 nm), suggesting a strong edge-on orientation with greatly increased film crystallinity.24,25 In contrast, the difluorinated ffBTzBTC12 film shows remarkable crystalline morphology in the as-cast film with up to (300) lamellar diffraction peaks in the OOP direction. The thermal annealed film shows a minimal change in the scattering patterns compared to that before thermal treatment. These GIWAXS data show a very good agreement with the temperature-dependent UV−vis absorption and measured OTFT characteristics of fBTz-BTC12 and ffBTzBTC12, which will be discussed in the following section. We

indicates that thermal annealing could not improve the interchain packing due to the highly amorphous characteristics of these two species and the lack of driving force to improve the backbone planarity and film crystallinity. It is interesting to compare the absorption spectra (Figure 3c,d) of ffBTz-BTCEH and ffBTz-BTC12 containing different chains on the 3 and 3′ positions of bithiophene; ffBTz-BTC12 with the linear ndodecyl chain shows a substantially red-shifted absorption, smaller bandgap, and structured absorption profile. This reveals that the steric hindrance generated by the linear n-dodecyl could be overcome, resulting in a planar backbone for ffBTzBTC12, whereas the slightly higher steric hindrance engendered by the branched 2-ethylhexyl leads to a twisted backbone and significantly different absorption for ffBTz-BTCEH. Among the polymers, thermal annealing resulted in the most drastic effects on the absorption profile of monofluorinated fBTz-BTC12 (Figure 3b and Figure S17). The λmax of this polymer was bathochromically shifted by ∼130 nm, accompanied by the development of a distinctive absorption shoulder 521

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ACS Macro Letters further studied the temperature-dependent GIWAXS characteristics of fBTz-BTC12 film with stepwise heating from 25 to 320 °C (Figure 5). Significant crystalline phase transition was

Figure 6. OTFT transfer characteristics of (a) monofluorinated fBTzBTC12 and (c) difluorinated ffBTz-BTC12 fabricated under the optimal condition; (b, d) evolution of hole mobility as a function of thermal annealing temperature; insets show the schematic, illustrating the change of backbone conformation, and interchain packing upon thermal treatment.

Figure 5. Temperature-dependent GIWAXS profiles of monofluorinated fBTz-BTC12: (a) In-plane scattering images and (b, c) its corresponding 1D line-cut profile, (d) out-of-plane scattering images, and (e) its corresponding 1D line-cut profile.

observed at 200−250 °C and the IP (Figure 5a) and OOP (Figure 5d) scattering peaks show the gradually reduced full width at half-maximum (fwhm) with increasing the annealing temperature, which indicates the increased long-range ordering of fBTz-BTC12.26,27 Top-gate/bottom-contact OTFTs were fabricated to investigate the charge transport properties of these BTR-based polymer semiconductors. Among them, nonfluorinated BTzBTC12 and difluorinated ffBTz-BTCEH containing branched 2-ethylhexyl chain were inactive in OTFTs under various device fabrication conditions, which is attributed to their twisted backbone and amorphous film morphology. For the monofluorinated fBTz-BTC12, the as-cast OTFTs exhibited a mediocre hole mobility (μh) of 0.004 cm2 V−1 s−1 (Figure S19 and Table S3). However, thermal annealing yielded remarkable effects on μh and Figure 6b illustrates the mobility evolution of fBTz-BTC12 as a function of the annealing temperature. After annealing at the temperature >200 °C, fBTz-BTC12 exhibited a markedly improved mobility. A μh > 0.1 cm2 V−1 s−1 was consistently attainable with the highest >0.5 cm2 V−1 s−1 (Table S3), which is attributed to its greatly increased backbone planarity and improved film crystallinity upon thermal annealing, as revealed by the UV−vis absorption (Figure S17) and GIWAXS data (Figure 5). For the difluorinated ffBTz-BTC12, the μh was much less sensitive to the annealing temperature (Figure 6d), where the μh ranged from 0.15 to 0.18 cm2 V−1 s−1 as the temperature was increased from 25 to 300 °C, which is consistent with the evolution of the film morphology. Compared to that of monofluorinated fBTzBTC12, the reduced μh of difluorinated ffBTz-BTC12 is likely attributed to the increased hole injection barrier due to its lower-lying HOMO,28 which also results in a larger threshold voltage (VT = −47 V) for the ffBTz-BTC12-based OTFTs. In addition, the strong aggregation of the difluorinated ffBTzBTC12 in solution may lead to polymer film with kinetically trapped structure, which could result in reduced charge carrier mobility in comparison to that of monofluorinated fBTzBTC12.

In conclusion, it was found that fluorination of the benzothiadiazole moiety can effectively tune the backbone conformation of the neighboring 3,3′-dialkyl-2,2′-bithiophene. Without fluorination, BTz-BTC12 adopts a twisted backbone and shows an amorphous morphology. Monofluorination leads to fBTz-BTC12 with a backbone conformation sensitive to the processing conditions. The twisted backbone is kinetically trapped in as-cast films and thermal annealing can result in the thermodynamically favored planar backbone. Therefore, the fBTz-BTC12 crystallinity is greatly improved after thermal annealing, resulting in markedly improved mobility. The difluorinated ffBTz-BTC12 features a planar backbone, narrow bandgap, and high crystallinity even without thermal treatment. However, the branched 2-ethylhexyl chain on BTR can also disrupt the planar conformation for the difluorinated polymer and ffBTz-BTCEH shows limited conjugation and an amorphous morphology. When incorporated into OTFTs, the polymer fBTz-BTC12 can attain an encouraging hole mobility >0.5 cm2 V−1 s−1. The results indicate that head-to-head bithiophene does not necessarily have to be twisted, and achieving planar conformation of BTR has profound implications in the materials design for organic semiconducting devices, yielding good solubility, reduced synthetic steps, improved opto-electrical properties. In addition, this study offers a new and effective approach to control backbone conformation, which is critical for developing high-performance organic semiconductors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00032. Experimental details, synthesis and characterization of monomers and polymers, optical spectra, details of OTFT fabrication, GIWAXS measurements, CV and TGA plots, DFT computational results, and OTFT performance data (PDF). 522

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Han Young Woo: 0000-0001-5650-7482 Xugang Guo: 0000-0001-6193-637X Author Contributions ‡

These authors contributed equally to this work (Q.L. and Y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.G. thanks NSFC (21774055 and 51573076), Shenzhen Peacock Plan (KQTD20140630110339343), Shenzhen Basic Research Fund (JCYJ20160530185244662), Shenzhen Key Lab funding (ZDSYS201505291525382), the Guangdong Natural Science Foundation (2015A030313900), and the South University of Science and Technology of China (FRGSUSTC1501A-72). M.A.U. and H.Y.W. are grateful to the financial support from the NRF of Korea (2016M1A2A2940911 and 20100020209).

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