New Donor−Donor Type Copolymers with Rigid and Coplanar Structures for High-Mobility Organic Field-Effect Transistors Soo-Young Jang, † In-Bok Kim, ‡ Jihong Kim, † Dongyoon Khim, † Eunhwan Jung, † Boseok Kang, ∥ Bogyu Lim, †,§ Yeong-A Kim, † Yun Hee Jang, † Kilwon Cho, ∥ and Dong-Yu Kim* ,† † School of Materials Science and Engineering and ‡ Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu, Gwangju, 500-712, Republic of Korea ∥ Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea * S Supporting Information R ecent advances in solution-processable conjugated poly- mers have been promoted in the field of organic field- effect transistors (OFETs) due to their applicability to flexible devices through low-cost manufacturing processes such as the printing technique. 1 Various types of semiconductors are being introduced as promising candidates for high-performance OFETs that exceed a field-effect mobility (μ FET ) of 1 cm 2 / (V·s) by tailoring their structural characteristics. 2,3 Most of these polymers require a high-level planarity in their conjugated backbones in an effort to achieve high charge-carrier mobility, 4 and such planarity is commonly accomplished by the acquisition of two main structural features. First, the monomeric units should have rigid and flat structures to inhibit bond rotation. 5−9 In addition, avoiding steric hindrance between monomeric units is also important for maintaining the planarity of backbones. 10−12 Recently, many researchers have demonstrated research results concerning high-mobility con- jugated polymers with planar backbones and found it obvious that the restriction of chemical bond torsion is indeed important in obtaining excellent charge-transport properties. 4,13 The second important structural feature for high-mobility conjugated polymers could be the donor−acceptor (D−A) configuration. Intrinsically different polarities of electron rich and electron deficient units allow this type of copolymers to have strong intermolecular interactions. 2−4,14 Some of the more popular types of units that are composed of the D−A conformation include 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) 2 and isoindigo, 12,15 which have exhibited an excellent μ FET of more than 1−10 cm 2 /(V·s). On the other hand, there also are conjugated polymers that are comprised of electron-donating units only, which also have potential for use in high-mobility OFETs. 6,16 In this paper, we classify these polymers as donor− donor (D−D) types, as opposed to the D−A types. These polymers are usually composed of a fused aromatic structure and are designed to have long-range intermolecular side chain interdigitations, leading to the formation of a three-dimensional lamellar π-stacking structure. By avoiding D−A type which is already a widely used conformational trend in conjugated polymers, we can further enlarge the diversity of monomeric combinations. However, most of the D−D-type polymers have shown a relatively lower degree of mobility compared with the D−A types because of the inevitable angular torsion 6 or a lack of structural stiffness 16 that diminishes the overall planarity in a conjugated system, resulting in a relatively lower μ FET (10 −5 ∼ 1 cm 2 /(V·s)) than D−A type polymers. To obtain high-mobility semiconductors for OFETs, further comprehensive analysis of these types of polymers should be conducted, including structural modifications by way of reinforcing the planarity of the conjugated backbones. This paper introduces high-mobility, all-thiophene-based D− D types of conjugated copolymers, poly[(E)-1,2-(3,3′-ditetra- decyl-2,2 ′ -dithienyl)ethylene- alt -dithieno(3,2- b :2 ′ ,3 ′ - d )- thiophene] (P14) and poly[(E)-1,2-(3,3′-dioctadecyl-2,2′- dithienyl)ethylene- alt -dithieno-(3,2- b :2 ′ ,3 ′ - d )thiophene] (P18), with thienylenevinylene (TV) and dithienothiophene (DTT) units. The vinyl group in the TV unit prevents angular torsion between two thiophenes resulting in coplanar structures in conjugated systems. 8 Also three fused thiophenes of the DTT unit lead to elevated structural rigidity in polymer backbones due to lack of structural distortion. 5 Indeed, both TV and DTT units are promising moieties in high-performance electronic materials because their rigid and coplanar structures contribute to well-extended and highly packed conjugated main chains which result in efficient charge transport. 5,8 Before this report, however, the combination of these two units have never been demonstrated. The outstanding electronic properties of the copolymers make them promising materials by maintaining a high level of planarity in their conjugated systems. By demonstrating a maximum μ FET of 3.91 cm 2 /(V·s), the copolymers suggest the potential of D−D type of polymers for use in high-performance OFETs even though they are comprised of electron-donating units only. As shown in Figure 1a, P14 and P18 were successfully synthesized through Stille polycondensation using distannyl functionalized TV (M1) and dibrominated DTT (M2), 17 and the synthetic details are described in the Supporting Information (S2). The thermogravimetry analysis of P14 and P18, shown in Supporting Information Figure S8, revealed their superb thermal stability at decomposition temperatures of 400 and 380 °C, respectively. Also, differential scanning calorimetry measurement demonstrated their thermal transitions in a low- temperature region (<50 °C) due to the melting and cooling behaviors of the linear side chains incorporated at the thienylenevinylene units (n-tetradecyl for P14 and n-octadecyl for P18). Received: July 8, 2014 Revised: November 27, 2014 Communication pubs.acs.org/cm © XXXX American Chemical Society A dx.doi.org/10.1021/cm502486n | Chem. Mater. XXXX, XXX, XXX−XXX