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Zhengran Yi, Shuai Wang,* and Yunqi Liu* To date, optimization of the molecular structure has become one of the mosteffective means to achieve excellent device performance. Thus, significant efforts have been invested in structure–property relationships for high-mobility polymer semiconductors. Many studies have proven that the molecular structures of both the polymer backbone and the side chains have great influence on the molecular packing, molecular weight, solubility, and microstructure of the corresponding polymers, finally affecting the OTFT performance. First, the backbone structure can affect the co-planarity, solubility, molecular weight, energy levels, and molecular packing of the corresponding polymers, for example.[14,23,24] Hence, some approaches have been adopted to develop new π-conjugated backbones with high planarity, close π–π stacking, and strong intermolecular interaction. Additionally, alkyl side chains are usually introduced into polymers to afford high molecular weight and satisfactory solubility for device fabrication. Subsequently, side-chain structures have been found to affect the morphology and molecule packing of the corresponding polymers.[25] Therefore, the selection of suitable alkyl side chains to use in π-conjugated systems is also crucial to the development of high-performance OTFT polymer semiconductors. In this Progress Report, we will summarize the synthetic strategy for high-mobility DPP-based polymers in respect of tuning the backbone and side chains. The relationships between the chemical structure, photophysical properties, molecular packing, OTFTs characteristics, etc., are also explored and discussed.

Since the report of the first diketopyrrolopyrrole (DPP)-based polymer semiconductor, such polymers have received considerable attention as a promising candidate for high-performance polymer semiconductors in organic thin-film transistors (OTFTs). This Progress Report summarizes the advances in the molecular design of high-mobility DPP-based polymers reported in the last few years, especially focusing on the molecular design of these polymers in respect of tuning the backbone and side chains, and discussing the influences of structural modification of the backbone and side chains on the properties and device performance of corresponding DPP-based polymers. This provides insights for the development of new and high-mobility polymer semiconductors.

1. Introduction Since the fabrication of the first organic thin-film transistors (OTFTs), π-conjugated polymer semiconductors for OTFTs have been widely studied due to their easily modified structure, solution processability, and inexpensive device fabrication, such as printing techniques.[1–5] Over the past years, great achievements in polymer-based OTFTs have been obtained through molecular design and device optimization.[6–10] For example, some polymers can exhibit high mobilities exceeding 1.0 cm2 V−1 s−1 and compete with the performance of amorphous silicon.[11–18] Amongst these polymers, diketopyrrolopyrrole (DPP)-based copolymers have gained much attention due to their promising application in OTFTs. Since the first DPP-based polymer semiconductor for OTFTs was reported in 2008,[19] these polymers have been reported to exhibit one of the highest mobilities observed, due to the remarkable aggregating properties of the DPP moieties.[20] The strong electron deficiency and good planarity of the DPP unit endow the polymer with strong intermolecular π–π interactions of its main chains and thus facilitate charge transport.[21,22]

Dr. Z. R. Yi, Prof. S. Wang College of Chemistry and Chemical Engineering Huazhong University of Science and Technology Wuhan 430074, PR China E-mail: [email protected] Prof. Y. Q. Liu Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, PR China E-mail: [email protected]

DOI: 10.1002/adma.201500401

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Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors

2. Tuning of the Conjugated Backbone of DPP-Based Polymers The DPP unit can be readily obtained by the reaction of an aryl carbonitrile (Ar-CN) and a succinic acid ester, with two aromatic substituents at the 3 and 6 positions of the DPP (Figure 1).[26] Therefore, a DPP core is generally flanked by two aromatic groups to form different electron-accepting building blocks. These aromatic groups comprise five-membered aromatic rings, six-membered aromatic rings, and fused heterocyclic rings (Ar1, Figure 1), which could effectively influence the backbone co-planarity, energy levels, crystallinity, and π–π stacking distance of polymers. All the reported high-mobility DPP-based polymers have been obtained by incorporation of these building blocks and other aromatic groups (Ar2, Figure 1). Herein, we classify high-performance DPP-based polymers by different

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electron-accepting building blocks and highlight the influences of the introduction of Ar1 and Ar2 into the polymer backbone on the properties and OTFT performance of the corresponding polymers.

2.1. DPP Flanked by Five-Membered Aromatic Ring 2.1.1. Dithienyl-DPP Five-membered rings including thiophene, furan, and selenophene have minimal steric effects on the DPP core and thus render these DPP-based building blocks highly co-planar. Dithienyl-DPP has been widely used for OTFT polymers (Figure 2). Crystal-structure analysis of dithienyl-DPP-based small molecules revealed H-bonding between the oxygen atoms of the DPP core and the β-hydrogen atoms of the neighboring thiophene rings.[27] The formation of hydrogen bonds can strengthen the co-planarity of the main chain, resulting in strongly intermolecular π–π stacking. Due to the excellent charge-transport properties and highly ordered packing of many thiophene-based polymers,[28,29] the introduction of a β-unsubstituted oligothiophene unit (such as 2T−7T) as the electron-donating unit into DPP-based polymers is a feasible way to acquire high-performance polymer semiconductors; namely, PDPPnT. In 2010, Janssen and co-workers synthesized PDPP2T (P1), exhibiting ambipolarity with the electron and hole mobilities both in the 10−4 cm2 V−1 s−1 range.[30] To induce additional planarity and enhance packing of P1, Janssen’s group first introduced an unsubstituted terthiophene unit into the DPP-based polymer to afford PDPP3T (P2a), which exhibited a higher hole mobility of 0.05 cm2 V−1 s−1 and electron mobility of 0.01 cm2 V−1 s−1, in comparison with that of PDPP2T.[31] In 2012, Lee et al. reported PDPP3T, with a longer alkyl chain on the DPP unit (P2b).[32] The authors found that introducing the longer alkyl chain contributed to the formation of more crystalline structures and enhanced charge transport. The hole and electron mobilities of P2b reach 1.57 cm2 V−1 s−1 and 0.18 cm2 V−1 s−1, respectively, which is the best device performance reported to date for PDPP3T. Li’s group reported DPP-quater-thiophene copolymer (PDPP4T, P3).[33] It was found that the incorporation of 4T into DPPbased polymers was favorable for self-assembly into highly ordered crystalline structures with close π–π stacking (3.75 Å), and thereby intermolecular charge hopping. Thus, P3 exhibited

Figure 1. A typical synthetic route for DPP compounds and a general formula for DPP-based polymers.

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Zhengran Yi received his B.S. degree in chemistry from Hubei University in 2004, and a Ph.D. degree in organic chemistry from Wuhan University in 2012. He is currently a postdoctoral researcher at Huazhong University of Science and Technology. His research focuses on the design and synthesis of polymer semiconductors for organic field-effect transistors. Shuai Wang received his B.S. and M.S. degrees in chemistry from Zhengzhou University, China, in 1997 and 2000, and a Ph.D. degree in chemistry from the Institute of Chemistry, CAS, China. He is currently a professor at the School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, China. His research interests include polymer semiconductors and devices. Yunqi Liu graduated in 1975 from the Department of Chemistry, Nanjing University, and received a doctorate from the Tokyo Institute of Technology, Japan, in 1991. He is currently a professor at the Institute of Chemistry, CAS, China. His research interests include molecular materials and devices.

high mobility of 0.89 cm2 V−1 s−1 even without thermal annealing. Subsequently, Yi et al. designed and synthesized an alternating copolymer PDPP5T containing DPP and a quintetthiophene unit (P4).[11] The authors demonstrated that the introduction of more β-unsubstituted thiophene groups into the main-chain of PDPP5T polymer not only makes the p-type behavior of the polymer more pronounced, but also slightly reduces the steric hindrance of the bulk side-chain groups in the DPP unit, which favors the construction of a well-ordered lamellar structure of π–π stacking after solution deposition and strengthening the molecular ordering capability of the polymer at low temperature; thus, it exhibited a very high holemobility of 1.08 cm2 V−1 s−1 with an on/off ratio of 105 without thermal annealing (Figure 3). In addition, XRD patterns and

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PROGRESS REPORT Figure 2. Chemical structures of representative dithienyl-DPP-based polymers (C6: n-hexyl; C8: n-octyl; C12: n-dodecyl; C6C10: 2-hexyldecyl; C8C12: 2-octyldodecyl; C10C14: 2-decyltetradecyl; C12C16: 2-dodecylhexadecyl).

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AFM images indicated that the annealing process at high temperature reduces the roughness of the thin film and increases the crystallinity of P4 chains (Figure 4). Thus, its hole mobility reaches 3.46 cm2 V−1 s−1, annealed directly in air (Figure 3), which is one of the highest results of reported OTFTs based on

DPP-oligothiophene copolymers. To research the influences of the longer β-unsubstituted oligothiophene unit (6T and 7T) on the molecular weight, film-forming fabrication, polymeric crystallization, intermolecular ordering capability and the OTFT performance of the corresponding copolymers, Yi et al. also

Figure 4. A) X-ray diffraction (XRD) patterns and B) AFM height images of P4 thin films without annealing (a) and annealed at 80 (b), 120 (c), 160 (d), 200 °C (e), respectively. Adapted with permission.[11] Copyright 2012, American Chemical Society.

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synthesized and characterized PDPP6T and PDPP7T (P5 and P6).[14] Remarkably high mobilities of 3.94 cm2 V−1 s−1 and 2.82 cm2 V−1 s−1 were obtained for PDPP6T and PDPP7T, respectively. Grazing-incidence X-ray diffraction (GIXRD) analysis revealed that the introduction of the longer β-unsubstituted oligothiophene unit favors the construction of a well-ordered lamellar structure of π–π stacking with edge-on orientations (Figure 5). However, the strengthening of the intrachain conjugation rigidity results in low molecular weight and poor solubility of the polymer, which is unfavorable for good OTFT performance. Therefore, the authors suggested that there is a particular number of thiophene groups in β-unsubstituted oligothiophene required for PDPPnT copolymers to exhibit the highest hole mobility and best OTFT performance; this also reveals the significance of regulating the balance between π–π stacking of intermolecular chains and molecular weight, as well as the solubility of the rigid main chain in the design of D–A copolymers for OTFTs. In order to extend co-planarity and promote intermolecular π–π stacking of DPP-based polymers, Liu, Yu, and coworkers combined highly π-extended (E)-2-(2-(thiophen-2-yl) vinyl)thiophene (TVT) units with the dithienyl-DPP unit in the main chain to yield polymers P7a (with short 2-octyldodecyl side chains) and P7b (with long 2-decyltetradecyl side chains).[12] Both polymer films exhibited very close stacking with π–π stacking distances of 3.72 Å for P7a and 3.66 Å for P7b, which are smaller than that of P3. The result indicated

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that intermolecular interaction between the electron-donating TVT (and thiophene units) and electron-accepting DPP could further shorten the distances between the polymer chains, and may also constitute convincing evidence to explain the higher field-effect mobility for both P7a and P7b relative to P3. Remarkably, OTFTs based on P7b exhibited a high hole mobility up to 8.2 cm2 V−1 s−1 with a high current on/off ratio of 105–107 (Figure 6), which was the highest mobility for polymer TFTs when the paper was published. On the basis of PDPPTVT, Kwon and co-workers introduced one nitrile group to the vinylene moiety to obtain PDPP-CNTVT (P7c).[34] It was found that the incorporation of the strong electron-withdrawing nitrile moiety can effectively decrease a polymer's highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level, which was in accord with density function theory calculations. The low-lying LUMO of P7c (−3.92 eV) is highly suitable for electron injection with the commonly used Au electrode. Moreover, computational calculations demonstrated that the planarity of the polymer backbone did not change obviously by incorporating the nitrile group in the vinyl spacer unit, implying that P7c showed moderately efficient intermolecular-packing properties despite the introduction of the nitrile group.The charge-transport properties revealed that P7c exhibited excellent n-type charge-transport behavior, with a record-breaking high electron mobility of 7.0 cm2 V−1 s−1. Film characterization demonstrated that P7c films annealed at high temperature (310 °C) only had a single-phase formation of crystallites, which was beneficial to charge-carrier transport. This result suggests that the incorporation of only one nitrile group as an electron-withdrawing function in the vinyl linkage is an easy approach to develop high-performance n-channel semiconducting polymers for OTFTs. To investigate the influence of the ratio of amount of thiophene to vinylene on the device performance of TVT-DPP-based polymers, Noh, Yang, and co-workers designed and synthesized three DPP-TVT-x polymers (P8a, x = 0; P8b, x = 1; P8c, x = 2), where x is the number of thiophene groups between the TVT units and the DPP units in the main chain of the polymers.[35] The OTFT characteristics revealed that P8a (DPP-TVT-0) exhibited ambipolar properties with nearly balanced hole and electron mobilities of 0.37 cm2 V−1 s−1 and 0.78 cm2 V−1 s−1, respectively. As the number of thiophene spacer units increased (from 0 to 2), the polarity of the charge carriers changed from ambipolar to p-type dominant because of the introduction of the electron-rich thiophene. Despite having larger π–π stacking distance and lamellar d-spacing, P8b exhibited a higher hole mobility relative to P8c (2.96 cm2 V−1 s−1 vs 1.88 cm2 V−1 s−1), which can be attributed to the smaller paracrystalline distortion parameter and narrower plane distribution for P8b. Recently, selenophene-based D–A copolymers have attracted substantial attention due to their promising application in OTFTs.[36–38] Compared with thiophene, selenophene endows the D–A copolymer with a stronger intermolecular interaction to construct a well-ordered lamellar structure and stronger electron affinity to improve electron injection.[39,40] P9a containing selenophene and dithienyl-DPP units had relatively high hole and electron mobilities up to 1.62 and 0.14 cm2 V−1 s−1, respectively.[41] The maximum absorption peak of 849 nm and narrow optical bandgap of 1.29 eV for a thin film of P9a indicated that

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Figure 6. a) Devices structure of the polymer TFTs with a PMMA encapsulation layer. b,c) Output (b), and transfer (c) characteristics of the P7b-based FET device. d) The mobility distributions based on different channel lengths. Reproduced with permission.[12] Copyright 2012, Wiley-VCH.

a strong intramolecular and intermolecular charge transfer exists among the D–A conjugated main chain. Moreover, X-ray diffraction (XRD) and atomic force microscopy (AFM) analysis demonstrated that P9a processed from 1,2,4-trichlorobenzene exhibited a dense nanofiber morphology with lamellar chain packing. Polymer P9b attached with the hybrid siloxane-solubilizing groups instead of alkyl chains (P9a) exhibited high hole and electron mobilities of 3.97 and 2.20 cm2 V−1 s−1, respectively, which is attributed to the closer π–π stacking distance of polymer with siloxane side chains, relative to polymer with alkyl chains (3.59 Å vs 3.74 Å).[42] Further tuning of the alkyl spacer length of the hybrid side chains resulted in high hole and electron mobilities of up to 6.16 and 3.07 m2 V−1 s−1, respectively, for polymer P9c and unprecedentedly high hole and electron mobilities of up to 8.84 and 4.34 m2 V−1 s−1 for polymer P9d.[43] On the basis of P5, the replacement of bi-thiophene with bi-selenophene as in polymer P10 slightly increased hole mobility from 1.04 to 1.5 cm2 V−1 s−1.[39] The UV–vis–near IR (NIR) absorption spectra of P10 in a thin film showed a redshift of almost 40 nm relative to that of P3, and the baseline floated significantly due to optical loss. The result suggests that selenophene-containing repeating groups are highly interactive, which contributes to forming self-assembled molecular domains through strong electronic coupling. P10 has a narrower optical bandgap than P3 (1.32 vs 1.39), due to the more-electron-rich nature of the selenium atom and stronger intermolecular interactions. Additionally, a more-significantly crystalline behavior could be observed in P10 film in the XRD patterns versus P3. As a result, despite having a relatively low

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Mn, P10 containing selenophene units had a higher mobility than P3 with thiophene units. P11, containing ter-selenophene units, was designed to extend the conjugation, resulting in a highly enhanced edge-on orientation and well-ordered lamellar structure in the annealed thin-film polymer.[44] A hole mobility as high as 5.0 cm2 V−1 s−1 was achieved for P11 OTFTs. In 2013, Kwon and co-workers incorporated selenophenylene vinylene selenophene into the backbones of dithienyl-DPP-containing polymers (P12a), exhibiting a high carrier mobility of 4.97 cm2 V−1 s−1.[40] Meanwhile, to study the influences of the replacement of thiophene with selenophene on OTFT performance, they also synthesized P7b containing thiophene units. In comparison to selenophene-containing P12a, P7b showed blueshifted absorption, indicating weaker interchain interactions in the thin film. Moreover, XRD analysis demonstrated that P7b has a slightly sparser packing versus P12a. P7b OTFTs displayed a relatively lower mobility of 2.77 cm2 V−1 s−1 relative to P12a. Therefore, replacement of the thiophene with selenophene can induce stronger intermolecular interactions, leading to denser packing and hence increased mobility. Subsequently, through tuning the side chains, higher hole mobilities of 12 cm2 V−1 s−1[16] and 17.8 cm2 V−1 s−1[45] were obtained for P12e. Unlike sulfur and selenium, tellurium is a metalloid and has the capability of forming hypervalent coordination complexes, which can promote interchain interactions of tellurophene-containing conjugated polymers.[46,47] Choi’s group first combined thiophene-flanked DPP and the tellurophene unit to afford P13.[48] The P13 exhibited a smaller optical band gap (Egopt = 1.25 eV) than thiophene-based PDPP3T (P2b,

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homologous heteroaromatic donor unit (TTTT), exhibited the highest hole mobility of up to 3.2 cm2 V−1 s−1, owing to the planar structure along the polymer backbone and a prominent edge-on orientation on the substrates. Based on BDT, a large aromatic ring (DTBDT) was incorporated into DPP-based polymer (P22) to make the polymer backbone more rigid, and thereby improve intramolecular charge transport in the polymer chain.[55] Hole mobility as high as 2.70 cm2 V−1 s−1 was achieved for P22 OTFTs, which is higher than that of BDT-DPP polymer (P18, 1.31 cm2 V−1 s−1). P23 with the NDF3 unit exhibited a high hole mobility up to 0.56 cm2 V−1 s−1, although there is no preferential orientation throughout the film.[56] Besides the above electron-donating moieties, electronaccepting moieties have also been incorporated into the backbones of DPP-base conjugated polymers to enhance the mobility, especially the electron mobility. Electron-accepting moieties have heteroatom-containing aromatic structures, including benzothiadiazole, fluorinated benzene, DPP, etc. On the one hand, intermolecular interactions can be improved through heteroatom contact to induce a high order of molecular organization, which is favorable for charge-carrier transport. On the other, the introduction of electron-accepting moieties lowers not only the HOMO energy level, which enhances oxidative stability, but also the LUMO energy level, facilitating easier electron injection from the electrode in an OTFT device. Sonar et al. combined dithienyl-DPP and 2,1,3-benzothiadiazole (BT) moieties to form P24 with the A−D−A’−D structure, having a lowlying LUMO (−4.0 eV) and a narrow bandgap (1.2 eV).[57] The film microstructure characterization indicated a highly ordered edge-on lamellar packing motif with close π–π stacking (3.73 Å) for P24. As a result, the polymer exhibited good OTFT performance with balanced hole and electron mobilities of 0.35 and 0.40 cm2 V−1 s−1, respectively. Sirringhaus and co-workers further improved the molecular weight of P24, leading to a higher electron mobility of 0.57 cm2 V−1 s−1.[58] Yuen et al. synthesized P25a and P25b consisting of diphenyl-DPP coupled with benzobisthiadiazole (BBT).[59] The strong accepting group of BBT lowers the LUMO values to −3.9 eV. Additionally, the strong heteroatom contacts (S–N) from benzobisthiadiazole favor the induction of an interchain interaction. Hence, excellent OTFT performance with high hole and electron mobilities was obtained for the two polymers. In particular, P25b has mobilities exceeding 1 cm2 V−1 s−1 for both hole and electron. Jo and co-workers reported four copolymers (P26–P29) composed of DPP and a fluorinated phenyl unit, in which varied numbers of fluorine atoms (0, 1, 2, and 4 F) were substituted on the phenyl unit.[60] It was found that the HOMO and LUMO energy levels of the polymers decreased on increasing the electron-withdrawing fluorine substitution, among which P29 containing 1,2,4,5-tetrafluorobenzene moieties had the lowest LUMO level of up to −4.18 eV. The low-lying LUMO of P29 can be attributed to the introduction of a very strong acceptor for tetrafluorobenzene, which was confirmed by dipole-moment calculations. The charge-transport properties of these polymers revealed that P26 and P27 showed clear ambipolar transport behavior with the hole and electron mobilities of 0.2–0.3 cm2 V−1 s−1, whereas P28 and P29 exhibited n-type dominant ambipolar transistor behavior. It is worth noting that P29 exhibited excellent n-type charge-transport behavior, with an electron mobility

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Egopt = 1.30 eV), due to the presence of the more-electron-rich tellurium atom. Compared with P2b, P13 exhibited a higher hole mobility (1.78 cm2 V−1 s−1 vs 0.62 cm2 V−1 s−1), which can be attributed to the better edge-on orientation of the polymer chains for P13 and the stronger intermolecular π–π interactions induced by the tellurophene units. The fused aromatic ring is another important building block for the design of high-performance polymer semiconductors. These fused ring aromatic structures have a strong tendency to form π–π stacks with a large overlapping area, which is preferable for charge-carrier transport through intermolecular hopping, and to induce a higher-order molecular organization that leads to large crystalline domains and fewer disordered domain boundaries.[29,49] In 2010, Li and co-workers reported P14 with the combination of fused ring aromatic moieties of thieno[3,2-b]thiophene (TT) and dithienyl-DPP; P14 has a narrow bandgap (1.23 eV) and a low-lying LUMO (−4.02 eV).[50] XRD analysis revealed that the thienothiophene polymer chain predominantly adopts an edge-on lamellar packing motif with a slightly smaller π–π stacking distance (3.71 Å) relative to P3 containing bithiophene (3.75 Å). The OTFTs annealed at 200 °C exhibited a high hole mobility of up to 0.94 cm2 V−1 s−1, which can be attributed to the small π–π distance and large intermolecular overlapping between the polymer backbones. Sirringhaus and co-workers found that Au source/drain electrode treatment greatly affects its work function, and hence the charge injection and measured mobility.[51] OTFTs for P14 using oxygen-plasma-cleaned Au electrodes showed high hole and electron mobilities of 0.93 and 0.29 cm2 V−1 s−1, whereas OTFTs using solvent-cleaned Au electrodes exhibited higher µh and µe of 1.36 and 1.56 cm2 V−1 s−1, respectively. Subsequently, through optimizing the reaction conditions, Ong, Liu and co-workers synthesized a highMnP14 with high carrier mobility of up to 10.5 cm2 V−1 s−1,[13] which was the first example of polymer TFTs having a mobility over 10 cm2 V−1 s−1. GIXRD analysis revealed that this highMn polymer showed a short π–π stacking distance (3.43 Å). Recently, Choi and co-workers reported a highly π-extended conjugated polymer, P15, containing di(thienothienyl)ethylene donor moieties.[52] High mobility (7.43 cm2 V−1 s−1) was achieved for P15 OTFTs fabricated by solution-shearing, due to the predominant edge-on orientation of the polymer chains on the substrates with close π−stacking (3.47 Å). Other fused rings such as naphthalene, pyrene, benzodithiophene (BDT), naphthodithiophene (NDT), dithieno[3,2-b:2′,3′-d]thiophene (DTT), thieno[3,2-b]thieno[2′,3′:4,5]-thieno[2,3-d]thiophene (TTTT), dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT), and naphtho[1,2-b;5,6-b′]difuran (NDF3) have also been used for constructing DPP-based polymers (P16–P23). For P16 with the naphthalene unit, hole mobilities of 0.65 cm2 V−1 s−1 and 0.98 cm2 V−1 s−1 were obtained in single- and dual-gate OTFT device geometries, respectively.[53] The π–π stacking distance in P16 (3.82 Å) is larger than that (3.71 Å) of P14, indicating weaker intermolecular interactions of P16. The weaker intermolecular donor–acceptor interaction is due to the lesser electron-donating nature of naphthalene than TT. Choi and co-workers reported five polymers P17–P21 containing pyrene, BDT, NDT, and TTTT units, respectively.[54] Hole mobilities of ca. 0.42–3.20 cm2 V−1 s−1 were obtained for these polymers. Among the five polymers, P21, containing a

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of 2.36 cm2 V−1 s−1. Film characterization demonstrated that the polymer chain predominantly adopts a face-on lamellar packing motif with close π–π stacking as the number of fluorine substitutions increases. P29 with four fluorine substitutions has both a face-on and an edge-on molecular orientation on the substrate, with high crystallinity and a long coherence length, which could contribute to its high electron mobility. Moreover, theoretical calculations indicated that P29 has a smaller torsional angle than fluorine-free polymer P26 does, implying that the introduction of fluorine may facilitate the formation of better co-planarity for π−π stacking and hence charge transport. Very recently, Cho’s group reported difluorobenzenebased polymer P30a and fluorine-free polymer P30b, and also synthesized P30c with the methoxybenzene moiety simultaneously, to research the influences of tailored conformational locks on the device performance of the polymers.[61] DSC analysis demonstrated that the thermal disruption of the crystalline domains for P30a is more difficult than those for P30b and P30c, indicating that the interchain interactions of P30a are strengthened by the introduction of the fluorine atoms. Moreover, film characterization revealed that polymer chains of P30a with the difluorobenzene moiety predominantly adopt an edge-on lamellar packing motif with much enhanced crystalline ordering, whereas the thin films of P30c with the methoxybenzene moiety have a prominently face-on orientation on the substrates, resulting in about six-times higher mobility for P30a and a lower mobility for P30c, relative to the P30b with the unmodified benzene moiety, respectively. The results demonstrate that tuning of the backbone orientation through chemical modification is a promising strategy to design high-mobiltiy polymer semiconductors. P31 was synthesized through copolymerization of dithienylDPP with dithienyl-DPP, resulting in extended absorption (up to 1000 nm) in both solid-state and solution absorption spectra and a short π–π stacking distance of 3.6 Å.[62] These results implied the presence of strong intermolecular interactions. Therefore, a high electron mobility of up to 3 cm2 V−1 s−1 was observed for P31 OTFTs. This result suggests that copolymerization of two acceptors is one of the most effective synthetic strategies for achieving high electron mobility.

2.1.2. Difuranyl-DPP Density functional theory calculations with Gaussian indicate both difuranyl-DPP and dithienyl-DPP have co-planar conformations in their energy-minimized states.[63] In difuranyl-DPP, the smaller oxygen atom of furan (versus the sulfur atom of thiophene) and the shorter C−O bond (relative to dithienyl-DPP) result in less steric hindrance between the DPP unit and the neighboring aryl group, and thereby allow for a more co-planar structure. Furthermore, furan substitution has been shown to improve the solubility of DPP-based polymers relative to thiophene, which enables the use of less-bulky solubilizing groups, and thus enhances the molecular packing of polymers.[23] Like dithienyl-DPP-based polymers (P3, P7b, P14 and P24), fused aromatic moieties including 2T, TT, TVT, and BT have been introduced into difuranyl-DPP-based polymers to give P32–P35, respectively (Figure 7). P32 showed a blue-shift of

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the absorption spectrum and a lower HOMO energy level than those of the dithienyl-DPP-based polymer P3, due to the reduced aromaticity of furan relative to thiophene.[64] XRD analysis demonstrated that P32 thin films are less crystalline and have a rather disordered chain orientation in the crystalline domains. However, excellent performance was observed, with high hole mobilities of up to 1.54 cm2 V−1 s−1 in bottom-gate, top-contact organic thin-film transistors, partially due to their strong intermolecular interactions and well-interconnected thin-film morphology. P33, containing thieno[3,2-b]thiophene, has a bandgap of 1.39 eV, which is larger than that of P14, which has a similar structure (1.23 eV).[65] XRD data indicated that the polymer chains in thin films predominantly adopted an edgeon chain orientation. It was found that the intensity of a broad peak at about 20° remained nearly constant with the increase of annealing temperature, likely due to the short-range ordering in the amorphous regions in the thin films. A hole mobility as high as 0.53 cm2 V−1 s−1 was achieved for P33 OTFTs. Based on P7b, Sonar et al. reported P34a, containing TVT.[66] AFM and TEM analyses of the polymer revealed a nodular terrace morphology with optimized crystallinity after 200 °C thermal annealing. Although the π–π stacking distance, optical bandgap and energy levels of this polymer are similar to those of P7b, its hole mobility (0.13 cm2 V−1 s−1) was much lower than that of P7b, which may be attributed to its lower molecular weight (Mn = 13 000 g mol−1).[66] In order to improve the molecular weight of difuranyl-DPP-TVT polymer, Ha and Sonar also synthesized P34b with longer alkyl chains (2-decyltetradecyl groups).[67] A hole mobility over 4.0 cm2 V−1 s−1 was obtained for P34b by using a unique device geometry with a recessed gate, short channel length, and low operating voltage; this is among the highest maximum mobilities reported to date for difuranylDPP-based polymers. Subsequently, Sonar et al. combined difuranyl-DPP moieties and 2,1,3-benzothiadiazole (BT) moieties to form P35.[68] Compared with a structural analogue polymer P24, P35 showed larger bandgap (1.26 eV vs 1.20 eV) and lower HOMO energy level (−5.37 eV vs −5.20 eV), which is also attributed to the introduction of furan into DPP-based polymer. The 2D-XRD pattern showed a broad peak located from 2θ = 18° to 28°, suggesting that this copolymer is not highly crystalline. Nevertheless, high mobilities of 0.20 cm2 V−1 s−1 and 0.56 cm2 V−1 s−1 were obtained for holes and electrons, respectively. In addition to BT, other electron accepting moieties such as fluorenone (FN), and diphenylfumaronitrile (DPFN) have also been incorporated into the backbones of difuranylDPP-based polymers (P36–P37), respectively. P36 with the FN moiety (PFN-DPPF) showed a blue-shift of the absorption spectrum compared to P35 containing BT, due to the weaker electron acceptor for FN relative to BT.[69] Moreover, the introduction of FN lowered the HOMO energy level (−5.45 eV), which enhanced the device stability. P36 OTFTs showed a high hole mobility of 0.15 cm2 V−1 s−1 in air, indicating that the incorporation of a weak electron acceptor of FN into polymers is an effective strategy for designing new high-performance and stable polymer semiconductors. According to this strategy, Sonar et al. first combined the novel weak electron acceptor of DPFN and difuranyl-DPP moieties to obtain P37.[70] OTFTs characteristics revealed that P37 exhibited a hole mobility of 0.20 cm2 V−1 s−1 in air. It is worth noting that this was the first report of a

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PROGRESS REPORT Figure 7. Chemical structures of representative difuranyl-DPP, diseleneyl-DPP, diTT-DPP, diphenyl-DPP, and dipyridinyl-DPP-based polymers (C12: dodecyl; C16: hexadecyl; C4C8: butyloctyl; C8C12: 2-octyldodecyl; C10C14: 2-decyltetradecyl).

DPFN-based copolymer for OTFTs with high mobility and good air stability. Recently, a thiophene-tetrafluorophenyl-thiophene (TFPT) D–A–D building block was used in combination with a difuranyl-DPP for synthesizing P38 (PDPPF-TFPT).[71] The optical and electrochemical properties showed that P38 has a low-lying LUMO (−4.11 eV) and a narrow bandgap (1.37 eV), mainly due to the incorporation of the strong electron-accepting tetrafluorophenyl moiety. XRD data indicated that the polymer chains in thin films adopted an edge-on chain orientation with a short π–π stacking distance of 3.71 Å. P38 exhibited clear ambipolar transport behavior with hole and electron mobilities of 0.40 cm2 V−1 s−1 and 0.12 cm2 V−1 s−1, respectively, implying that incorporating TFPT moieties into difuranyl-DPP-based polymer favors balancing the electron-withdrawing and electron-donating building blocks in the polymer backbone. To study the influence of the acene size on the optical, electrochemical, and device performance properties of difuranylDPP-based polymers, three donor blocks of phenylene, naphthalene, and anthracene have also been incorporated into the polymers to give P39–P41, respectively.[72] The UV–vis–NIR

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absorption spectra in solution revealed that P39 showed an absorption maximum (λmax) at 744 nm, whereas P40 and P41 exhibited their maxima at 713 nm and 707 nm, respectively, which could be attributed to the large torsion angle between the difuranyl-DPP unit and the acene moiety, leading to localization of the π-electron wave functions and hence reduction of conjugation along the polymer backbone. The HOMO energy levels calculated for P39, P40, and P41 are −5.40 eV, −5.34 eV and −5.33 eV, respectively. The lower HOMO level of P39 relative to P40 or P41 is mainly due to the weaker electron-donation of phenylene in comparison to naphthalene and anthracene. Like P35, similar broad peaks (010) were observed in XRD patterns of these polymers, indicating the existence of an amorphous phase. The diffraction peak maxima (010) were located at 2θ = 20.74°, 21.18°, and 20.08° for P39, P40, and P41, corresponding to face-to-face packing distances of 4.27 Å, 4.18 Å, and 4.41 Å, respectively. These large π–π stacking distances are not favorable for intramolecular interactions and hence charge-carrier transport. Therefore, P39–P40 OTFTs showed a low mobility of 0.7−0.11 cm2 V−1 s−1. Among these polymers,

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the highest hole mobility of 0.11 cm2 V−1 s−1 was achieved for P40, due to its more co-planar backbone structure and relatively closer π–π stacking compared with those of P39 or P41. Recently, Chen et al. introduced furan units into difuranyl-DPP-based polymers with branched 2-butyloctyl side chains (P42a) or linear hexadecyl (P42b) to probe structural effects on polymer-packing alignment.[23] P42a with high solubility showed a hole mobility of 0.56 cm2 V−1 s−1, while P42b with low solubility provided a higher hole mobility up to 2.25 cm2 V−1 s−1. Analysis of the films by AFM and GIXRD revealed that P42b, with linear side chains, exhibited larger crystalline domains, closer π–π stacking (3.52 Å), and aligned with a great preference for in-plane π–π packing. The influences of linear and branched side chains on OTFT performance of the corresponding polymers will be discussed in the following section.

2.1.3. Diseleneyl-DPP Compared with difuranyl-DPP and dithienyl-DPP, diseleneylDPP contains the much larger selenium atom, which seems to suggest increased steric hindrance. However, theoretical calculations indicate that diseleneyl-DPP tends to have a completely co-planar conformation with an intramolecular Se–O distance of 2.94 Å, which is much smaller than the sum of the van der Waals radii of 3.42 Å.[63] Heeney’s group reported three diseleneyl-DPP-base polymers containing selenophene, TT, and BT monomers, namely P43, P44, and P45, respectively (Figure 7).[58,73] The selenium atom is expected to stabilize and delocalize the LUMO to a larger extent than sulfur in these structures, which should be favorable for electron transport. Electrochemical characterization revealed that P43 and P45 showed lower LUMO levels than those of respective thiophene analogues (P9a, P24), while the LUMO level of P44 was similar to that of analogue P14. P43 had mobilities exceeding 0.1 cm2 V−1 s−1 for both holes and electrons. The mobility of P43 was much lower than that of P9a, which could be attributed to weaker intermolecular interactions and the lower crystalline of P43. Hole and electron mobilities as high as 1.1 cm2 V−1 s−1 and 0.14 cm2 V−1 s−1 were achieved for P44 OTFTs. In the XRD patterns of P44, there were no peaks observable in the π–π region, indicating that the polymer backbone may be well aligned with respect to the substrate. Besides the lower LUMO level, P45 showed red-shifts of the absorption spectra in thin film relative to P24, implying that replacement of thiophene with selenophene contributes to enhanced ordering in the solid state. Mobilities for P45 of 0.46 cm2 V−1 s−1 and 0.84 cm2 V−1 s−1 were observed for holes and electrons, respectively. Increasing the molecular weight (from 30 000 to 130 000 g mol−1) resulted in a significantly reduced hole mobility of 0.20 cm2 V−1 s−1 and a slightly increased electron mobility of 0.97 cm2 V−1 s−1. Noticeably, the maximum electron mobility of P45 is nearly double that of P24, which can be attributed to the lower LUMO level of P45 relative to P24 (−3.84 eV vs −3.77 eV), and the increased intermolecular contact due to the larger size of selenium atoms. These results indicate that the replacement of thiophene with selenophene is a promising synthesis method to develop highmobility ambipolar polymers.

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2.2. DPP Flanked with a Six-Membered Aromatic Ring The first reported DPP-based polymers consisting of sixmembered aromatic rings are diphenyl-DPP-based polymers, which were applied in photorefractive devices by Yu and coworkers in 1993.[74] Until recent years, some studies on the semiconducting properties of diphenyl-DPP polymers were reported.[75–78] Unfortunately, the OTFT performance for this type of polymer was not ideal. Amongst these polymers, P46, containing bithiophene units, exhibited the best OTFT performance with hole mobility of up to 0.04 cm2 V−1 s−1 (Figure 7).[78] Quantum-chemical calculations of the DPP-diphenyl unit predict a dihedral angle of 27° between the phenyl group and the DPP unit.[29] Moreover, the backbone twist of the polymer and the low degree of backbone co-planarity are expected to hinder strong intermolecular π–π interactions, ordered packing, and charge transport, resulting in a poor OTFT performance for this class of polymer. On the basis of P46, Li and co-workers replaced phenyl with 2-pyridinyl to give P47 (Figure 7), which can effectively avoid its steric interaction with the DPP moiety, and thus result in a highly co-planar structure for P47, as revealed by computer simulations.[15] It can be seen from absorption spectra that P47 shows a red-shift of almost 125 nm (λmax) compared with that of P46 in solution and in thin film, implying that P42 has more co-planarity of the backbone in both solution and the solid state. The co-planar polymer backbone of P47 could effectively promote molecular packing with a short π–π stacking distance (0.36 nm) and high crystallinity. Furthermore, the relatively electron-deficient characteristic of pyridine helps to reduce the LUMO energy level (−4.33 eV), which is deeper than that of P46 (−3.68 eV). As a result, P47 exhibited excellent charge-transport performance with a remarkable high electron mobility of 6.30 cm2 V−1 s−1 in OTFT devices.

2.3. DPP Flanked with Fused Heterocyclic Rings Flanking DPP with fused heterocyclic rings can extend polymer co-planarity and also promote a more-delocalized HOMO distribution along the backbone, which would enhance intermolecular charge-carrier hopping. Bronstein et al. reported a novel DPP-based monomer containing flanking thieno[3,2-b] thiophenes (diTT-DPP) and subsequently synthesized the homopolymer (P48) and the thiophene copolymer (P49) (Figure 7).[79] P48 exhibited hole and electron mobilities up to 0.052 cm2 V−1 s−1 and 0.39 cm2 V−1 s−1, respectively. Remarkably, a much higher hole mobility of 1.95 cm2 V−1 s−1 was achieved for P49 OTFTs. The authors suggested that the introduction of a thiophene unit into the main-chain of P48 resulted in an increased intermolecular distance between the solubilizing alkyl chains, and thus reduced steric interactions, which could affect the nature of polymer packing in the solid state and the corresponding OTFT performance. XRD and DSC studies indicated that these polymers were semicrystalline. Relative to the corresponding dithienyl-DPP homopolymer (P1 and P2b), OTFT performance for P48 and P49 was improved significantly, which can be attributed to the incorporation of TT, and thus enhancement of intermolecular charge-carrier hopping.

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3. Tuning of the Side Chain of DPP-Based Polymers The existence of strong π–π interactions in π-conjugated systems of DPP-based polymers significantly decreased their solubility. Hence, to afford satisfactory solubility for purification and device fabrication of DPP-based polymers, the introduction of side chains is necessary. Moreover, many studies have demonstrated that side chains play an important role in defining the physical properties and molecular packing, and thereby the device performance of the corresponding DPP-based polymers. Therefore, tuning of the side chain is believed to be an efficient strategy for acquiring excellent device performance. We herein highlight recent investigations on the effects and roles of side chains in high-mobility DPP-based polymers, which will provide a better understanding of the relationships between the structure of the side chains and the properties of the corresponding polymers.

3.1. Branched and Linear Alkyl Chains Generally, branched alkyl chain substitution has been shown to improve the solubility of DPP-based polymers. However, the use of bulky solubilizing groups increases the repulsive interactions of the polymer backbone and thus affects the polymer packing in blend films.[23] Fréchet and co-workers found that smaller side chains could promote closer polymer packing in blend films and substantially improve the photovoltaic performance of DPPbased polymers.[80] In respect of OTFTs, systematic research by Chen et al. reported that the size of alkyl chain could have a significant effect on aggregation, solid-state order, and device performance of four DPP-based polymers attached to branched 2-butyloctyl side chains (P42a, P50a) or linear hexadecyl (P42b, P50b) (Figure 8).[23] In the GIXRD pattern, the π–π spacing peaks are visible as a ring or partial arc at q = 1.7 Å−1, corresponding to π-stacking distances of 3.82 and 3.85 Å for P42a and P50a films, respectively, whereas films of P42b and P50b showed reduced π–π spacings of 3.52 and 3.68 Å, respectively. These results suggested that the polymers substituted with linear (P42b and P50b) versus branched alkyl chains (P42a and P50a) achieved

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Figure 8. DPP-based polymers with branched and linear alkyl chains.

tighter π–π packing. In XRD data, the length (LC) reflects the crystal direction of the polymer. In polymer films, a reduction in the variability of the chain position and rotation corresponds to a longer LC when the full width at half-maximum of scattering peaks is determined. Films of P42a and P50a displayed a π–π LC of 1.4 and 1.1 nm, while those of P42b and P50b showed LC values of 3.9 and 2.2 nm, respectively, indicating that linear alkyl chains allow the polymer backbones to form crystalline nanostructures with greater order, relative to branched alkyl chains. Additionally, branched and linear alkyl chain substitution can affect the orientation of π-stacking with respect to the substrate. Films of P42a with branched side chains showed a slight preference for in-plane orientation, while P50b with linear side chains showed that nearly all the π-packing occurs in-plane with the substrate. From AFM height images of films, polymers substituted with linear alkyl chains (P42b and P50b) showed bigger root-mean square roughness (RRMS) values than those with branched alkyl chains (P42a and P50a), which may be the result of increased polymer crystallization during film formation. Moreover, NMR, variabletemperature UV–vis–NIR, and dynamic light scattering (DLS) spectroscopy of these polymer solutions demonstrated that the polymers with linear side chains have a stronger propensity for aggregation in solution relative to those with branched side chains, leading to the formation of films with solid-state properties, and thus excellent OTFT performance. As a result, higher mobilities were achieved for polymers substituted with linear alkyl chains versus branched alkyl chains. Remarkably, in comparison to the analogous thiophene-containing DPP-based polymers, the furan moieties in the backbones of these polymers provide better solubility for the polymers, which enables the use of linear alkyl chains to afford a satisfactory molecular weight and facile device fabrication for the polymers. Therefore, the structure of the polymer backbone has a great influence on the selection of branched and linear alkyl chains. Overall consideration of the structures of both backbones and side chains is believed to be an effective assurance for developing highmobility DPP-based polymers.

3.2. Length of Alkyl Chains For the length effect of alkyl chains on OTFT performance, poly(3-alkylthiophene) is one of the most extensively studied polymer semiconductors. It was found that the length of the alkyl chains has a great influence on the molecular packing and thus the OTFT performance of the corresponding polymers.[81] Poly(3-alkylthiophene) with hexyl groups has been found to exhibit the best device performance, with a hole mobility up to 0.1 cm2 V−1 s−1, due to edge-on molecular packing.[28] In DPP-based polymer systems, tuning the alkyl chain length can also effectively optimize molecular packing and thereby enhance device performance. Liu and co-workers found that

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Figure 9. a) Grazing incidence X-ray scattering pattern (out-of-plane) of polymer P7a (PDVT-8) and P7b (PDVT-10) films annealed at 180 °C. b) Inplane X-ray scattering pattern of polymer P7a (PDVT-8) and P7b (PDVT10) films annealed at 180 °C. Reproduced with permission.[12] Copyright 2012, Wiley-VCH.

the length of the alkyl chain could affect the interchain π–π stacking distance of DPP-based polymers, and hence the OTFT performance.[12]P7b, with longer alkyl chains (2-decyltetradecyl groups), exhibited a higher mobility of up to 8.2 cm2 V−1 s−1, while P7a, with shorter 2-octyldodecyl side chains, showed a relatively lower mobility of 4.2 cm2 V−1 s−1. In the in-plane XRD pattern of P7a and P7b (Figure 9), strong diffraction peaks at 2θ = 23.8° and 24.3° were observed, corresponding to a π–π stacking distance of 3.72 Å for P7a and 3.66 Å for P7b in thin films. The smaller π–π stacking distance of P7b indicated that a stronger intermolecular interaction existed among the D–A conjugated main chain in thin films, resulting in a better OTFT performance. Additionally, AFM images showed the formation of corn-leaf like interconnected networks in P7b film (annealed at 180 °C, Figure 10), indicating that P7b exhibited larger crystalline domains than P7a. This result reveals that the morphology of the polymer films can be affected by the length of alkyl chains.

Figure 10. AFM topography images of polymer films on OTS-modified SiO2/Si substrates. The P7a thin films: a) without annealing, and b) with annealing at 180 °C. The P7b thin films: c) without annealing, and d) with annealing at 180 °C. Reproduced with permission.[12] Copyright 2012, Wiley-VCH.

while the OTFT mobility of P3’ was almost one order of magnitude lower, at 0.1 cm2 V−1 s−1, which is largely attributed to the steric-hindrance effect caused by the alkyl groups.[19] Additionally, alkylation of thienothiophenes in DPP-based polymers also affects their OTFT performance. Janssen and co-workers reported P51, containing thienothiophenes, and P51’, with alkylated thienothiophenes (Figure 11).[83] In comparison to P51, P51’ showed a blue-shift of the absorption spectrum and a larger bandgap (1.33 vs 1.31 eV), which can be attributed to the twisting between adjacent units induced by the alkyl chains on the thienothiophene, and hence decreased orbital overlap and conjugation. This steric hindrance also influences their OTFT performance. P51’ OTFTs exhibited hole and electron mobilities up to 1 × 10−2 cm2 V−1 s−1 and 2 × 10−3 cm2 V−1 s−1, respectively, which were several times lower than those (0.03 and 0.009 cm2 V−1 s−1 respectively) of P51 without alkyl chains on the thienothiophene.

3.4. Functionalized Alkyl Chains 3.3. Substitution Position of Alkyl Chains Many studies have demonstrated that the substitution position of the alkyl chains can exert an influence on the carrier mobility through tuning the steric hindrance effect or rotational angle, which is attributed to the effective conjugation length being limited by the torsional disorder along the polymer backbone.[82] Similar results have also been observed in DPP-based polymers. As shown in Figure 11, P3 and P3’ have identical D–A polymer backbones. Based on P3, two additional dodecyl chains were introduced near the thienyl–thienyl connecting position to give P3’, which caused obviously steric repulsion, and thereby increased the dihedral angles in solid state. OTFT mobility of 0.98 cm2 V−1 s−1 was obtained for P3,[33]

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It was found that functionalized side chains could modulate the carrier injection of organic semiconductors. Among functionalized side chains, siloxane-terminated alkyl side chains and triethylene glycol side chains have been the most widely used for OTFTs. Yang, Oh, and co-workers firstly reported DPP-based polymers (P9b) containing siloxane-terminated side chains, and also synthesized P9a with alkyl chains to elucidate the influence of the alkyl sides on the molecular packing and OTFT performance of the corresponding polymers.[42] The UV–vis–NIR absorption spectra of P9a and P9b in solution revealed that there was a red-shift of almost 40 nm (λmax) in going from P9a to P9b, which could be attributed to the minimization of steric effects between siloxane-terminated side chains, and thereby the increased co-planarity of the backbone for P9b compared with

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Patil, Anthopoulos, and co-workers reported a DPP–DPP copolymer with engineered triethylene glycol (TEG) side chains on the DPP unit (P31) to allow optimized chain packing and formation of large crystalline domains in the solid state.[62] Remarkably high electron mobilities of up to 3 cm2 V−1 s−1 with a short π–π stacking distance of 3.60 Å were obtained for P31 OTFTs. This result suggested that TEG chains may endow organic semiconductors with other novel properties; this is believed to a promising side chain in the design of organic semiconductors.

3.5. Alkyl Chain Branching Position Branched alkyl chains which are commonly used in organic semiconductors include 2-ethylhexyl, 2-octyldodecyl, 2-decyltetradecyl groups, etc. However, these readily available Figure 11. Investigation of the substitution position of alkyl chains in DPP-based polymers; alkyl chains, branched at the second position, will result in steric hindrance for polymer the arrows indicate the rotational angles caused by the alkyl chains. backbones and affect the corresponding performance, due to the close distance between the branching points and the conjugated backbone.[25] To avoid P9a. In the out-of-plane XRD pattern, the d(001)-spacing value of P9b thin film annealed at 220 °C was 23.54 Å (2θ = 3.75°), the steric-hindrance effect, Pei and co-workers developed three novel branched alkyl chains (3-decyltridecyl, 4-decyltetrawhich was relatively larger than that (20.95 Å) of P9a thin film. decyl, and 5-decylpentadecyl) to modify the isoindigo-based This was likely due to the longer bond length of Si–O (1.64 Å) polymer.[84] They found that moving the branching point of relative to C–C (1.53 Å). Moreover, an additional and remarkably strong peak was observed at 2θ ≈ 25° in the diffractograms of these alkyl chains farther away from the backbone would result in gradually decreased π–π stacking distances for the correthe P9b thin films, which could be attributed to the formation of efficient π–π stacking (π-stacking distance ≈ 3.6 Å) with facesponding polymers. Subsequently, Meager, McCulloch, and coworkers systematically studied the influences of the side-chain on orientations. The authors expected that P9b films would be branching point on the OTFT performance for DPP-based polyable to adopt 3D conduction channels that would enhance charge mers (P49, P49a, and P49b, Figure 12).[85] It was found that transport over those of polymer films with only perpendicular π–π planes. Noticeably, the π-stacking distance of P9b (ca. 3.6 Å) was smaller than that of P9a (ca. 3.7 Å), indicating that a stronger intermolecular interaction exists among the D–A conjugated main chain for P9b in thin films. The AFM images revealed that P9a thin films formed fine and isotropic “granular” domains with a root-mean-square (rms) roughness value of 0.88 nm, whereas P9b thin film formed rugged granular structures with larger sizes, implying the formation of moreefficient pathways for charge-carrier transport in the P9b film. As a result, high hole and electron mobilities of up to 3.97 cm2 V−1 s−1 and 2.20 cm2 V−1 s−1 were obtained for P9b containing siloxane-terminated side chains, respectively, while P9a with alkyl chains showed relatively lower hole and electron mobilities of 1.62 and 0.14 cm2 V−1 s−1, respectively. Compared with alkyl chains, oligo(ethylene glycol) chains exhibit better solubility in organic solvents. Recently, Figure 12. DPP-based polymers attached branched alkyl chains with different branching points.

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www.MaterialsViews.com Table 1. OTFT performances of DPP-based polymers. Polymers

dπ-stacking [Å]

Ref.

HOMO [eV]

LUMO [eV]

Deposition Process

Max µh [cm2 V−1 s−1]

P1

−4.85

−3.55

spin-coating

0.00039

0.00016

P2a

−5.17

−3.61

spin-coating

0.04

0.001

P2b

−5.3

−4.0

spin-coating

1.57

0.08

P3

−5.2

−4.0

spin-coating

0.97

106

P4

−5.12

−3.55

spin-coating

3.46

8

10

3.65

[11]

P5

−5.10

−3.49

spin-coating

3.94a)

106–107

3.69

[14]

−5.09

−3.49

spin-coating

1.05b)

107

−5.15

−3.73

spin-coating

2.82

a)

3.63

[14]

−5.11

−3.70

spin-coating

0.048b)

105–106 [12]

P6

Ion/Ioff

Max µe [cm2 V−1 s−1]

Ion/Ioff

[30] [31]

106–107

3.65

[32]

3.75

[33]

P7a

−5.30

spin-coating

4.5

105–107

3.72

P7b

−5.28

spin-coating

8.2

105–107

3.66

P7c

−5.77

−3.92

spin-coating

0.82

7.00

P8a

−5.28

−3.55

spin-coating

0.37

0.78

3.89

P8b

−5.18

−3.39

spin-coating

2.96

0.36

3.84

[35]

P8c

−5.13

−3.23

spin-coating

1.88

0.04

3.80

[35]

P9a

−5.09

−3.46

spin-coating

1.62

5.3 × 103

P9b

−5.09

−3.41

solution-sheared

3.97

104

2.2

10

P9c

−5.17

−3.56

solution-sheared

6.16

104

3.07

103

[43]

P9d

−5.10

−3.49

solution-sheared

8.84

104

4.34

10

[44]

P10 P11

−5.15

P12a

−5.26

−3.79

spin-coating

1.5

106

spin-coating

5.0

106

spin-casting

4.97

1.55 × 107

0.14

[12] [34]

1.4 × 106

[35]

[41] 3.59

[42]

[39] 3.75

[44] [40]

−5.15

−3.84

spin-casting; drop-cast; solution-sheared

3.29; 4.56; 4.59

105; 105; 106

3.60

[45]

P12b

−5.13

−3.86

spin-casting; drop-cast; solution-sheared

0.37; 0.42; 0.50

105; 106; 107

3.61

[45]

P12c

−5.19

−3.93

spin-casting; drop-cast; solution-sheared

2.25; 4.14; 4.23

106; 107; 103

3.61

[45]

P12d

−5.17

−3.92

spin-casting; drop-cast; solution-sheared

0.43; 0.48; 0.54

105; 106; 106

3.57

[45]

P12e

−5.14

−3.91

spin-casting; drop-cast; solution-sheared

4.69; 7.42; 13.90

107; 105; 105

3.60

[45]

P12e

−5.27

spin-coating

12.04

106

3.62

[16]

P12f

−5.16

−3.93

spin-casting; drop-cast; solution-sheared

5.38; 7.74; 10.30

104;

105; 106

3.61

[45]

P12g

−5.12

−3.87

spin-casting; drop-cast; solution-sheared

3.86; 5.78; 7.27

106; 106; 105

3.61

[45]

P12h

−5.17

−3.93

spin-casting; drop-cast; solution-sheared

3.71; 4.69; 4.75

105; 105; 104

3.57

[45]

P13

−5.13

−3.88

spin-coating

1.78

105–106

4.02

[48]

P14

−5.25

−3.40

spin-coating

0.94

10

3.71

[50]

−5.33

−4.07

spin-coating

1.36

105–106

3.8

[51]

spin-coating

10.5

>106

3.43

[13]

TGSS

7.43

>105

3.67

[52]

spin-coating

0.98

2 × 107

3.82

[53]

0.68

>107

−5.2

−3.5

P15 P16 P17

−5.29 −5.36

−3.3 −3.75

spin-coating

6

1.56

105–106

[54] Continued

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Polymers

HOMO [eV]

LUMO [eV]

Deposition Process

Max µh [cm2 V−1 s−1]

Ion/Ioff

P18

−5.36

−4.02

spin-coating

1.31

>106

P19

−5.34

−3.98

spin-coating

P20

−5.22

−3.91

spin-coating

P21

−5.21

−3.91

spin-coating

P22

−5.39

−3.38

spin-coating

2.7

P23

−5.18

−3.57

spin-coating

0.56

P24

−5.2

−4.0

spin-coating

0.35

Max µe [cm2 V−1 s−1]

Ion/Ioff

dπ-stacking [Å]

Ref.

3.74

[55]

0.78

10

7

3.69

[54]

2.31

106

3.54

[54]

3.20

>106

3.60

[54]

4.2

[55]

0.40

3.73

[57]

3.65

106–107

[56]

−5.25

−3.77

spin-coating

0.33

0.57

P25a

−4.55

−3.9

spin-coating

0.8

1.36

[59]

P25b

−4.55

−3.9

spin-coating

1.17

1.32

[59]

P26

−5.36

−3.56

spin-coating

0.40

0.24

3.93

P27

−5.45

−3.57

spin-coating

0.30

0.26

3.93

[60]

P28

−5.57

−3.56

spin-coating

0.03

0.12

3.93

[60]

P29

−5.65

−3.64

spin-coating

0.25

P30a

−5.20

−3.71

spin-coating

1.32

105

P30b

−5.11

−3.62

spin-coating

P30c

−5.03

−3.50

spin-coating

P31

−5.38

−3.66

spin-coating

P32

−5.32

−3.91

spin-coating

1.54

106–107

P33

−5.28

−3.89

spin-coating

0.53

106

P34a

−5.22

−3.81

spin-coating

0.13

106–107

spin-coating

4.2

107

P34b

2.36

[58]

[60]

3.85

[60]

1.17

105

3.60

[61]

0.24

105

0.21

105

3.90

[61]

1.12

105

0.11

105

3

104

3.6

[62]

[61]

[64] [65] 0.078

[66] [67]

P35

−5.37

−3.74

spin-coating

0.61

103

P36

−5.45

−3.83

spin-coating

0.15

104

P37

−5.50

−3.95

spin-coating

0.20

P38

−5.48

−4.11

spin-coating

0.40

103

P39

−5.40

−3.88

spin-coating

0.04

P40

−5.34

−3.85

spin-coating

P41

−5.33

−3.80

P42a

−5.45

−3.59

P42b

−5.42

−3.61

P43

−5.2

−4.02

P44

−5.1

−3.92

spin-coating

1.1

0.14

P45

−5.16

−3.84

spin-coating

0.46a) (0.20b))

0.84a)(0.97b))

P46

−5.4

−3.5

spin-coating

0.04

0.67

103

[68] [69] [70]

0.12

102

3.71

[71]

105

4.27

[72]

0.11

105

4.18

[72]

spin-coating

0.07

105

4.41

[72]

spin-coating

0.56

105

3.85

[23]

spin-coating

2.25

106

spin-coating

0.1

0.1

3.62

[23]

3.8

[73]

3.64

[56]

[73]

[78]

P47

−5.69

−4.33

spin-coating

2.78

103

6.30

103

P48

−5.04

−3.76

spin-coating

0.052

105

0.39

104–105

[79]

P49

−5.06

−3.68

spin-coating

1.95

105

0.07

103–104

[79]

a)With

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Table 1. Continued

[15]

the respective low-molecular-weight polymer; b)With the respective high-molecular-weight polymer.

P49b, with the largest alkyl chain, showed the best solubility and highest number-average molecular weight (up to 80 000 Da) among these polymers. UV–vis–NIR absorption spectra revealed that the alkyl-chain branching point was further away from the DPP core (P49a and P49b), which allowed stronger intermolecular π–π stacking of polymer backbones, ultimately leading to enhanced molecular aggregation in solution and

Adv. Mater. 2015, 27, 3589–3606

similar absorption features in solution and the solid state. However, P49, with short alkyl chains, showed a significant redshift in going from solution to the solid state. The authors considered that the proximity of the alkyl-chain branching point to the DPP core in P49 possibly hindered the π–π stacking and aggregation in solution, and this was overcome by intermolecular forces in the solid state, planarizing the backbone.

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GIXRD analysis indicated that P49 exhibited a pronounced face-on character, while both P49a and P49b had significant edge-on character, suggesting that the location of the branching point can influence the orientation of the conjugated backbone plane and the more-distant branching points in P49a and P49b seem to encourage an edge-on morphology in thin films. P49b OTFTs exhibited the best OTFT performance with the highest hole mobility (up to 0.066 cm2 V−1 s−1), higher than those of P49 and P49a. These results demonstrate that the variation of the branching point of the alkyl chain is an effective strategy for modulating molecular packing and OTFT performance. According to this strategy, Kwon and co-workers synthesized P12e, in which the branched alkyl chain has a relatively longer alkyl spacer (C6) between the branching point and the backbone.[16] Compared with short branched alkyl chains attached to P12a, P12e had a much higher hole mobility of 12 cm2 V−1 s−1 and a shorter π–π stacking distance of 3.58 Å. Recently, Oh, Kim and co-workers found that odd–even numbers of linear spacer groups in branched alkyl chains have great influences on OTFT performance for DPP-based polymers.[45] The polymers with an even number of carbon atoms in the linear spacer groups (C2, P12a; C4, P12c; C6, P12e) showed one order of magnitude higher charge mobilities than those with an odd number of carbon atoms (C3, P12b; C5, P12d), due to shorter lattice spacing. Remarkably high hole mobility of up to 17.8 cm2 V−1 s−1 was obtained for solution-sheared P11e film. However, along with the increase of the spacer group length (C7, P12f; C8, P12g; C9, P12h), the corresponding polymers deviated from the above odd–even trend, which can be attributed to increased unpredictable intermolecular interactions between the alkyl chains. For functionalized alkyl chains, such as siloxane-terminated alkyl chains, their branching position was also found to influence the OTFT performance of DPP-base polymers. Yang, Oh and co-workers reported DPP-based polymers with different siloxane-terminated points, such as butyl units (P9c), pentyl units (P9d), and hexyl units (P9b).[43] The authors found that the branching position in the hybrid side chains could be delicately tuned to induce dense molecular packing and facilitate charge transport through 3D conduction channels. Although both crystallinity and solubility were enhanced with increasing alkyl spacer length, the shorter side chains induced smaller lamellar spacing, while retaining a close π–π stacking distance, leading to enhanced charge transport. Notably, P9d with pentyl spacers exhibited the best OTFT performance with high hole and electron mobilities of 8.84 and 4.34 cm2 V−1 s−1, respectively. Table 1 shows the OTFT performances of the most of the DPP-based polymers discussed in this Progress Report.

4. Conclusions and Outlook Compared with other types of polymer semiconductors, more DPP-based polymers have been reported to exhibit high mobilities of up to 1 cm2 V−1 s−1. The reported mobility even reaches 17.8 cm2 V−1 s−1,[45] which can compete with the performance of amorphous silicon. In addition to device optimization, molecular design is crucially important for achieving excellent device performance. In this Progress Report, we summarize the

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advances in molecular design and OTFT performance of highmobility DPP-based polymers reported in the last few years. For the optimization of DPP-based polymer structures, modulating the building blocks appears to be an effective approach to obtain high device performance for DPP-based polymers. These building blocks consist of five-membered aromatic rings, sixmembered aromatic rings, and fused heterocyclic rings, which will principally determine the backbone co-planarity, bandgap, and energy levels, and thus affect the molecular packing and device performance of the corresponding polymers. Moreover, structural modifications of the polymer side chains, such as alterations in the shape, length, substitution position, branching position, and connected functionalized groups, play a significant role not only in improving the solubility and the achievable molecular weight of the DPP-based polymers, but also in influencing their intermolecular interactions, thin-film morphology, and charge carrier transport. Therefore, overall consideration and engineering of both π-conjugated backbones and side chains is believed to constitute an effective strategy for acquiring excellent device performance of DPP-based polymers. We believe that this synthetic strategy has immediate applicability to other types of polymer semiconductors and will greatly facilitate the development of organic semiconductors.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Project No. 51173055), the National Program on Key Basic Research Project (973 Program, Grant No. 2013CBA01600), and the China Postdoctoral Science Foundation (No. 2013M542009). Received: January 26, 2015 Revised: April 1, 2015 Published online: May 15, 2015

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Adv. Mater. 2015, 27, 3589–3606

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Adv. Mater. 2015, 27, 3589–3606

Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors.

Since the report of the first diketopyrrolopyrrole (DPP)-based polymer semiconductor, such polymers have received considerable attention as a promisin...
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