Synthesis and Properties of Soluble Fused Thiophene

Nov 15, 2018 - Previously, we reported a DPP-FT4 polymer with molecular weight up to 30 kDa. A new design and synthesis was required to overcome this ...
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Synthesis and Properties of Soluble Fused Thiophene Diketopyrrolopyrrole-Based Polymers with Tunable Molecular Weight Weijun Niu,† Hung-Chin Wu,‡ James R. Matthews,† Adama Tandia,† Yang Li,† Arthur L. Wallace,† Jenny Kim,† Hongxiang Wang,† Xin Li,† Karan Mehrotra,† Zhenan Bao,*,‡ and Mingqian He*,† †

Corning Incorporated, One River Front Plaza, Corning, New York 14831, United States Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, United States

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‡

S Supporting Information *

ABSTRACT: It is challenging to realize both a fully conjugated rigid polymer backbone and high molecular weight at the same time. Previously, we reported a DPPFT4 polymer with molecular weight up to 30 kDa. A new design and synthesis was required to overcome this limitation. Here, we report the successful synthesis of a conjugated semiconducting polymer with tunable molecular weight over a wide range. Through molecular design and synthesis control, our new polymer can be selectively prepared with numberaveraged molecular weight (Mn) ranging from approximately 20 to 100 kDa, realizing both high molecular weight and high solubility at the same time. Four polymers within this range were investigated, with particular emphasis on Mn of 50 kDa (P2) and 97 kDa (P4). The relationships between molecular weight and polymer properties, molecular packing, and electrical behavior are explored in detail. All the polymers in this series are fully soluble in nonchlorinated solvents at room temperature, which is promising for large-area advanced electronic device applications. The effect of molecular weight on the charge-transport performance of our new polymer was investigated using bottom-gate/top-contact field-effect transistor devices. Stable device characteristics with high on/off ratios up to 107 were obtained. Of particular interest is the discovery that the hole mobility of P2 (lower Mn) is higher than that of P4 (higher Mn). This is mainly due to morphological manipulation as demonstrated by atomic force microscopy and grazing-incidence X-ray diffraction.

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“What is the number of repeat units required to have the best device performance?” Because of the reduction of conformational freedom, larger size linear fused aromatics in the conjugated polymers generally lead to high-performance OFETs.12 However, high-molecular-weight organic semiconducting polymers containing rigid larger fused aromatics often have poor solubility. To overcome the poor solubility of linear fused aromatics in such polymeric semiconductors, more and/or longer alkyl side chains may be added to the polymer repeat unit. However, this frequently results in lower mobility. Therefore, it is challenging to have both efficient charge transport and high molecular weight at the same time. Previously, we reported an organic semiconducting polymer containing β-(linear-alkyl)-substituted larger fused thiophene and diketopyrrolopyrrole (DPP) units (PTDPPTFT4, Figure 1) that meets the requirements and demonstrated hole mobilities in excess of 2 cm2 V−1 s−1, on/off ratio of >106, and threshold voltage 105. The thermal annealing (at 200 °C for 1 h) process was introduced to further improve the polymer interchain organization and packing and consequently the charge-carrier transport efficiency. The mobilities of the studied thin films were significantly increased by this process. The annealing process increased the average hole mobility of P2-based transistors to 2.04 cm2 V−1 s−1. Note that well-defined linear and saturation regions and good current modulations were observed from the output characteristics, as shown in Figure 5c,d. Thin-film transistors prepared using lower molecular weight (P1 and P2) showed relatively higher mobilities and source-to-drain currents than their higher molecular-weight D

DOI: 10.1021/acs.macromol.8b01760 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. (a) Illustration of a top-contact/bottom-gate FET device. (b) Transfer curves of P2TDPP2TFT4-based FETs. The source/drain voltage was set at −80 V for all the measurements. Output characteristics of FET devices based on annealed (c) P2 and (d) P4 thin films, respectively.

the polymers with higher molecular weights, their intermolecular-packing structure is more disordered, which leads to lower charge-carrier mobilities, confirmed by the FET device performances. Similar to molecular ordering, packing structures (face-on or edge-on packing and packing spacing) relative to the other device components (source and drain) can affect charge transport. The π−π stacking (010) signal was investigated to determine the packing features in the polymer films. For the pristine polymer thin films, the intensity of the (010) peak in the qz direction is significantly stronger than the annealed thin films, indicating the percentage of face-on packing structure in the films is higher than that in the non-annealed films. Face-on packing is less suitable for the charge transport in the FET devices because the charge is delivered in the channel horizontally between source and drain contacts, while faceon packing leaves the π−π stacking perpendicular to this charge-transport direction. Especially for the pristine thin films of higher molecular weight (P3 and P4), the face-on packing dominates the thin-film structure, resulting in a lower carrier mobility within the thin-film transistor devices. Once treated by thermal annealing, the edge-on packing fraction is greatly increased and a better charge-transport ability was achieved. The lamellar packing spacing is affected by the packing structures. With increasing polymer molecular weight, more face-on packing fraction appeared in the thin films and a smaller lamellar spacing was exhibited. Smaller spacing reflects that the polymer chains can be stacked closer to each other, which may facilitate a more efficient charge transport. By integration of all morphological factors, face-on/edge-on packing features, stacking ordering (i.e., fwhm), and packing distance, as summarized in Figure 5d, an optimal molecular weight of approximately 50 kDa is suggested to achieve the highest charge-carrier mobility, with this particular polymer system.

Table 2. Electrical Properties of P2TDPP2TFT4-Based Transistor Devices Fabricated from p-Xylene Solutions annealing P1 P2 P3 P4

pristine 200 °C pristine 200 °C pristine 200 °C pristine 200 °C

average mobility (cm2 V−1 s−1)

on/off ratio

threshold voltage (V)

± ± ± ± ± ± ± ±

∼105 ∼105 ∼107 ∼107 ∼106 ∼106 ∼107 ∼107

2 −10 −4 −2 −5 −6 −2 −3

1.16 1.71 1.38 2.04 0.235 1.33 0.476 1.08

0.25 0.35 0.32 0.41 0.080 0.13 0.059 0.068

the 2D GIXD patterns in both qz and qxy directions are depicted in (b) and (c), respectively, of Figure 5 to clarify the detailed packing features. For lower molecular-weight P1- and P2-based thin films, a highly ordered lamellar crystalline structure was found in the qz direction even for the pristine thin films. After a 200 °C thermal annealing, the lamellar packing was further improved with higher diffraction intensity. In comparison, diffraction signals with relatively low intensity were obtained from the higher molecular-weight thin films. This is postulated to have implications for interchain charge transport. Moreover, we investigated the packing orientation by determining the full-width at half-maximum (fwhm) of the (200) peak (summarized in Table 3). The fwhm for all studied polymers became smaller after annealing, meaning that the thermal treatment can enhance the lamellar packing orientation and further improve the electrical performance. More importantly, the fwhm value increased significantly as the polymer molecular weight increased, suggesting the lower molecular-weight polymers in this system can form better oriented lamellar packing as well as longer range ordering. For E

DOI: 10.1021/acs.macromol.8b01760 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. (a) 2D GIXD patterns of P1−P4 thin films with and without thermal treatment. Clear diffraction signals were obtained in both out-ofplane and in-plane directions. 1D crystallographic profiles of 200 °C annealed thin films, extracted from the 2D patterns along the (b) out-of-plane (qz) and (c) in-plane (qxy) directions, respectively. (d) Relationships between lamellar packing distance, structural ordering, and charge-transport properties.

(normally 50−60 kDa) to allow the polymer chains to connect the disorder aggregates for efficient charge transfer at the segmental level.18−20 Instead of increasing crystallinity in ordered domain, controlling of order/disorder features in πaggregates and connectivity of aggregates plays a crucial role for achieving high mobility. In our system, P2 possesses a molecular weight around 50 kDa and exhibits both face-on and edge-on ordering structures and moderate molecular-ordering and -packing distance, leading to high charge-carrier mobility. Such electrical performance is consistent with that of classical donor−acceptor conjugated polymers in the literature. A plausible conclusion from the literature is that the higher the molecular weight of an organic semiconducting copolymer, the higher the carrier mobility. This is particularly evident at smaller molecular weights.21 This holds true up to a critical molecular weight, beyond which mobility is essentially independent of molecular weight.22 Yet the solubility and other factors can also affect the charge-transport properties. Sirringhaus and co-workers studied P3HT over a much wider molecular-weight range (Mn = 15−270 kDa) and found that increasing the molecular weight of that polymer increased the mobility, up to ∼50 kDa, yet the mobility did not increase further with higher Mn.17 The anticipated increase in mobility of higher weight polymers due to improved interaction between domains, as well as their preferred parallel arrangement with respect to each other, is offset by an increased degree of disorder with increasing molecular weight. Müllen and co-workers have carried out systematic studies on CDTBTZ semiconducting copolymers and also found a dependence of mobility on molecular weight.23 Ong and co-workers synthesized high-molecular-weight OSC polymers (PDBT-co-TT) containing both non-βsubstituted fused thiophene and diketopyrrolopyrrole (DPP) units.16 The PDBT-co-TT copolymers with the highest molecular weight (Mn = 110 kDa) (measured by using GPC

Table 3. Crystallographic Parameters of P2TDPP2TFT4 Films with Different Molecular Weights

P1 P2 P3 P4

annealing

lamellar spacing (Å)

out-of-plane (200) fwhm

π−π stacking (Å)

pristine 200 °C pristine 200 °C pristine 200 °C pristine 200 °C

29.53 29.49 29.48 29.40 29.63 29.37 29.02 29.31

0.0625 0.0514 0.0608 0.0518 0.0656 0.0618 0.0718 0.0623

3.65 3.66 3.65 3.67 3.66 3.66 3.65 3.67

The viscosities of dilute solutions were measured at 25 °C (Figure S5 in Supporting Information), and a large difference in viscosity was found between these polymers with different molecular weights. The viscosity increases almost 300-fold as the molecular weight was increased from 20 to 100 kDa. Such greatly increased viscosity of high-molecular-weight polymers indicates they probably possess significant main-chain entanglements between polymer chains. Those entanglements induced by long polymer chain length may form some chain folding or kinks, which could interrupt the molecular-packing ordering and domain-to-domain connection when dried into a thin film. This may in turn lead to a less efficient charge-carrier mobility in transistor devices.17 The above-discussed evidence suggests that the interchain ordering structure and charge-carrier mobility can be manipulated by rational design and selection of the polymer molecular weight. It has been reported that the molecular weight can tailor the polymorphism in the film state, affecting charge-transport behaviors. Both edge-on and face-on packing structures are desired at the same time to facilitate 3D chargetransport networks, and the molecular weight needs to be high F

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Macromolecules at 160 °C) had the highest hole mobility at >10 cm2 V−1 s−1 and an on/off ratio of >106. Given the structural similarities between PDBT-co-TT and our polymer (P2TDPP2TFT4), we expected that polymers such as P3 and P4 with higher molecular weight than P1 and P2 should be able to achieve better charge-transport performance. However, the mobility of our fused thiophene DPP copolymers was not improved by boosting the polymer molecular weight above about 50 kDa (P2), even to ∼100 kDa (P4). In contrast, P4 actually has lower charge mobility than P2, in the thin-film transistor devices tested. This implies that although higher molecular weight (i.e., longer polymer chains) can help to enhance πaggregate connectivity, a limitation still exists to the charge transfer between polymer chains and domains.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Carrie Hogue for help and support with NMR data acquisition. This work was partially performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152. GIXD experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL), a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

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ABBREVIATIONS PTDPPTFT4,poly[(3,7-bis(heptadecyl)thieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene-2,6-diyl)(2,5-bis(hexadecyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4dione)-5,5′-diyl)] P2TDPP2TFT4,poly[((2,6-bis(thiophen-2-yl)-3,7-bis(9octylnonadecyl)thieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene)-5,5′-diyl)(2,5-bis(8-octyloctadecyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione)-5,5′-diyl)] DPP,diketopyrrolopyrrole FT4,[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene RT,room temperature CB,chlorobenzene mXY,m-xylene fwhm,full-width at half-maximum AFM,atomic force microscopy GIXD,grazing incidence X-ray diffraction DSC,differential scanning calorimetry

CONCLUSIONS Conjugated organic semiconducting polymers (P2TDPP2TFT4, Scheme 1) with repeat units containing four branched alkyl chains were synthesized with controlled molecular weights and characterized. Polymers P1−P4, with Mn from ∼20 kDa to ∼100 kDa respectively, which are fully soluble in nonchlorinated solvents such as p-xylene at RT, have been used to demonstrate the relationship between the hole mobility and molecular weight of this conjugated organic semiconducting polymer. To our surprise, given earlier results for PDBT-co-TT,16 the hole mobilities of lower Mn P1 and P2 are higher than those of higher Mn P3 and P4 in thin-film transistors. AFM, GIXD, and the difference in the solution viscosity are used to explain the device performance differences. Soluble fully conjugated organic semiconducting polymers with controlled molecular weights ranging from ∼20 kDa up to ∼100 kDa open these organic materials up to potential different applications such as OPV and OFETs, with processing tuned to the application.

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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/acs.macromol.8b01760.

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REFERENCES

(1) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 1999, 401, 685−688. (2) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Organic semiconductors for solution-processable field-effect transistors (OFETs). Angew. Chem., Int. Ed. 2008, 47, 4070−4098. (3) Bao, Z.; Locklin, J.; Organic Field-Effect Transistors, 1st ed.; CRC Press: Boca Raton, FL, 2007. (4) Katz, H. E. Recent Advances in Semiconductor Performance and Printing Processes for Organic Transistor-Based Electronics. Chem. Mater. 2004, 16, 4748−4756. (5) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. Thieno[3,2-b]thiophene-diketopyrrolopyrrole-containing polymers for high-performance organic field-effect transistors and organic photovoltaic devices. J. Am. Chem. Soc. 2011, 133, 3272−3275. (6) Li, Y.; Singh, S. P.; Sonar, P. A high mobility P-type DPPthieno[3,2-b]thiophene copolymer for organic thin-film transistors. Adv. Mater. 2010, 22, 4862−4866. (7) Kang, I.; Yun, H.-J.; Chung, D. S.; Kwon, S.-K.; Kim, Y.-H. Record High Hole Mobility in Polymer Semiconductors via SideChain Engineering. J. Am. Chem. Soc. 2013, 135, 14896−14899. (8) Yun, H.-J.; Choi, H. H.; Kwon, S.-K.; Kim, Y.-H.; Cho, K. Conformation-Insensitive Ambipolar Charge Transport in a Diketopyrrolopyrrole-Based Co-polymer Containing Acetylene Linkages. Chem. Mater. 2014, 26 (13), 3928−3937.

Characterization methods, polymer synthesis and purification procedure, DSC of polymers, AFM of polymer thin films, molecular modeling of the polymer repeat unit, viscometry measurements of polymer solutions, and NMR(PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Hung-Chin Wu: 0000-0001-6492-0525 Zhenan Bao: 0000-0002-0972-1715 Mingqian He: 0000-0002-9387-9928 Author Contributions

W.N and H.-C W. contributed equally to this publication. The manuscript was written through contributions of all authors. Funding

Funding was provided by Corning Incorporated. G

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Macromolecules (9) Sun, B.; Hong, W.; Yan, Z.; Aziz, H.; Li, Y. Record High Electron Mobility of 6.3 cm2V−1s−1 Achieved for Polymer Semiconductors Using a New Building Block. Adv. Mater. 2014, 26 (17), 2636−2642. (10) Holliday, S.; Donaghey, J. E.; McCulloch, I. Advances in Charge Carrier Mobilities of Semiconducting Polymers Used in Organic Transistors. Chem. Mater. 2014, 26 (1), 647−663. (11) Street, R. A. Unraveling Charge Transport in Conjugated Polymers. Science 2013, 341 (6150), 1072−1073. (12) Khim, D.; Luzio, A.; Bonacchini, G. E.; Pace, G.; Lee, M.-J.; Noh, Y.-Y.; Caironi, M. Uniaxial Alignment of Conjugated Polymer Films for High-Performance Organic Field-Effect Transistors. Adv. Mater. 2018, 30 (20), 1705463. (13) Matthews, J. R.; Niu, W.; Tandia, A.; Wallace, A. L.; Hu, J.; Lee, W.-Y.; Giri, G.; Mannsfeld, S. C. B.; Xie, Y.; Cai, S.; Fong, H. H.; Bao, Z.; He, M. Scalable Synthesis of Fused Thiophene-Diketopyrrolopyrrole Semiconducting Polymers Processed from Nonchlorinated Solvents into High Performance Thin Film Transistors. Chem. Mater. 2013, 25 (5), 782−789. (14) He, M.; Matthews, J. R.; Sorensen, M. L. Solvent mixture for molecular weight control. U.S. Patent 8,642,719, February 4, 2014. (15) Lu, C.; Lee, W.-Y.; Gu, X.; Xu, J.; Chou, H.-H.; Yan, H.; Chiu, Y.-C.; He, M.; Matthews, J. R.; Niu, W.; Tok, J. B.-H.; Toney, M. F.; Chen, W.-C.; Bao, Z. Effects of Molecular Structure and Packing Order on the Stretchability of Semicrystalline Conjugated Poly(Tetrathienoacene-diketopyrrolopyrrole) Polymers. Adv. Electron. Mater. 2017, 3 (2), 1600311−1600320. (16) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. A stable solutionprocessed polymer semiconductor with record high-mobility for printed transistors. Sci. Rep. 2012, 2, 754. (17) Chang, J. F.; Clark, J.; Zhao, N.; Sirringhaus, H.; Breiby, D. W.; Andreasen, J. W.; Nielsen, M. M.; Giles, M.; Heeney, M.; McCulloch, I. Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 115318. (18) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A general relationship between disorder, aggregation and charge transport in conjugated polymers. Nat. Mater. 2013, 12, 1038−1044. (19) Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. Siloxane-Terminated Solubilizing Side Chains: Bringing Conjugated Polymer Backbones Closer and Boosting Hole Mobilities in ThinFilm Transistors. J. Am. Chem. Soc. 2011, 133, 20130−20133. (20) Mei, J.; Wu, H.-C.; Diao, Y.; Appleton, A.; Wang, H.; Zhou, Y.; Lee, W.-Y.; Kurosawa, T.; Chen, W.-C.; Bao, Z. Effect of Spacer Length of Siloxane-Terminated Side Chains on Charge Transport in Isoindigo-Based Polymer Semiconductor Thin Films. Adv. Funct. Mater. 2015, 25, 3455−3462. (21) Zhang, R.; Li, B.; Iovu, M. C.; Jeffries-EL, M.; Sauvé, G.; Cooper, J.; Jia, S.; Tristram-Nagle, S.; Smilgies, D. M.; Lambeth, D. N.; McCullough, R. D.; Kowalewski, T. Nanostructure Dependence of Field-Effect Mobility in Regioregular Poly(3-hexylthiophene) Thin Film Field Effect Transistors. J. Am. Chem. Soc. 2006, 128 (11), 3480−3481. (22) Khim, D.; Luzio, A.; Bonacchini, G. E.; Pace, G.; Lee, M.-J.; Noh, Y.-Y.; Caironi, M. Uniaxial Alignment of Conjugated Polymer Films for High-Performance Organic Field-Effect Transistors. Adv. Mater. 2018, 30 (20), 1705463. (23) Tsao, H. N.; Cho, D. M.; Park, I.; Hansen, M. R.; Mavrinskiy, A.; Yoon, D. Y.; Graf, R.; Pisula, W.; Spiess, H. W.; Müllen, K. Ultrahigh Mobility in Polymer Field-Effect Transistors by Design. J. Am. Chem. Soc. 2011, 133, 2605−2612.

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DOI: 10.1021/acs.macromol.8b01760 Macromolecules XXXX, XXX, XXX−XXX