Research Article www.acsami.org
A Physicochemical Approach Toward Extending Conjugation and the Ordering of Solution-Processable Semiconducting Polymers Minjung Lee,†,§ Hyeonyeol Jeon,‡,§ Mi Jang,† and Hoichang Yang*,† †
Department of Applied Organic Materials Engineering, Inha University, Incheon 22212, Republic of Korea Center for Materials Architecturing, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Poly(3-hexylthiophene)s (P3HTs) were synthesized with a well-controlled molecular weight (Mw) and degree of regioregularity; additionally, π-conjugated P3HT structures in both solutions and films were systematically investigated. Conjugated P3HT phases in spin-cast films significantly changed from ordered nanorods, -fibrils, and -ribbons to lessordered granules, depending on the conformation of the P3HT chains in solutions. The chain conformations could be physicochemically adjusted by modifying chain lengths (from 5 to 45 kDa), solvents, and ultrasonication. Highly extended conformations of the P3HT in ultrasound-treated solutions yielded longer degree of conjugation both the intra- and intermolecularly. When toluene was used as a marginal solvent, ultrasonicated 0.1 wt % 29 kDa P3HT solutions could be used to yield highly ordered aggregates in spin-cast films, including nanoribbons or nanosheets, with field-effect mobility (μFET) up to ∼0.1 cm2 V−1 s−1 being measured for organic field-effect transistors (OFETs). However, ultrasonicated chloroform systems with good P3HT solubility (for P3HT Mw ≥ 20 kDa) yielded featureless conducting layers even at 0.4 wt % P3HT content. However, these film-based OFETs yielded μFET values up to 0.04 cm2 V−1 s−1, which were much greater than 0.004 cm2 V−1 s−1 for the nonultrasonicated systems. KEYWORDS: organic field-effect transistor, poly(3-hexylthiophene), conjugation, self-assembly, ultrasonication, nanoribbon
1. INTRODUCTION π-Conjugated semiconducting polymers are emerging as materials with wide electronic applications due to their tunable conductivity properties via controlling the direction of πconjugation and through nano/microscale self-assembly. Electronic applications include light-emitting diodes, field effect transistors (FETs), photovoltaics, and sensors, and are continually expanding into many new areas.1 One critical factor determining the function of organic-based electronics is the π-conjugated ordering of semiconducting polymers into a solid state from a dilute solution. Chain conformations of the dissolved (or isolated) polymers in organic solutions are significantly affected by intrinsic molecular structures, specifically, backbone rigidity, molecular weight (Mw), and solventcompatible components.2−10 Structural control of π-conjugated polymers in the solid state is essential to the improvement of charge-carrier transport along the intra- and intermolecular conjugated regions. Most semiconducting polymers with broad Mw distributions do not readily assemble to form long-range ordered crystal structures.3 However, Salleo et al. recently reported that the limiting charge transport step in high-Mw semiconducting polymers was often trapped by lattice disorder, though the short-range intermolecular aggregates of well-conjugated (i.e., less-twisted) backbone chains were sufficient for efficient long-range charge transport.11 © 2016 American Chemical Society
One form of the alkyl-substituted semiconducting polymers, poly(3-hexylthiophene) (P3HT) preferentially grows into onedimensional (1D) nanofibrils of π-conjugated “edge-on” chains on gate dielectrics under general conditions,12 with exceptions for specific environments.13 P3HT-based OFETs have yielded field-effect mobility (μFET) values ranging from 10−4 to 0.1 cm2 V−1 s−1, depending on the degree of regioregularity (RR),14 Mw,2,4−8,10 and processing condition.15−17 Kline et al. reported that the electrical properties of cast P3HT films in OFETs were significantly affected by the degree of connectivity between the fibrillar crystallites: low-Mw P3HT ( 105 > 105 > 105 > 105 ∼ 106 > 106
0.006 0.012 0.003 0.042 0.016 0.067
0.003 0.004 0.001 0.005 0.002 0.030
μFET (cm2V−1s−1)
Vth (V)
ION/IOFF
± ± ± ± ± ±
0.0 −4.0 −7.0 +3.0 −12.0 −2.50
> 104 ∼ 105 > 105 > 105 ∼ 105 > 105
0.003 0.006 0.004 0.035 0.003 0.045
0.001 0.002 0.001 0.003 0.001 0.003
transfer curves for Mw-dependent P3HT OFETs. Table 2 summarizes the μFET, Vth, and the ION/IOFF values for the OFETs fabricated from both toluene and CF systems. As expected from the morphological results (Figures 3a and 5a,b,d,e), the 13, 20, and 29 kDa P3HT OFETs fabricated from the ultrasonicated toluene systems yielded the average μFET values of 0.01, 0.04, and 0.07 cm2 V−1 s−1 (up to ∼0.10 cm2 V−1
polymer chain ordering was improved in ultrasonicated solution and the corresponding films exhibited different structures after ultrasonication. These results will be interestingly matched with the performance of OFETs later. Electrical properties of the ultrasonic-driven crystallites for the 13, 20, and 29 kDa P3HTs were investigated by fabricating the corresponding OFETs. Figure 7 shows the typical ID−VG 4824
DOI: 10.1021/acsami.5b12552 ACS Appl. Mater. Interfaces 2016, 8, 4819−4827
Research Article
ACS Applied Materials & Interfaces s−1), respectively. In particular, the maximum μFET values for the 29 kDa P3HT OFETs were 3 times greater than 0.03 cm2 V−1 s−1 of the best OFETs including conjugated P3HT fibrils fabricated from the nonultrasonicated solutions. In contrast, the 13 kDa P3HT OFETs with short nanorods percolated layers exhibited similar μFET values as those of the nonultrasonicated system (Figure S5 and Table S1). Similar to the toluene system, enhanced OFET performance was yielded via ultrasonication of the chloroform system. As expected from the UV−vis spectra (Figure 2b), the ordered aggregates induced via ultrasonication resulted in correspondingly high performance of OFETs. Particularly, the 20 and 29 kDa P3HT OFETs yielded 9 and 15 times greater average μFET values after ultrasonication. These results demonstrated that relatively volatile and well-dissolving P3HT solvents could be used to obtain higher electrical properties in OFETs, only through facile ultrasonication of the polymer solution for a short period of time (≤10 min). Additionally, Mw-dependent variation of the μFET for the above OFET results can be seen in Figure 8. For both solvent
sonication, the conformational transition from a twisted and coil-like chain to an extended and planar chain in solution increased the coplanar stacking between the extended chains, yielding dimensional expansion in the crystal structures. The resulting 29 kDa P3HT film exhibited excellent electrical OFET properties, yielding values up to ∼0.1 cm2 V−1 s−1.
4. CONCLUSION A well-defined poly(3-hexylthiophene) (P3HT) series was synthesized to investigate the Mw-dependent conjugation extension and crystallization behaviors of the polymer chains. A synthesized P3HT series had a Mn ranging from 5 to 45 kDa, a narrow PDI < 1.30, and a high RR greater than 95%. As expected, percolated nanorods of the low-Mw P3HTs (less than 13 kDa) in the spin-cast films yielded poor μFET values of less than 0.002 cm 2 V −1 s −1 for OFETs, while the interconnected fibrillar structures of P3HTs with sufficient Mw (≥13 kDa) improved the μFET values up to 0.04 cm2 V−1 s−1. Furthermore, direct self-assembly of P3HT chains in solution pretreated via ultrasonication were significantly investigated to expand the dimension of crystallites in conjugated structures using different two solvent systems, toluene and chloroform. Interestingly, enhanced crystallites of P3HT could be developed directly onto the gate dielectrics via spin-casting an ultrasonic-treated 0.1 wt % toluene solution: 1D fibrils and 2D nanoribbons. In particular, nanoribbons developed from the 29 kDa P3HT exhibited excellent electrical OFET properties, yielding μFET values up to ∼0.1 cm2 V−1 s−1.
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Figure 8. Mw-dependent field-effect mobility (μFET) variation of P3HT films before and after ultrasonication in a solution containing: (a) 0.1 wt % toluene or (b) 0.4 wt % CF.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12552. GPC, DSC, 1H NMR, AFM, etc. (PDF)
systems, enhancement of the μFET was greater with the order of Mw. This could be explained by the fact that the longer chains with limited ordering due to entanglements or twisting in solution are possible to form well-ordering after disentanglement and extension of the chains in solution via ultrasonication, similar to prior studies.28−33 By controlling the conjugation and ordering of the welldefined P3HT series with Mw values ranging from 5 to 45 kDa, we confirmed that the chain conformation and molecular interaction of P3HTs with a twistable thiophene backbone was significantly affected by the conjugated chain lengths, that is, Mw. Chain entanglement of P3HTs dissolved in the solutions was accelerated with an increase in Mw for general systems. By ultrasonicating the dilute P3HT solutions, however, the chain conformation and molecular interaction of P3HTs were changed differently depending on the polymer Mw. First, the low-Mw polymer chains seemed to be completely dispersed in the ultrasonicating solutions because the short chains possessed weaker intermolecular π−π interactions than the dissipation energy imparted via ultrasonication. By contrast, during ultrasonication, middle-Mw and high-Mw P3HTs showed that the twisted or entangled chain conformation of the P3HTs changed to a stretched or elongated chain conformation. Each extended chain could then directly form self-assembled aggregates in the ultrasonic-treated solutions, with strong π−π interactions. These conjugated aggregates could grow into various crystalline structures in dilute solutions: 1D nanorod, fibril, microscale whisker and 2D nanoribbons or nanosheets. Particularly, ultrasonication against the dilute 29 kDa P3HT solution was very effective toward directly forming long-range and highly conjugated 2D crystallites. During further ultra-
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +82-32-860-7494. Author Contributions §
M.L. and H.J. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by grants from the Inha University Research Grant, the Center for Advanced Soft Electronics under the Global Frontier Research Program (2012M3A6A5055225) and General Research Program (2013R1A12063963) of the Ministry of Education, Science, and Technology (MEST), Korea.
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ABBREVIATIONS P3HT poly(3-hexylthiophene) PS-Si(CH3)2Cl dimethylchlorosilane-terminated polystyrene gPS grafted polystyrene SiO2 silicon dioxide Mw molecular weight Mn number-average molecular weight Mw weight-average molecular weight PDI polydispersity index RR regioregularity 4825
DOI: 10.1021/acsami.5b12552 ACS Appl. Mater. Interfaces 2016, 8, 4819−4827
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ACS Applied Materials & Interfaces UVO3 GB bp GPC NMR UV−vis AFM GIXD OFET
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