J. Phys. Chem. C 2009, 113, 3903–3908
3903
ARTICLES Lithium Perchlorate-Doped Poly(styrene-b-ethylene oxide-b-styrene) Lamellae-Forming Triblock Copolymer as High Capacitance, Smooth, Thin Film Dielectric Jihua Chen, C. Daniel Frisbie,* and Frank S. Bates* Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455, USA ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: December 12, 2008
A symmetric poly(styrene-b-ethylene oxide-b-styrene) (SOS) triblock copolymer with block molecular weights (Mn) of 7k-14k-7k was synthesized by anionic polymerization. Various amounts of lithium perchlorate salt ([O/Li] ) 3, 6, 12, 24, and 48) were incorporated into the SOS block copolymer to achieve an elevated charge carrier density, which also led to lamellae formation and reduced crystallinity of the poly(ethylene oxide) block according to small-angle X-ray scattering and differential scanning calorimetry. Spin-coated SOS/LiClO4 capacitors of 30 nm thickness exhibit very high capacitances (up to 4.5 µF/cm2 at 10 Hz and 1.5 µF/cm2 at 1000 Hz with an amplitude of 0.1 V) and rather smooth surfaces (root-mean-square roughness of 0.5-2 nm). We expect to optimize these thin-film capacitors for high-performance organic thin film transistor applications. I. Introduction Concurrent with the ongoing search for new organic semiconductors of high charge carrier mobility, efforts to improve gate dielectrics have become a recent research focus to further enhance the performance of current organic thin film transistors (OTFTs).1-4 High-capacitance dielectric materials, including self-assembling monolayers,5,6 polar polymers,7-15 organic/ inorganic composite films,16,17 ion gels,18,19 and inorganic dielectrics2,3 have been used to significantly decrease the operation voltage of OTFTs. Stand-alone poly(ethylene oxide) (PEO) and PEO-containing block copolymers, together with ionic salts of alkali metals (e.g., lithium perchlorate LiClO4), are well-known as high-conductivity solid polymer electrolytes and potential candidate materials for batteries and fuel cell membranes.20-22 These materials also have exceptionally large specific capacitances in the range of 5-110 µF/cm2 at low frequencies. Correspondingly, PEO/ LiClO4 gated organic thin film transistors have been shown to have very high gate-induced hole concentrations (up to 1015 charges/cm2), which leads simultaneously to both low voltage operation and high current output.23-27 However, the uncontrolled morphology of stand-alone PEO films (in particular, the presence of gigantic spherulites (spherical semicrystalline regions)) causes a very rough surface with excessive topographic features visible by optical microscopy. Thus, only the top-gate configuration has been achieved to this date in PEO-gated OTFTs.23-27 We seek to make smooth, high-capacitance PEObased films that allow bottom-gate OTFT configurations, because this will enhance the variety of organic semiconductor films that can be paired with this high-capacitance dielectric. The coupling of polystyrene (PS) blocks with PEO was shown to effectively confine the crystallization of PEO blocks between * To whom correspondence should be addressed. E-mail addresses:
[email protected] (C.D.F.),
[email protected] (F.S.B.).
PS microdomains.28-33 Previous study by Epps and co-workers demonstrated that selective doping of the PEO domains with LiClO4 tended to drive microphase separation toward stronger segregation in both poly(styrene-b-isoprene-b-ethylene oxide) and poly(isoprene-b-styrene-b-ethylene oxide), which consequently led to a dramatic increase in the order-disorder transition temperature as well as the disappearance of network phases.34,35 In this work, through lithium perchlorate doping of a symmetric SOS triblock copolymer, a combination of high capacitance and surface smoothness was obtained in solid polymer electrolyte thin films (Figure 1). The specific capacitances achieved in this work (up to 4.5 µF/cm2 at 10 Hz and 1.5 µF/cm2 at 1000 Hz with an amplitude of 0.1 V), although lower than those of ion gels (40 µF/cm2 at 10 Hz)18,19 and conventional PEO/LiClO4 systems (5-110 µF/cm2 at 0.1-0.01 Hz),23-27 are superior to those typically obtained for silicon dioxide (1 V). Possibilities of using a super-thin (∼2 nm) and smooth encapsulation layer on top of the SOS/LiClO4 films are currently under investigation to further improve the SOS/LiClO4 surface smoothness and minimize the effect of moisture without significantly sacrificing the capacitance performance. IV. Conclusion Sequential living anionic polymerization was used to synthesize a poly(styrene-b-ethylene oxide-b-styrene) (SOS) triblock copolymer (Mn ) 7k-14k-7k). After doping with lithium perchlorate salts ([O/Li] ) 3, 6, 12, 24, and 48), the SOS block copolymer formed lamellar structures, as evidenced by smallangle X-ray scattering and atomic force microsopy. Spin-coated SOS/LiClO4 capacitors have very high specific capacitances (up
to 4.5 µF/cm2 at 10 Hz and 1.5 µF/cm2 at 1000 Hz with an amplitude of 0.1 V), depending upon the lithium concentration. A rather smooth surface (root-mean-square roughness < 1 nm) was achieved in SOS/LiClO4 films with proper spin conditions, which will be important for potential application of this material as a gate dielectric in organic thin film transistors. Acknowledgment. Support for this work was provided by the Department of Energy through a subcontract to UT-Battelle (No. 4000041622). Parts of this work were carried out in the University of Minnesota I.T. Characterization Facility, which receives support in part from NSF through the NNIN program. J.C. is grateful for constructive discussions with and technical help from Jian Qin, Dr. Yiyong He, Adam Meuler, Mike Bluemle, Dr. Chris Ellison, Sangwoo Lee, C. Guillermo Alfonzo, Dr. Guillaume Fleury, Kevin Davis, Zach Thompson, Ameara Mansour, Dr. Wei Fan, Dr. Jiyoul Lee, Dr. Jung-Yong Kim, David Ellison, Vivek Kalihari, Yan Liang, Yu Xia, Bryan Boudouris, Derek Stevens, Dr. David Gilles, and all other Bates group and Frisbie group members. Supporting Information Available: Supporting Information includes (1) differential scanning calorimetry data of SOS/ lithium perchlorate complexes as a function of [O/Li] ratio, and (2) variable-temperature small-angle X-ray scattering results of SOS/ lithium perchlorate with [O/Li] ) 3:1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sirringhaus, H. AdV. Mater. 2005, 17 (20), 2411–2425. (2) Facchetti, A.; Yoon, M. H.; Marks, T. J. AdV. Mater. 2005, 17 (14), 1705–1725. (3) Veres, J.; Ogier, S.; Lloyd, G.; de Leeuw, D. Chem. Mater. 2004, 16 (23), 4543–4555. (4) Chua, L. L.; Zaumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434 (7030), 194–199. (5) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schutz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature 2004, 431 (7011), 963–966. (6) Zschieschang, U.; Halik, M.; Klauk, H. Langmuir 2008, 24 (5), 1665–1669. (7) Kim, C.; Wang, Z. M.; Choi, H. J.; Ha, Y. G.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130 (21), 6867–6878. (8) Liu, P.; Wu, Y. L.; Li, Y. N.; Ong, B. S.; Zhu, S. P. J. Am. Chem. Soc. 2006, 128 (14), 4554–4555. (9) Yang, S. Y.; Kim, S. H.; Shin, K.; Jeon, H.; Park, C. E. Appl. Phys. Lett. 2006, 88 (17), 173507. (10) Yoon, M. H.; Yan, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127 (29), 10388–10395. (11) Schroeder, R.; Majewski, L. A.; Grell, M. AdV. Mater. 2004, 16 (7), 633. (12) Chua, L. L.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. Appl. Phys. Lett. 2004, 84 (17), 3400–3402. (13) Chua, L. L.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. AdV. Mater. 2004, 16 (18), 1609. (14) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. AdV. Mater. 2002, 14 (23), 1717. (15) Halik, M.; Klauk, H.; Zschieschang, U.; Kriem, T.; Schmid, G.; Radlik, W.; Wussow, K. Appl. Phys. Lett. 2002, 81 (2), 289–291. (16) Cao, Q.; Xia, M. G.; Shim, M.; Rogers, J. A. AdV. Funct. Mater. 2006, 16 (18), 2355–2362. (17) Hwang, D. K.; Lee, K.; Kim, J. H.; Im, S.; Kim, C. S.; Baik, H. K.; Park, J. H.; Kim, E. Appl. Phys. Lett. 2006, 88 (24), 243513. (18) Lee, J.; Panzer, M. J.; He, Y. Y.; Lodge, T. P.; Frisbie, C. D. J. Am. Chem. Soc. 2007, 129 (15), 4532. (19) Cho, J. H.; Lee, J.; He, Y.; Kim, B.; Lodge, T. P.; Frisbie, C. D. AdV. Mater. 2008, 20, 686. (20) Przyluski, J.; Such, K.; Wycislik, H.; Wieczorek, W.; Florianczyk, Z. Synth. Met. 1990, 35 (1-2), 241–247. (21) Ferloni, P.; Chiodelli, G.; Magistris, A.; Sanesi, M. Solid State Ionics 1986, 18-9, 265–270. (22) Nagaoka, K.; Naruse, H.; Shinohara, I.; Watanabe, M. J. Polym. Sci., Part C: Polym. Lett. 1984, 22 (12), 659–663. (23) Panzer, M. J.; Frisbie, C. D. J. Am. Chem. Soc. 2007, 129 (20), 6599–6607.
3908 J. Phys. Chem. C, Vol. 113, No. 10, 2009 (24) Panzer, M. J.; Frisbie, C. D. AdV. Funct. Mater. 2006, 16 (8), 1051– 1056. (25) Panzer, M. J.; Frisbie, C. D. Appl. Phys. Lett. 2006, 88 (20), 203504. (26) Panzer, M. J.; Newman, C. R.; Frisbie, C. D. Appl. Phys. Lett. 2005, 86 (10), 103503. (27) Panzer, M. J.; Frisbie, C. D. J. Am. Chem. Soc. 2005, 127 (19), 6960–6961. (28) Zhu, L.; Cheng, S. Z. D.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Lotz, B. Polymer 2001, 42 (13), 5829–5839. (29) Zhu, L.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Cheng, S. Z. D.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Liu, L. Z.; Lotz, B. Macromolecules 2001, 34 (5), 1244–1251. (30) Xu, J. T.; Yuan, J. J.; Cheng, S. Y. Eur. Polym. J. 2003, 39 (11), 2091–2098. (31) Sun, L.; Zhu, L.; Ge, Q.; Quirk, R. P.; Xue, C. C.; Cheng, S. Z. D.; Hsiao, B. S.; Avila-Orta, C. A.; Sics, I.; Cantino, M. E. Polymer 2004, 45 (9), 2931–2939. (32) Li, Y. J.; Kaito, A. Eur. Polym. J. 2006, 42 (9), 1986–1993. (33) Huang, P.; Zhu, L.; Guo, Y.; Ge, Q.; Jing, A. J.; Chen, W. Y.; Quirk, R. P.; Cheng, S. Z. D.; Thomas, E. L.; Lotz, B.; Hsiao, B. S.; AvilaOrta, C. A.; Sics, I. Macromolecules 2004, 37 (10), 3689–3698. (34) Epps, T. H.; Bailey, T. S.; Pham, H. D.; Bates, F. S. Chem. Mater. 2002, 14 (4), 1706–1714. (35) Epps, T. H.; Bailey, T. S.; Waletzko, R.; Bates, F. S. Macromolecules 2003, 36 (8), 2873–2881. (36) Pinto, J. C.; Whiting, G. L.; Khodabakhsh, S.; Torre, L.; Rodriguez, A. B.; Dalgliesh, R. M.; Higgins, A. M.; Andreasen, J. W.; Nielsen, M. M.; Geoghegan, M.; Huck, W. T. S.; Sirringhaus, H. AdV. Funct. Mater. 2008, 18 (1), 36–43.
Chen et al. (37) Koo, C. M.; Hillmyer, M. A.; Bates, F. S. Macromolecules 2006, 39 (2), 667–677. (38) Epps, T. H.; Cochran, E. W.; Bailey, T. S.; Waletzko, R. S.; Hardy, C. M.; Bates, F. S. Macromolecules 2004, 37 (22), 8325–8341. (39) Ruzette, A. V. G.; Soo, P. P.; Sadoway, D. R.; Mayes, A. M. J. Electrochem. Soc. 2001, 148 (6), A537–A543. (40) Spangler, L. L.; Torkelson, J. M.; Royal, J. S. Polym. Eng. Sci. 1990, 30 (11), 644–653. (41) Flack, W. W.; Soong, D. S.; Bell, A. T.; Hess, D. W. J. Appl. Phys. 1984, 56 (4), 1199–1206. (42) Lawrence, C. J. Phys. Fluids 1988, 31 (10), 2786–2795. (43) Podgrabinski, T.; Hrabovska, E.; Svorcik, V.; Hnatowicz, V. J. Mater. Sci.-Mater. Electron. 2005, 16 (11-12), 761–765. (44) Robitaille, C. D.; Fauteux, D. J. Electrochem. Soc. 1986, 133 (2), 315–325. (45) Gray, F. M.; Vincent, C. A.; Kent, M. J. Polym. Sci., Part B: Polym. Phys. 1989, 27 (10), 2011–2022. (46) Bailey, T. S.; Hardy, C. M.; Epps, T. H.; Bates, F. S. Macromolecules 2002, 35 (18), 7007–7017. (47) Marzantowicz, M.; Dygas, J. R.; Krok, F.; Florjanczyk, Z.; ZygadloMonikowska, E. J. Non-Cryst. Solids 2006, 352 (42-49), 5216–5223. (48) Berthier, C.; Gorecki, W.; Minier, M.; Armand, M. B.; Chabagno, J. M.; Rigaud, P. Solid State Ionics 1983, 11 (1), 91–95. (49) Liu, X. J.; Osaka, T. J. Electrochem. Soc. 1996, 143 (12), 3982–3986. (50) Young, W.-S.; Brigandi, P. J.; Epps, T. H. Macromolecules 2008, 41 (17), 6276–6279.
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