Measuring Molecular Order in Poly(3-alkylthiophene) Thin Films with Polarizing Spectroscopies Marc C. Gurau, ² Dean M. Delongchamp, ‡ Brandon M. Vogel, ‡ Eric K. Lin, ‡ Daniel A. Fischer, § Sharadha Sambasivan, § and Lee J. Richter* ,² Surface and Microanalysis Science DiVision, Chemical Science and Technology Laboratory, Polymers DiVision, Materials Science and Engineering Laboratory, and Ceramics DiVision, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, 100 Bureau DriVe, Mail Stop 8372, Gaithersburg, Maryland 20899 ReceiVed June 30, 2006. In Final Form: October 5, 2006 We measured the molecular order of poly(3-alkylthiophene) chains in thin films before and after melting through the combination of several polarized photon spectroscopies: infrared (IR) absorption, variable angle spectroscopic ellipsometry (SE), and near-edge X-ray absorption fine structure (NEXAFS). The data from the various techniques can be uniformly treated in the context of the dielectric constant tensor ǫ for the film. The combined spectroscopies allow determination of the orientation distribution of the main-chain axis (SE and IR), the conjugated π system normal (NEXAFS), and the side-chain axis (IR). We find significant improvement in the backbone order of the films after recrystallization of the material at temperatures just below the melting temperature. Less aggressive thermal treatments are less effective. IR studies show that the changes in backbone structure occur without significant alteration of the structure of the alkyl side chains. The data indicate that the side chains exhibit significant disorder for all films regardless of the thermal history of the sample. Introduction Interest in organic semiconductors has increased significantly because of their potential use in low cost, high volume electronics applications such as radio frequency identification tags, bio- sensors, or photovoltaics. The development of solution pro- cessable organic semiconductors has made it possible to take advantage of fabrication methods such as spin coating, dip coating, or ink-jet printing onto flexible substrates. 1 Of the early solution processable polymers, semiconducting poly(3-alkylthiophenes) (P3ATs) provide relatively high field effect mobilities and have been employed as p-type semiconductors in organic field effect transistors (OFETs) and photovoltaic devices. Reports establish that there exists substantial variability in the device performance that can be achieved with P3AT films. 2-12 Some of the observed variability has been attributed to the development of different film morphologies. 5,8,9 Studies on spin cast P3AT thin films have shown that changes in field effect mobility correlate to changes in the alkyl side-chain length, 2 polymer molecular weight, 9,10 solvent, 11 casting method, 13 and the thermal history of the sample. 10 The regioregular P3ATs are nominally semicrystalline, rigid- rod polymers that are thought to adopt a π stacked, lamellar structure in ordered regions. In early grazing X-ray diffraction studies of poly(3-hexylthiophene) P3HT films, it was found that mobility strongly correlated with molecular orientation. Face-on orientation, where the conjugated backbone lays parallel to the substrate surface, gives rise to lower mobilities than edge-on orientation. 14 For consistently edge-on orientations, higher mobilities were correlated with greater regioregularity via the development of improved π stacking, evidenced by the develop- ment of an interchain exciton-polaron at 2.03 eV. 14,15 The critical structural motif for charge transport has been questioned in recent studies of the molecular weight dependence of the performance of P3HT. Low molecular weight (MW) material has an extremely high crystalline order, forming ribbon-like nanocrystals. However, its mobility is significantly lower than that of high MW material, with small, poorly ordered domains. This observation has led to proposals that either grain boundary effects 9 or conjugation length effects 10 play a dominant role. The problem of optimizing the material parameters of P3ATs is complicated by the realization that the films are rarely in an equilibrium structure. Variations in performance with deposition method (spin vs cast) and conditions (solvent, spin speed) have been reported. 7,11,13,14 In * Corresponding author. E-mail: lee.richter@nist.gov. Phone: (301) 975- 4152. ² Surface and Microanalysis Science Division. ‡ Polymers Division. § Ceramics Division. (1) Gamota, D. R. Printed organic and molecular electronics; Kluwer Academic: Boston, 2003. (2) Babel, A.; Jenekhe, S. A. Synth. Met. 2005, 148 (2), 169-173. (3) Raja, M.; Lloyd, G. C. R.; Sedghi, N.; Eccleston, W.; Di Lucrezia, R.; Higgins, S. J. J. Appl. Phys. 2002, 92 (3), 1441-1445. (4) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280 (5370), 1741- 1744. (5) 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. Synth. Met. 2000, 111, 129-132. (6) Chua, L. L.; Zaumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. Nature (London) 2005, 434 (7030), 194-199. (7) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69 (26), 4108-4110. (8) Kim, D. H.; Park, Y. D.; Jang, Y. S.; Yang, H. C.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S. J.; Chang, T. Y.; Chang, C. W.; Joo, M. K.; Ryu, C. Y.; Cho, K. W. AdV. Funct. Mater. 2005, 15 (1), 77-82. (9) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J. S.; Frechet, J. M. J.; Toney, M. F. Macromolecules 2005, 38 (8), 3312-3319. (10) Zen, A.; Pflaum, J.; Hirschmann, S.; Zhuang, W.; Jaiser, F.; Asawapirom, U.; Rabe, J. P.; Scherf, U.; Neher, D. AdV. Funct. Mater. 2004, 14 (8), 757-764. (11) Chang, J. F.; Sun, B. Q.; Breiby, D. W.; Nielsen, M. M.; Solling, T. I.; Giles, M.; McCulloch, I.; Sirringhaus, H. Chem. Mater. 2004, 16 (23), 4772- 4776. (12) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Frechet, J. M. J. Appl. Phys. Lett. 2005, 86 (12). (13) DeLongchamp, D. M.; Vogel, B. M.; Jung, Y.; Gurau, M. C.; Richter, C. A.; Kirillov, O. A.; Obrzut, J.; Fischer, D. A.; Sambasivan, S.; Richter, L. J.; Lin, E. K. Chem. Mater. 2005, 17 (23), 5610. (14) 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. Nature (London) 1999, 401 (6754), 685-688. (15) Brown, P. J.; Thomas, D. S.; Kohler, A.; Wilson, J. S.; Kim, J. S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Phys. ReV.B 2003, 67 (6). 834 Langmuir 2007, 23, 834-842 10.1021/la0618972 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/19/2006