Grafting of Conjugated Polymers on Oxide Surfaces

1. UW MRSEC DMR-0520527 Juan J. de Pablo, PI University of Wisconsin-Madison Materials Research Science & Engineering Center on Nanostructured Interfaces Grafting of Conjugated Polymers on Oxide Surfaces P. Paoprasert, J. Spalenka, D.L. Peterson, R. Ruther, R.J. Hamers, P.G. Evans, P. Gopalan Despite tremendous progress made in the synthesis of surface-grafted polymer brushes, there have been few reports of polymer brushes in organic electronics. Polymer brushes are promising for organic electronics because they exhibit a higher degree of ordering and better uniformity than spin-cast films. In the conventional “grafting-from” method, the synthesis of conjugated polymers with controlled structure and composition usually requires reaction conditions and techniques that cannot be easily employed on inorganic surfaces. We report a versatile “grafting-to” approach based on click chemistry to prepare poly(3-hexylthiophene) (P3HT) brushes on oxide substrates. P3HT is an important polymeric semiconductors in electronic applications such as field-effect transistorsand photovoltaics.Regioregular ethynyl-terminated P3HT with molecular weight of 5900 g/mol and polydispersity of 1.2 was synthesized by catalyst-transfer polycondensation using Grignard metathesis mediated by a nickel-based catalyst. The azide monolayer was prepared from bifunctional molecules containing an azide click reaction precursor and a siloxane surface linker. The P3HT brushes were characterized by atomic force microscopy, ellipsometry, X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and UV-visible spectroscopy. The grafting of P3HT brushes was studied as a function of click reaction time and the growth of the brushes is governed by a diffusion-controlled process. P3HT brushes were prepared on the pre-fabricated field-effect transistor structures in order to probe the electrical properties of the brushes. The saturation hole mobility for P3HT brushes measured in an FET structure was 5 x 10-5 cm2/Vs, which is comparable to hole mobilities in FETs with monolayer-thick dip-coated P3HT. We have further shown the generalization of this chemistry to the wide-bandgap semiconductor ZnO, which is especially challenging as it requires mild conditions to prevent etching ZnO. The versatile synthetic methodology developed in this work can be generalized to prepare a wide variety of semiconducting conjugated polymer brushes on oxide surfaces relevant to organic electronic devices. c) d) Figure Caption: a) Illustration of the bottom-contact FET device configuration with P3HT brushes, b) transistor characteristic curves of P3HT brushes measured at drain voltage of -20 V, c) UV-visible spectra of P3HT brushes on glass substrate, and d) thicknesses of P3HT brushes measured by ellipsometry as a function of reaction times. Figure Caption: Synthetic scheme for a) 3-azidopropyltrimethoxysilane (1).b)2,5-dibromo-3-hexylthiophene (2) and ethynyl-terminated P3HT (3).c)anchoring of ethynyl-terminated P3HT(3) by click chemistry to a SAM of azide (1) . Paoprasert, P.; Spalenka, J. W.; Peterson, D. L.; Ruther, R. E.; Hamers, R. J.; Evans, P. G.; Gopalan, P, J. Mater. Chem.2010. 20, 2651-2658.