Octadecyltriethoxysilane Surface Modification of Zinc Oxide
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Octadecyltriethoxysilane Surface Modification of Zinc Oxide Cary G. Allen1, Darick J. Baker1, Thomas E. Furtak1, Reuben T. Collins1, Matthew S. White2, Dane T. Gillaspie2, Dana C. Olson2, and David S. Ginley2 1 Physics, Colorado School of Mines, 1523 Illinois St., Golden, CO, 80401 2 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401 ABSTRACT Zinc Oxide (ZnO) is actively investigated for hybrid organic inorganic device applications. The interface greatly influences the electronic properties of these devices. Molecular surface modification of ZnO is being investigated for its potential to control the alignment of energy levels, charge transfer, as well as, interfacial chemical characteristics that influence device fabrication. In this study, octadecyltriethoxysilane (OTES) treatments of thin film ZnO produced by sol-gel decomposition were explored. The ZnO films were hydroxylated and then modified using OTES in solution. The condensation reaction of the OTES at the surface was promoted by the addition of a protoamine catalyst. Contact angle and infrared spectroscopy studies confirmed the surface modification and indicated that the coverage of the OTES was submonolayer. The modified ZnO films were reproducible and stable for long periods. The effects of the modification on subsequently spin-cast poly[3-hexylthiophene](P3HT) and on hybrid ZnO/P3HT organic solar cell performance are discussed. INTRODUCTION Conducting metal oxides are used as a component in organic electronic devices such as polymeric [1] and Gratzel solar cells [2] and organic light emitting diodes [3]. The roles of metal oxide in these devices extend from more passive transparent electrodes [4, 5] to active involvement in charge separation and collection [1, 6, 7]. In each instance, optimizing charge transfer between the organic and metal oxide is often essential to device performance. ZnOpolymer solar cells are members of an important class of these devices and are presently under active investigation [1, 6, 8, 9]. In hybrid polymer solar cells, photoexcitation creates excitons within the polymer. An interface between an electron acceptor (ZnO) and the polymer (P3HT) allows the excitons, which have a relatively large binding energy, to dissociate into free carriers. Because the exciton diffusion length is quite short (~10 nm or less), the active region of the device is limited to very near the interface [10]. Optimizing the morphology of the polymer at the interface without negatively impacting charge transfer could ultimately result in improved hybrid solar cell performance. Self-assembled or covalently bonded organic monolayers [11, 12, 13] are a common surface modification strategy and have been studied widely on silicon dioxide [14, 15]. Here we explore octadecylsiloxane (ODS) modification of the ZnO surface. ODS modification of ZnO is motivated by results indicating that alkyl chains can improve local polymer structure. For example, P3HT thin film transistors formed on alkyl- modified SiO2 show large increases
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