Comparison of Molecular Monolayer Interface Treatments in Organic-inorganic Photovoltaic Devices
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Comparison of Molecular Monolayer Interface Treatments in Organic-inorganic Photovoltaic Devices Jamie M. Albin1, Darick J. Baker1, Cary G. Allen1, Thomas E. Furtak1, Reuben T. Collins1,2, Dana C. Olson3, David S. Ginley3, Christian C. Weigand4, Astrid-Sofie Vardoy4, Cecile Ladam5 1 Colorado School of Mines, Golden, CO; 2Renewable Energy Materials Research Science and Engineering Center, Golden, CO; 3National Renewable Energy Laboratory Golden, CO; 4 Norwegian University of Science and Technology, Trondheim, Norway; 5SINTEF, Trondheim, Norway ABSTRACT In this study, we explore the effects of alkyl surface terminations on ZnO for inverted, planar ZnO/poly(3-hexylthiophene) (P3HT) solar cells using two different attachment chemistries. Octadecylthiol (ODT) and octadecyltriethoxysilane (OTES) molecules were used to create 18-carbon alkyl surface molecular layers on sol gel-derived ZnO surfaces. Molecular layer formation was confirmed and characterized using water contact angle measurements, infrared (IR) transmission measurements, and X-ray photoelectron spectroscopy (XPS). The performances of the ZnO/P3HT photovoltaic cells made from ODT- and OTES-functionalized ZnO were compared. The ODT-modified devices had higher efficiencies than OTES-modified devices, suggesting that differences in the attachment scheme affect the efficiency of charge transfer through the molecular layers at the treated ZnO surface. INTRODUCTION Excitonic hybrid organic-inorganic solar cells are gaining viability as alternatives to p-n junction photovoltaics. Although hybrid cells typically have lower efficiencies than their inorganic counterparts, they are more compatible with inexpensive manufacturing techniques such as spray deposition and roll-to-roll processing, which can reduce the fabrication cost per photovoltaic watt. Polymer devices with nanostructured ZnO as the electron-accepting layer have the potential to improve carrier collection and power conversion efficiency in the bulk heterojunction approach to organic solar cells [1]. The ZnO/polymer interface, however, is not optimal and properties such as polymer ordering, wetting at the interface, and charge transfer across the interface need improvement. Functionalization of the ZnO surface with molecular monolayers has the potential to resolve these issues [2,3]. Monson (Hsu is not the first author) et al. observed increased current density with alkyl chain length in alkylthiol-modified ZnO/P3HT devices despite the fact that the molecules are expected to lengthen the electron tunneling barrier [2]. In this study, we demonstrate formation of surface molecular layers on sol gel-derived ZnO using octadecylthiol (ODT) and octadecyltriethoxysilane (OTES). Both molecules create an 18carbon chain alkyl surface termination allowing a comparison of ZnO surfaces functionalized with the same end group but using different attachment chemistries. Molecular layer formation was characterized using water contact angle measurements, infrared (IR) transmission measurements, and X-ray photoel
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