Silver Nanowire Arrays : Fabrication and Applications

Abstract

Nanowire arrays have increasingly received attention for their use in a variety of applications such as surface-enhanced Raman scattering (SERS), plasmonic sensing, and electrodes for photoelectric devices. However, until now, large scale fabrication of device-suitable metallic nanowire arrays on supporting substrates has seen very limited success. This thesis describes my work rst on the development of a novel successful processing route for the fabrication of uniform noble metallic (e.g. Ag and Au) nanowire arrays on ITO (and other) substrates. Ag nanowire arrays are studied in detail. I use an in situ oxygen plasma cleaning process and a sputtered Ti layer to enhance the adhesion between the anodic aluminum oxide (AAO) template and the ITO glass. An ultrathin gold layer (2 nm) is deposited as a nucleation layer for the electrodeposition of silver. Furthermore, a stable cyanide-free electrolyte compatible with the AAO templates is developed. Ultimately, an unprecedented high level of uniformity and control of the nanowire diameter, spacing, and length has been achieved. Moving forward, new architectures for organic solar cells are employed, based on such nanostructured electrodes, covered with a separate zinc oxide (ZnO) layer. Two types of solar cells are presented in this thesis. One is a traditional opaque solar cell with a thick (100 nm) Ag back contact. The other is a semi-transparent solar cell with a transparent top electrode (consisting mainly of Ca:Ag). Complementary characterization tools enable further in-depth investigation of the optical and electronic properties of such devices. The results show that the Ag nanowire arrays seem to enhance the charge collection e ciency. However, using too thick a polymer lm (which is necessary to ensure a working device) combined with the increased nanowire-polymer interface, introduces extended defect sites for recombination, which limits the e ciency of the nanowire solar cell. Finally, this work yields a clearer design route for Ag nanowire organic solar cells, suggesting a use of a proper semiconductor layer with low recombination rate as well as a very thin polymer layer to conformally coat the Ag nanowire arrays. In this way, the nanowire solar cells not only make use of the charge collection and light trapping advantages of Ag nanowires, but also minimize recombination losses. Additional techniques explored on metallic nanowire arrays as part of this project included Raman and low-temperature (< 80 K) angle-dependent UV-Vis, which pave the way for potential applications in SERS, temperature sensing, and many other areas.