Fabrication of two-dimensional metal nanoparticle arrays was achieved both by thermal annealing and nanosphere lithography techniques, which are both very fast and economic. The surface plasmon resonance of the fabricated nanostructure was analyzed experimentally by optical absorption spectroscopy and theoretically by three-dimensional finite-difference time-domain (FDTD) calculation. The surface plasmon resonance frequency of the metal nanos- tructure can be modified to match the solar spectrum by adjusting several processing parameters. These results will provide a database of the surface plasmon resonance of different metal nanoparticle arrays with different sizes and shapes.

Nanoscale optical characterization of the metal nanoparticles was performed using near-field scanning optical microscopy (NSOM), and the effects of the surface plasmon were studied. Strong near-field electric field was detected near the metal nanoparticle when using light that matched the surface plasmon resonance of the metal nanoparticles. The induced high electric field only exists at the metal-dielectric interface and decays

exponentially when moving away from the interface. Therefore, the optical absorption should be greatly enhanced near the metal nanoparticles. The scientific impact of these measurements provided necessary evidence as to whether effects of surface plasmon can be used to enhance the power conversion efficiency.

The metal nanoparticle arrays were fabricated on top of transparent conducting oxides (ITO). A standard organic solar cell was subsequently fabricated on top of the array. The metal and the organic layer were very close, and the plasmonic effects can enhance the optical absorption. Standard incident photon-to-current conversion efficiency (IPCE) measurements were performed to determine the plasmonic effects on the conversion efficiency at different optical wavelengths. An organic thin-film solar cell with enhanced conversion efficiency is expected in this study.

Gold nanoparticles fabricated by thermal annealing of gold thin films were studied using NSOM. The transmitted intensity by an NSOM operating at illumination mode was recorded when using light sources with different wavelengths. The transmitted intensity contrast between the gold nanoparticle and the background is higher when the localized surface plasmon resonance of the gold nanoparticle matches the wavelength of the light source. This phenomenon was theoretically confirmed by the three-dimensional FDTD simulations. Therefore, gold nanoparticles with controlled sizes can be distinguished by NSOM in this study, while conventional atomic force microscopy (AFM) can only recognize the existence of nanoparticles. By modifying the surface of metal nanoparticles differently according to their sizes, one can obtain material-specific NSOM images and reveal more material information compared to the current AFM images. Further development of this technique will be beneficial for future nano-imaging of biomolecular studies.

This work was done by Yia-Chung Chang of the Academia Sinica for the Asian Office of Aerospace Research and Development. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Manufacturing & Prototyping category. AFRL-0154

This Brief includes a Technical Support Package (TSP).
Surface-Plasmon-Enhanced Organic Thin-Film Solar Cells

(reference AFRL-0154) is currently available for download from the TSP library.

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This article first appeared in the October, 2010 issue of Defense Tech Briefs Magazine.

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