Nanosphere lithography of nanostructured silver films on thin-film silicon solar cells for light trapping
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1153-A07-14
Nanosphere lithography of nanostructured silver films on thin-film silicon solar cells for light trapping B. Ozturk1, E. A. Schiff1, Hui Zhao1, S. Guha2, Baojie Yan2, and J. Yang2 1 2
Department of Physics, Syracuse University, Syracuse, NY 13244-1130 U.S.A. United Solar Ovonic LLC, 1100 W. Maple Rd., Troy, MI 48084 U.S.A.
ABSTRACT Sparse arrays of evaporated silver nanodisks were fabricated with nanosphere lithography (NSL) on glass substrates and on hydrogenated nanocrystalline silicon solar cells. The optical transmittance spectra for arrays on glass vary substantially with film thickness, and were reasonably consistent with previous work. The quantum efficiency spectra of hydrogenated nanocrystalline silicon solar cells show spectral shifts due to coupling of surface plasmons in the metal nanodisks to the planar waveguide modes of the cells, with overall photocurrent enhancement up to 10%. INTRODUCTION Nanostructured metal films prepared on top of thin silicon photodiodes can give surprisingly strong enhancements of the diode photocurrents at some wavelengths. The effect was discovered about ten years ago by Stuart and Hall [1] for silicon-on-insulator (SOI); it is due to the coupling of the surface plasmon excitations in the metal nanostructures to the planar waveguide modes in the photodiodes. The effect is now being assessed by several laboratories for its possible utility in thin-film solar cells [2,3,4]; it is an alternative to the use of textured substrates in solar cells, which produce enhanced photocurrents through stochastic light-trapping [5]. Surface plasmon resonance is the collective oscillation of the conduction electrons near metal surfaces. The resonant frequency of the sub-wavelength-size metallic particles depends on their size and shape as well as the dielectric into which they are embedded. At this frequency, particles interact with light and result in an extinction dip in the transmission spectrum reflecting both scattering of light and absorption (thermalization) within the nanoparticle. Absorption dominates if the particle size is very small compared to the wavelength [6] (i.e., d ≤ 10 nm). The absorbed light is dissipated as heat to the system and this is not desirable for the solar cell application. Scattering dominates with the increasing size and can be utilized to couple certain wavelengths of the incident light into the waveguide modes of a thin-film solar cell. A schematic diagram of the thin film solar cell cross section is shown in Figure 1. There are several technologies for creating nanostructured metal films on top of solar cells. Most previous research on plasmonic photocurrent enhancements in photodiode structures has used evaporated silver films that are subsequently annealed above 200°C to produce irregular nanostructured films. This method is the only one that has given substantial photocurrent enhancements (about tenfold) in photodiodes and silicon solar cells [1,3] but the high-temperatures of annealing are a difficulty for use with hydrogenated amorphous
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