Plasmonic Light-trapping and Quantum Efficiency Measurements On Nanocrystalline Silicon Solar Cells and Silicon-On-Insul
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1245-A03-02
Plasmonic Light-trapping and Quantum Efficiency Measurements on Nanocrystalline Silicon Solar Cells and Silicon-On-Insulator Devices Hui Zhao1, Birol Ozturk1, E. A. Schiff1, Baojie Yan2, J. Yang2 and S. Guha2 1 2
Department of Physics, Syracuse University, Syracuse, New York 13244-1130, U.S.A. United Solar Ovonic LLC, Troy, Michigan 48084, U.S.A.
ABSTRACT Quantum efficiency measurements in nanocrystalline silicon (nc-Si:H)solar cells deposited onto textured substrates indicate that these cells are close to the "stochastic lighttrapping limit" proposed by Yablonovitch in the 1980s. An interesting alternative to texturing is "plasmonic" light-trapping based on non-textured cells and using an overlayer of metallic nanoparticles to produce light-trapping. While this type of light-trapping has not yet been demonstrated for nc-Si:H solar cells, significant photocurrent enhancements have been reported on silicon-on-insulator devices with similar optical properties to nc-Si:H. Here we report our measurements of quantum efficiencies in nc-Si:H solar cells and normalized photoconductance spectra in SOI photodetectors with and without silver nanoparticle layers. As was done previously, the silver nanoparticles were created by thermal annealing of evaporated silver thin films. We observed enhancement in the normalized photoconductance spectra of SOI photodetectors at longer wavelengths with the silver nanoparticles. For nc-Si:H solar cells, we have not yet observed significant improvement of the quantum efficiency with the addition of annealed silver films. INTRODUCTION Light trapping is essential in thin film solar cells due to the limited optical pathlength of photons at longer wavelengths. Their limited thickness does not allow complete light absorption at these wavelengths. Conventionally, both surfaces are textured in order to increase the scattering angles of the light from these interfaces that leads to stochastic light trapping. For a material with refractive index n, perfect texturing will enable as high as 4n2 times pathlength enhancement (about 50 for Si) as predicted by Yablonovitch [1]. Plasmonic light trapping is an emerging alternative to stochastic light trapping. A deep understanding of the plasmonic light trapping mechanism has not been presented yet, however experimental data shows that this light trapping scheme can lead to significant enhancements in semiconductor devices and solar cells. Stuart and Hall demonstrated a 20-fold photocurrent enhancement on silicon-on-insulator (SOI) photodetectors with the deposition of silver nanoparticles on the top silicon layer [2]. This experiment was reproduced by Pillai, et al. using SOI substrates with thicker top silicon layers [3], where they observed 18 times photocurrent enhancement at λ= 1200 nm. This group recently reported six times photocurrent enhancement on a 20 micron thick crystalline silicon solar cell [4]. Schaadt, et al. deposited colloidal gold nanoparticles on a silicon photodiode and observed up to 1.8 times photocurrent enhancement [5
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