Thin film silicon solar cell photovoltaic panels offer the possibility of harvesting almost limitless electromagnetic energy radiated from the sun. However, widespread adoption of these solar cells requires that greater conversion efficiencies can be attained at cheaper costs than that available with current bulk silicon solar cell technologies. In this regard, new nanophotonic materials and thin-film silicon solar cell technologies coupled with emerging processing techniques able to reliably manufacture cheap nanostructured materials offers great promise to improve conversion efficiencies available in solar cells through improved optical design. In particular, metallic nanoparticle arrays on the surface of silicon solar cells are able to dramatically increase the absorption of solar energy within thin film silicon solar cells, by relying on the large scattering cross section afforded by nanoparticles.
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"We have recently shown that rear-located Ag nanoparticle arrays can provide effective light trapping near the band edge for Si solar cells1. The ability to decouple the fields scattered by the nanoparticles from the total fields expedites the analysis of these systems greatly, allowing us to calculate the efficiency of the light trapping provided by the nanoparticles in a simple and effective way."
F. Beck, Australia National University
1F. J. Beck et al., Journal of Applied Physics 105, 114310 (2009).
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Step 1: Construct a realistic, three-dimensional optical design in FDTD Solutions of the silicon solar cell device accurate over a wide wavelength range
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Solar cell device consisting of a periodic array of silver nanoparticles located on the upper surface of the silicon solar cell.
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The three-dimensional solar cell model is composed of a silicon substrate, and a periodic array of silver nanoparticles on the upper surface of the photovoltaic. Multi-coefficient broadband material models accurate over the solar spectrum from 400 to 1100 nm are generated automatically from the built-in material database within FDTD Solutions. The resulting material models offer excellent accuracy (e.g. RMS error of 0.05) over the entire bandwidth for silicon, and allow one to calculate the absorption across the entire spectrum for a single three-dimensional solar cell device geometry in a single simulation.
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Step 2: Generate a movie of the nanostructured solar cell photovoltaic dynamics and check that the optical design is operating as expected
The movie monitor within FDTD Solutions allows one to capture the field dynamics of the metallic nanoparticles on the upper silicon surface of the solar cell being illuminated. The left-hand figure below shows a movie of the x component of the electric field, and the figure to the bottom right shows the steady-state electric field intensity averaged over the solar spectrum from 400 to 1100nm. As the absorption in the silicon is proportional to the electric field intensity, the figure to the bottom right indicates that the majority of the absorption takes place in the silicon regions directly underneath the nanoparticles as desired.
Movie of scattering from silver nanoparticles on upper surface of a thin film silicon solar cell
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Spectral average over solar spectrum of electric field intensity of incident and scattering radiation from silver nanoparticles on solar cell surface
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Step 3: Calculate the impact of silver and gold nanoparticles on the spectral transmittance into the silicon surface over the entire solar spectrum
To optimize solar cell conversion efficiency, the extent of the solar cell spectrum must be accounted for in any viable optical device design. With a single simulation and the broadband material models generated by FDTD Solutions, it is possible to measure the fraction of light transmitted into the silicon solar cell over a wide wavelength range corresponding to the solar spectrum.
The figure to the bottom left shows the amount of light transmitted into the silicon for both silver and gold nanoparticles normalized to the light transmitted into a bare silicon surface. A quantity in excess of unity corresponds to increased transmittance relative to the bare surface, while a quantity less than unity corresponds to less transmittance. Because the plasmon resonance in silver occurs at much shorter wavelengths than it does in gold, silver exhibits a enhancement over much more of the solar spectrum than gold and as such is a better choice for the optical design.
Solar cell spectrum over a wavelength range from 400 to 1100 nm.
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Simulated thin film silicon solar cell device performance as a function of wavelength over the solar spectrum. The plot shows the relative transmission of light transmitted into the silicon for silver and gold nanoparticles of 180nm diameter relative to a bare silicon surface.
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Step 4: Calculate the total improvement to solar cell conversion efficiency as a function of the nanoparticle diameter
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 Relative conversion efficiency improvement of a nanostructured
solar cell over a bare silicon surface as a function of nanoparticle
diameter.
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The figure to the left shows the relative conversion efficiency enhancement afforded by silver and gold nanoparticles of varying diameter, at a period of 400nm. Given the large enhanced transmission into the silicon available with silver over almost all wavelengths, silver nanoparticles on the upper surface enhance total conversion efficiency up to 20% depending on the diameter of the nanoparticle used in the solar cell design. Meanwhile, the decreased transmission of light into the silicon surface with gold nanoparticles over the short wavelength end of the solar spectrum (where most of the energy is) results in gold nanoparticles offering worse conversion efficiencies than that available with a bare silicon surface.
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