Optical design of solar cells: modeling a nanostructured thin film silicon solar cell photovoltaic in FDTD Solutions
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 in the visible part of the electromagnetic spectrum.
"We have recently shown that rear-located Ag nanoparticle arrays can provide effective light trapping near the band edge for Si solar cells. 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
Step 1: Construct a realistic, three dimensional optical design in FDTD Solutions of the silicon solar cell device accurate over a wide wavelength range
The surface-plasmon enhanced solar cell three-dimensional model is composed of a silicon substrate, and a periodic array of spherical 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 to accurately describe the dispersive properties of the constituent materials. 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.
3D FDTD Solutions model of solar cell device consisting of a periodic array of silver nanoparticles located on the upper surface of the silicon solar cell.
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 figure below shows a movie of the x component of the electric field.
Movie of scattering from silver nanoparticles on upper surface of a thin film silicon solar cell.
The figure below 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 indicates that the majority of the absorption takes place in the silicon regions directly underneath and nearby the nanoparticles as desired.
Plot at a wavelength of 621 nm of electric field intensity of incident and scattering radiation from silver nanoparticles on solar cell surface.
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 full 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 in a single simulation.
Solar cell spectrum over a wavelength range from 400 to 1100 nm.
The figure to the bottom shows the amount of light transmitted into the silicon for gold nanoparticles normalized to the light transmitted into a bare silicon surface for two different diameters of nanoparticles. 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.
Relative transmission of light transmitted into the silicon for gold nanoparticles of 100nm and 200nm diameter relative to a bare silicon surface.
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.
Relative conversion efficiency improvement of a nanostructured solar cell over a bare silicon surface for silver nanoparticles.
Step 4: Calculate the total improvement to solar cell conversion efficiency as a function of the nanoparticle diameter
The figure below shows the relative conversion efficiency enhancement afforded by silver nanoparticles of varying diameter and period. 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 approaching 40% 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.
Conversion efficiency enhancement of silver nanoparticles on a silicon surface for varying periods and diameters.
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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 cells. 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.
