Increasing upconversion by metal and dielectric nanostructures
2012
Upconversion (UC) of sub-band-gap photons can increase solar cell efficiencies. Up to now, the achieved efficiencies are
too low, to make UC relevant for photovoltaics. Therefore, additional means of increasing UC efficiency are necessary.
In this paper, we investigate both metal and dielectric photonic nanostructures for this purpose. The theoretical analysis
is based on a rate equation model that describes the UC dynamics in β-NaYF 4 : 20% Er 3+ . The model considers ground
state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multi phonon relaxation.
For one, this model is coupled with results of Mie theory and exact electrodynamic theory calculations of plasmon
resonance in gold nanoparticles. The effects of a 200 nm gold nanoparticle on the local field density and on the transition
rates within in the upconverter are considered. Calculations are performed in high resolution for a three dimensional
simulation volume. Furthermore, the effect of changed local fields in the proximity of grating waveguide dielectric
nanostructure is investigated. For this purpose FDTD simulation models of such structures are coupled with the rate
equation model of the upconverter. The results suggest that both metal nanoparticles and dielectric nanostructures can
increase UC efficiency.
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