Increasing upconversion by metal and dielectric nanostructures

Abstract

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-NaYF4 : 20% Er3+. 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.