Increasing Upconversion by Plasmon Resonance in Metal Nanoparticles—A Combined Simulation Analysis

Upconversion (UC) of subbandgap photons has the potential to increase solar cell efficiencies. In this paper, we first review our recent investigations of silicon solar cell devices with an attached upconverter based on β-NaYF₄ :20%Er³⁺. Such devices showed peak external quantum efficiencies of 0.64% under monochromatic excitation at 1523 nm and an irradiance of 2305 Wm ^-2. Under broad spectrum illumination, an average UC efficiency of 1.07 ± 0.13% in the spectral range from 1460 to 1600 nm was achieved. The measured quantum efficiency corresponds to a relative efficiency increase of 0.014% for the used bifacial silicon solar cell with 16.70% overall efficiency. This increase is too small to make UC relevant in photovoltaics. Therefore, additional means of increasing the UC efficiency are necessary. In this paper, we investigate plasmon resonance in metal nanoparticles in the proximity of the UC material, with the aim of increasing UC efficiency. The local field enhancement by the plasmon resonance positively influences UC efficiency because of the nonlinear nature of UC. Additionally, the metal nanoparticles also influence the transition probabilities in the upconverter. To investigate the effects, we combine different simulation models. We use a rate equation model to describe the UC dynamics in β-NaYF₄ :20%Er³⁺. The model considers ground state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multiphonon decay. The rate equation model is coupled with Mie theory calculations of the changed optical field in the proximity of a gold nanoparticle. The changes of the transition rates both for radiative and nonradiative processes are calculated with exact electrodynamic theory. Calculations are performed in high resolution for a 3-D simulation volume. The results suggest that metal nanoparticles can increase UC efficiency.