Increasing upconversion efficiency by plasmon resonance in metal nano-particles

Upconversion (UC) of sub-band-gap photons has the potential to increase solar cell efficiencies. In this paper, we present investigations of silicon solar cell devices with attached upconverter based on NaYF₄:20% Er³⁺. Such devices showed peak external quantum efficiencies of 0.64% under monochromatic irradiation 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 higher efficiency under broad spectrum illumination is attributed to the parallel resonant excitation of the two most important involved optical transitions. The measured quantum efficiency corresponds to a relative efficiency increase of 0.014% for the used bifacial silicon solar cell with 16.7% overall efficiency. This increase is too small to make upconversion relevant in photovoltaics. Therefore, additional means of increasing the UC efficiency are necessary such as plasmon resonance in metal nano-particles in the proximity of the upconverting material. The higher local field intensities caused by plasmon resonance positively influence upconversion efficiency because of the non-linear nature of UC. Additionally, the metal nano-particles also influence transition probabilities in the upconverter. To investigate the impact of such nano-particles, we used a rate equation model of the up-converting material. It is based on Einstein coefficients of the relevant transitions, which were determined from absorption measurements. The model describes the UC dynamics and considers ground state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multi phonon relaxation. The results show good agreement with photoluminescence measurements. The model was coupled with Mie theory calculations of the changes of the electric field and of the transition probabilities. The calculations suggest that metal nano-particles are a- le to increase UC efficiency significantly by at least a factor of eight.