Broadband excitation of upconversion in lanthanide doped fluorides for enhancement of Si solar cells

The use of upconverters (UC) to harvest light with photon energy below the bandgap of a photovoltaic cell is one possible route to overcome the Shockley-Queisser limit for single junction devices. The materials which have shown potential to enhance the performance of silicon (Si) cells are rare earths (RE) such as trivalent erbium (Er ³⁺). Er ³⁺ is limited by a low absorption cross section over a narrow bandwidth which requires high excitation powers to achieve good efficiencies due to its non-linear response. This material has predominantly been investigated under monochromatic excitation at 1523nm as this achieves strong resonance with the equidistant energy levels although, is not representative of its application under a spectrally broad solar irradiance. In this paper we show the importance of using broadband excitation (12nm and 38nm bandwidths) as a method to characterise these materials and understand their possible benefits. Using an oxyfluoride ceramic with active YF ₃:Er ³⁺ _10% nano-crystals (NC), and increasing the bandwidth by a factor of 3.17, lead to a 55 fold increase in emission for the same solar concentration. This is equivalent to achieving the same level of emission with a factor of 7.6 less Suns.