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 (Er3+). Er3+ 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 YF3:Er3+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.
Up- and down-conversion (UC, DC) constitute two singular routes to achieve improved energy harvesting of sunlight by
changing its shape of the solar spectrum. To obtain a significant conversion rate two main challenges have to be
overcome: i) the excited lanthanide ions have to emit efficiently, a target which has been better accomplished for DC
materials; ii) the absorption in the lanthanide-based UC and DC layers has to be high to ensure a sizeable fraction of
photons can be harvested. In this paper, we review such materials and their use as spectral converters for photovoltaics
(PV), paying special attention to the UC and DC processes in lanthanide glasses in fluoride matrices. We discuss the
challenges that need to be overcome in order to implement these materials in real PV devices. Finally, we will present
the synthesis of erbium (Er3+) doped YF3 nano-crystals embedded in transparent glass ceramic (TGC) by melt
quenching. This material presents a low phonon energy environment for the Er3+ ions due to the fluoride crystals, while
the silica glass provides chemical and mechanical stability to the compound.
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