Upconverter materials and upconverter solar devices were recently investigated with broad-band excitation revealing the great potential of upconversion to enhance the efficiency of solar cell at comparatively low solar concentration factors. In this work first attempts are made to simulate the behavior of the upconverter <i>β</i>-NaYF<sub>4</sub> doped with Er<sup>3+</sup> under broad-band excitation. An existing model was adapted to account for the lower absorption of broader excitation spectra. While the same trends as observed for the experiments were found in the simulation, the absolute values are fairly different. This makes an upconversion model that specifically considers the line shape function of the ground state absorption indispensable to achieve accurate simulations of upconverter materials and upconverter solar cell devices with broadband excitations, such as the solar radiation.
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<sup>3+</sup>). Er<sup>3+</sup> 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<sub>3</sub>:Er<sup>3+</sup>
<sub>10%</sub> 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-conversion (UC) and down-conversion (DC) of sunlight are two possible routes for improving energy harvesting
over the whole solar spectrum. Via such processes it could be possible to exceed the Shockley-Queisser limit for a
single-junction photovoltaic (PV) device. The effect of adding DC and UC layers to the front and rear of a solar cell,
respectively, is to modify the incident solar spectrum. One of the materials more extensively studied for these propose
have been the lanthanides or rare-earth systems, due to the suitability of their discrete energy levels for photon
conversion inside a wide variety of host materials. While high quantum yields of 200% have been demonstrated with
DC materials, there remain several barriers to realising such a layer that is applicable to a solar cell. These are, firstly,
weak absorption of the lanthanide ions and, secondly, the competing loss mechanism of non-radiative recombination.
For UC, these two barriers still exist, however an additional challenge is the non-linear nature of the UC process, thus
favouring operation under concentrated sunlight. In this paper, we review the application of UC and DC to PV,
discussing the material systems used and optical characterisation.