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.