  Soiling, which consists of dust, dirt and particles accumulated on the
surface of conventional or concentrator photovoltaic modules, absorbs,
scatters, and reflects part of the incoming sunlight. Therefore, it reduces the
amount of energy converted by the semiconductor solar cells. This work focuses
on the effect of soiling on the spectral performance of multi-junction (MJ)
cells, widely used in concentrator photovoltaic (CPV) applications. Novel
indexes, useful to quantify the spectral impact of soiling are introduced, and
their meanings are discussed. The results of a one-year experimental
investigation conducted in Spain are presented and are used to discuss how
soiling impacts each of the subcells of a MJ cell, as well as the cell
current-matching. Results show that soiling affects the current balance among
the junctions, i.e. the transmittance losses have found to be around 4% higher
in the top than in the middle subcell. The spectral nature of soiling has
demonstrated to increase the annual spectral losses of around 2%. Ideal
conditions for the mitigation of soiling are also discussed and found to be in
blue-rich environments, where the higher light intensity at the shorter
wavelengths can limit the impact of soiling on the overall production of the
CPV system.
