Too Many Junctions? A Case Study of Multijunction Thin-Film Silicon Solar Cells

The benefit of two-terminal multijunction solar cells in regard to the number
of junctions (subcells) is critically evaluated. The optical and electrical losses
inherent in the construction of multijunction cells are analyzed using information
from thin-film silicon photovoltaics as a representative case. Although
the multijunction approach generally reduces the thermalization and nonabsorption losses, several types of losses rise with the number of subcells.
Optical reflection and parasitic absorption are slightly increased by adding
supporting layers and interfaces. The output voltages decline because of the
tunnel recombination junctions, and more importantly of the illumination
filtered and reduced by the top subcell(s). The loss mechanisms consume
the potential gains in efficiency of multijunction cells. For thin-film silicon,
the triple-junction is confirmed to be the best performing structure. More
generally, only when each component subcell shows a high ratio between the
output voltage and the bandgap of the absorber material, a multijunction cell
with a large number of subcells can be beneficial. Finally, the high voltage
and low current density of multijunction cells with a large number of subcells
make them difficult to optimize and manufacture, vulnerable to any changes
in the solar spectrum, and thus less practical for the ordinary terrestrial
applications.