  The maximal concentration limit of solar radiation for photovoltaic
applications is assumed to be constant, at Cmax\approx46,000 suns. This limit
is easily found via a geometrical application of the 2nd law of thermodynamics
to a radiation transfer system [A. Rabl, Sol. Energy vol. 18, pp. 93-111,
1976]. However, previous analysis did not include the generation of entropy in
photovoltaic conversion. Here, we show a bandgap dependence of the maximal
concentration limit for a semiconductor solar cell when taking this entropy
generation into account, and show that the limit is reduced for low bandgap
materials. This new concentration limit lies in contrast to the assumed
invariance of the concentration limit, and we attribute this difference to a
breakdown in the assumptions used to derive the traditional detailed-balance
model.
