  A simple thermodynamic argument related to a (weakly absorbing) finite
dielectric slab illuminated by sunlight- originally suggested by Yablonovich-
leads to the conclusion that the absorption in a dielectric can at best be
increased by a factor 4n2. Therefore, the absorption in these materials is
always imperfect; the Shockley-Queisser limit can be achieved only
asymptotically. In this paper, we make the connection between the degradation
in efficiency and the Yablonovich limit explicit and re-derive the 4n2 limit by
intuitive geometrical arguments based on Snell's law and elementary rules of
probability. Remarkably, the re-derivation suggests strategies of breaking the
traditional limit and improving PV efficiency by enhanced light absorption.
