Fundamentals of PV Efficiency: Limits for Light Absorption
  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. 
M. Ryyan Khan, Xufeng Wang, and Muhammad A. Alam
physics.gen-ph