  Current methods for evaluating solar cell efficiencies cannot be applied to
extremely thin cells where phenomena from the realm of near field optics
prevail. We overcome this problem by offering a rigorous electromagnetic
calculation of solar cell efficiencies based on the fluctuation dissipation
theorem. Our approach is demonstrated by calculating the efficiency of a GaAs
solar cell with an Au back reflector for thicknesses well below the typical
wavelength of the solar flux. It is shown that near field optics affect the
performance of low dimensional solar cells.
