  We introduce a limit for the strength of coupling light into the modes of
solar cells. This limit depends on both a cell's thickness and its modal
properties. For a cell with refractive index n and thickness d, we obtain a
maximal coupling rate of 2c*sqrt(n^2-1)/d where c is speed of light. Our method
can be used in the design of solar cells and in calculating their efficiency
limits; besides, it can be applied to a broad variety of resonant phenomena and
devices.
