  We use a rigorous electromagnetic approach to analyze the fundamental limit
of light-trapping enhancement in grating structures. This limit can exceed the
bulk limit of 4n^2, but has significant angular dependency. We explicitly show
that 2D gratings provide more enhancement than 1D gratings. We also show the
effects of the grating profile's symmetry on the absorption enhancement limit.
Numerical simulations are applied to support the theory. Our findings provide
general guidance for the design of grating structures for light-trapping solar
cells.
