  In this paper, we calculate the material specific absorption accurately using
a modal method by determining the integral of the Poynting vector around the
boundary of a specific material. Given that the accuracy of our method is only
determined by the number of modes included, the material specific absorption
can be used as a quality measure for the light-trapping performance. We use
this method to investigate metallic gratings and find nearly degenerate
plasmons at the interface between metal and amorphous silicon (a-Si). The
plasmons cause large undesired absorption in the metal part of a grating as
used in a-Si cells. We explore ways to alleviate the parasitic absorption in
the metal by appropriate choice of the geometry. Separating the diffraction
grating from the back reflector helps, lining silver or aluminum with a
dielectric helps as well. Gratings with depth > 60nm are preferred, and periods
> 600nm are not useful. Maximum absorption in silicon can occur for less thick
a-Si than is standard. We also find that geometric asymmetry can improve the
absorption for PV. Lastly, we investigate the modal contributions to
absorption, and find that for the most part, the absorption can not be
attributed to single modes, but comes from the interplay of a few important
modes.
