  The rigorous coupled wave approach (RCWA) was implemented to investigate
optical absorption in a triple-p-i-n-junction amorphous-silicon solar cell with
a 2D metallic periodically corrugated backreflector (PCBR). Both total and
useful absorptances were computed against the free-space wavelength $\lambda_o$
for both s- and p-polarized polarization states. The useful absorptance in each
of the three p-i-n junctions was also computed for normal as well as oblique
incidence. Furthermore, two canonical boundary-value problems were solved for
the prediction of guided-wave modes (GWMs): surface-plasmon-polariton waves and
waveguide modes. Use of the doubly periodic PCBR enhanced both useful and total
absorptances in comparison to a planar backreflector. The predicted GWMs were
correlated with the peaks of the total and useful absorptances. The excitation
of GWMs was mostly confined to $\lambda_o < 700$ nm and enhanced absorption. As
excitation of certain GWMs could be correlated with the total absorptance but
not with the useful absorptance, the useful absorptance should be studied while
devising light-trapping strategies.
