  The rigorous coupled-wave approach (RCWA) and the differential evolution
algorithm (DEA) were coupled in a practicable approach to maximize absorption
in optical structures with three-dimensional morphology. As a model problem,
optimal values of four geometric parameters and the bandgaps of three i-layers
were found for an amorphous-silicon, multi-terminal, thin-film tandem solar
cell comprising three p-i-n junctions with a metallic hexagonally corrugated
back-reflector. When the optical short-circuit current density was chosen as
the figure of merit to be maximized, only the bandgap of the topmost i-layer
was significant and the remaining six parameters played minor roles. While this
configuration would absorb light very well, it would have poor electrical
performance. This is because the optimization problem allows for the
thicknesses and bandgaps of the semiconductor layers to change. We therefore
devised another figure of merit that takes into account bandgap changes by
estimating the open-circuit voltage. The resulting configuration was found to
be optimal with respect to all seven variable parameters. The RCWA+DEA
optimization approach is applicable to other types of photovoltaic solar cells
as well as optical absorbers, with the choice of the figure of merit being
vital to a successful outcome.
