  We report on the role of plasmonic resonances in determining the delicate
balance between scattering and absorption of light in nanometric particle
arrays applied to the front surface of solar cells. Strong parasitic absorption
is shown to be dependent upon the excitation of localized surface plasmon
resonances and prohibits efficient scattering into the underlying
semiconductor. Via detailed analytical and numerical investigations we obtain
the dependence of scattering and absorption in nanoparticles upon their complex
refractive index. These results provide an insight into the optimum material
properties required to minimize parasitic optical absorption, while maintaining
high scattering cross-section efficiency, thus providing a general design
guideline for efficient light trapping with scattering nanoparticles. The work
is extended to include comprehensive optoelectronic simulations of plasmonic
solar cells in which the scattering metals are made from either Au, Ag or Al.
We show that Al particles provide the closest approximation to the optimized
particle refractive index and therefore exhibit the smallest parasitic
absorption and correspondingly lead to the greatest solar cell efficiency
enhancements. Indeed, for the Al particles we report a full-band enhancement of
external quantum efficiency over the reference device.
