Plasmonic nanoparticles were once sought to harness enormous potential for light-trapping in inorganic thin-film photovoltaics. However, the incorporation of such metallic nanostructures near solar cell absorbing layers without inducing overall harm to performance has proven to be a major obstacle. Herein, we demonstrate a solar cell design which integrates a periodic array of plasmonic Ag nanoparticles within the p-i-n structure of a-Ge:H ultrathin optical cavity solar cells. The plasmonic solar cells showed a 33% short-circuit current density increase relative to geometrically identical cells where the Ag nanoparticles were replaced by SiO2. We experimentally mapped the localized surface plasmon excitations on the surface of Ag nanoparticles embedded in the optoelectronic device using electron energy loss spectroscopy and correlated the results to the device performance. Using three-dimensional optical simulations, we further explored the light-trapping mechanisms responsible for the observed performance enhancements. The nanostructured cells produced localized and tunable charge carrier generation enhancements while maintaining the planar geometry of the ultrathin absorbing layer. Therefore, this design concept provides a direct and useful avenue for initial light-trapping efforts in next-generation photovoltaics based on ultrathin nanoabsorbers, such as few layer transition metal dichalcogenides.