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Plasmonic thin film solar cells (modified with metallic nanostructures) often display enhanced light absorption due to surface plasmon resonance (SPR). However, the plasmonic field localization may not be significantly beneficial to improved photocurrent conversion efficiency for all types of cell configurations. For instance, the integration of random metallic nanoparticles (NPs) into thin film solar cells often introduces additional texturing. This texturing might also contribute to enhanced photon-current efficiency. An experimental systematic investigation to decouple both the plasmonic and the texturing contributions is hard to realize for cells modified with randomly deposited metallic nanoparticles. This work presents an experimental and computational investigation of well-defined plasmonic (Ag) nanoparticles, fabricated by nanosphere lithography, integrated to the back contact of hydrogenated amorphous silicon (a-Si:H) solar cells. The size, shape, periodicity and the vertical position of the Ag nanoparticles were well-controlled. The experimental results suggested that a-Si:H solar cells modified with a periodic arrangement of Ag NPs (700 nm periodicity) fabricated just at the top of the metal contact in the back reflector yields the highest improvement in terms of current density (JSC). Finite-difference time-domain (FDTD) simulations also indicated that Ag nanoparticles located at the top of the metal contact in the back reflector is expected to lead to the most efficient light confinement inside the a-Si:H absorber intrinsic layer (i-layer).