A back reflector (BR) that can efficiently scatter weakly absorbed light is essential to obtain high-efficiency thin-film silicon solar cells. We present the design routes of plasmonic BR based on self-assembled silver nanoparticles (Ag NPs) for high-efficiency thin-film silicon solar cells. Both optical and electrical effects on solar cells are considered. The shape of Ag NPs, the thickness of ZnO:Al spacer layers, materials on top of Ag NPs, and nanoparticle size are crucial for the performance of plasmonic BR. Increased annealing temperature lead to the formation of more appropriate shapes (more spherical and regular shapes) for a good light scattering and, thus, increase the photocurrent. The ZnO:Al layer between the Ag NPs and the Ag planar film has an optical effect on solar cells, while the ZnO:Al layer between the Ag NPs and the doped a-Si:H has both optical and electrical influence on the device. Larger NPs have less parasitic absorption and can preferentially scatter light into larger angles, thus increasing the spectral response in the solar cell. However, for larger Ag NPs, the fill factor deteriorates due to the rougher surface in the plasmonic BR, indicating a compromise between light trapping and electrical performance. Following the design routes, we obtained 8.4% high-efficiency plasmonic a-Si:H solar cell.