In order to improve the efficiency of multi-crystalline silicon (mc-Si) solar cells, ablation by laser radiation has been adopted as a non-contact tool for various processes. With ultra-short pulsed laser radiation the reflectivity of silicon is decreased by altering the surface topology and increases the efficiency of the solar cells. The formation of structures in the ìm-range on the surface of silicon is investigated with respect to the focal diameters at constant laser parameters. The most promising laser parameters reducing the reflectivity of the solar cells are identified. Multicrystalline silicon solar wafers with an edge length of 5 inch and a laser generated ìm-scale surface structures, so called {\textquotedbl}Black Silicon{\textquotedbl}, were finally processed into cells. The achieved absolute efficiency gain has been measured to Δη = 0.21 \%.