Down conversion Ce3+-Yb3+co-doped YAG phosphors are used to enhance concentrator mono-crystalline silicon solar cells. The coating of polymethyl methacrylate (PMMA) mixed with the phosphors is deposited on the surface of the solar cells by spin-coating method. It is found that the solar cells with the phosphor coating always have higher conversion efficiency than the bare solar cells under different illumination intensities. This is attributed to the down conversion effect of the phosphors and the reduced reflection (especially in the wavelength range 350–550 nm). The reflection of the light emitted from the phosphor’s particles at the air/PMMA interface also contributes to the improvement. The relative growth in the conversion efficiency of the solar cells with the phos-phor coatings increases with the illumination intensity from 4.86% under 100 mW/cm2 to 6.04% under 400 mW/cm2 because the increase in the emission from the phosphors is faster than that of the illumination intensity when the illumination intensity increases.