The approach to efficiency improvement of space 3-Junction and 2-Junction solar cells, that involves a layer of down-converting material on the top of the cell, is presented. The layer consists of a polymer nanocomposite coating impregnated with the nanoparticles of inorganic compounds (fluorides or oxides) doped with rare earth ions down-converting solar UV light to visible matching the spectrum of the responsivity of the cell. Computational model of a solar cell is presented and used for processing the experimental data. Experimental results demonstrate an increase of the generated current and photoelectric efficiency.