Nanocrystals of up-conversion phosphor，rare earth-doped sodium yttrium fluoride were prepared by hydro-thermal method for the application in thin film solar cells. Effect of Yb3+ and/or Er3+ doped materials on the crystal structure and optical properties were studied. Up-conversion materials with Yb3+/Er3+ co-doped absorb the infrared near 980 nm and 1530nm and emit red (653nm)and green (520nm，540nm)light，which is well adapted for use in thin film solar cells. Nanoparticels with efficient near-infrared to visible up-conversion fluorescence with appropriate doping concentrations of 18% Yb3+ and 2% Er3+ were prepared.