Abstract: To improve the photoelectric conversion efficiency of the single junction amorphous silicon (a-Si) solar cells and alleviate the increasingly serious energy and environmental problems, Yb ³⁺, Ho ³⁺doped NaYF ₄ phosphors were prepared by high temperature solid state method. The transformation of Yb ³⁺ and Ho ³⁺ co-doped NaYF ₄ phosphors on photoluminescence were studied under the heat treatment process, and the effect of the transformation on surface morphology and phase structure were analyzed. The samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) testing. Results show that under the 980nm excitation, three up conversion emission bands 541,649 and 750nm are clearly resolved. The material can be used for promoting the conversion efficiency of a-Si solar cells. Further study shows that by changing the annealing temperature to modulate the surface morphology and phase structure of the samples, their PL density can be enhanced up to a great extent nearly 40 times, which is the strongest PL density of these samples at an annealing temperature of 700℃.