To improve the light conversion efficiency of the upconversion phosphor NaYF₄:Yb3+, Tm3+ for use in solar cells, its phosphor surface was treated by removing the NaF phase from the surface of the NaYF₄ host crystals. The surface of the phosphor was treated by a simple procedure consisting of washing, filtering, and drying. It is confirmed that the NaF compound is efficiently eliminated from the surface of NaYF₄-based phosphors by this surface treatment. The experimental results strongly suggest that the removal of NaF from the surface of the NaYF₄:Yb3+, Tm3+ phosphor reduces surface defects and enhances the photoluminescence characteristics of upconversion phosphors.