Er-doped fluoride crystals for up and down conversion in single junction solar cells

We describe the growth and characterization of Er-doped BaY₂F₈, KY₃F₁₀ and NaYF₄ crystals, intended as up and down converters to enhance the efficiency of single junction solar cells. Direct visual observation of upconversion is supported by photoluminescence spectroscopy indicating the presence of both upconversion and downconversion processes. Unlike powders, our samples are polished single crystals and are therefore especially useful for optical characterization and for the development of an understanding of the physics underlying upconversion and downconversion processes. The application of rare earth ion-doped crystals to Si solar cells is discussed, including limitations of rare earth absorption and ways to overcome these limitations.