High-conversion-efficiency solar cell using fluorescence of rare-earth ions

An experiment was performed to obtain a high-conversion-efficiency solar cell by shifting UV-region sunlight to the visible region, in which the optical absorption and emission properties of rare-earth Eu activator ions were exploited. A thin disc of CaF2:Eu single crystal was used as all alternative to the antireflection (AR) film in a solar cell. The effectiveness of rare-earth ions was examined. Their relative efficiencies were found to be higher at Eu concentrations from 0.001 to 2 mol%, with a maximum value oi 1.5 at 0.05 mol%, than those of nondoped crystal when subjected to low illuminance by a Xe-lamp-based solar simulator. For the purpose of putting the crystal to practical use as an AR film, rare-earth ions were implanted into CaF2 crystal at a dose of 10(14)-10(15) ions/cm(2) and an energy of up to 140 keV.