Investigation On Rare Earth Doped Oxylfuoride Glass Ceramics Based On Broad Solar Spectrum

Abstract：Through modifying the incoming solar spectrum on the silicon solar cellsby the upconversion or downconversion luminescence of rare earth ions, it couldenlarge efficiently the solar cell spectrum response range, thus it is the photoelectricconversion efficiency of solar cells may be further improved. In comparison withother luminescent materials, the rare earth doped transparent oxyfluoride glassceramics are the research focus for the application on silicon solar cells.Based on the binary borosilicate glasses system, rare earth doped transparentoxyfluoride glass ceramics containing different fluoride nanocrystals weresuccessfully prepared. The thermal stability, the crystal phases and sizes of theprecipitated fluoride nanocrystals, the upconversion/downconversion luminescenceand the fluorescence lifetime of oxyfluoride glass ceramics were studied byDifferential Thermal Analysis （DTA）, X-ray Diffraction （XRD）, transmissionelectron microscopy（TEM）, absorption spectrum, fluorescence spectrum and so on.Er³⁺single doped and Er³⁺/Yb³⁺codoped transparent oxyfluoride glass ceramicscontaining NaLnF₄（Ln=Y, Gd） nanocrystals were prepared, respectively. The resultsshow that the rare earth ions doped oxyfluoride glass ceramics maintain hightransparency and exhibit excellent upconversion luminescence properties. Theupconversion emission of Er³⁺ions in the oxyfluoride glass ceramics containingNaLnF₄nanocrystals is enough strong to be observed with the naked eye with thepumping power smaller than10mW. These materials show potential application insilicon solar cells for improving their performance.Tb³⁺/Yb³⁺, Tm³⁺/Yb³⁺codoped transparent oxyfluoride glass ceramicscontaining Ba₂YbF₇nanocrytals were fabricated, respectively. The results show thatthe energy transfer efficiency from Tb³⁺or Tm³⁺to Yb³⁺increases obviously, whenthe doping concentration of YbF3changes from0mol%to6mol%. Moreover, whenthe doping concentration of YbF3is6mol%, the efficiencies of energy transfer forTb³⁺and Tm³⁺are40%,82.9%. The excellent downconversion characteristic of rareearth ions indicates these materials may be used in the front of the solar cells bymodifying the short wavelength solar spectrum.Eu³⁺/Eu²⁺doped oxyfluoride glass ceramics containing Ba₂GdF₇nanocryastals and Eu³⁺/Eu²⁺/Tb³⁺triply doped oxyfluoride glass ceramics containing Sr₂GdF₇nanocrystals were prepared, respectively, and show good optical emission properties.Especially, part of Eu³⁺was reduced to Eu²⁺under reductive atmosphere. Based onthe change of fluorescence lifetimes for rare earth ions before and after thermalheatment, the same charge and the similar ions radius, the conclusions may beconcluded that Eu³⁺and Eu²⁺ions enter into the precipitated fluoride nanocrystals byselectively substitution for Gd³⁺and Ba²⁺(or Sr²⁺) ions, respectively.