The objective of the present work is to develop luminescent materials with a quantum yield higher than 100% for improving the conversion efficiency of photovoltaic solar cells. The study is focused on rare-earth-doped sulfide-based materials with low phonon energy. The multiplication of photon is firstly studied in rare-earths doped Ga₂S₃-GeS₂-CsCl glasses. It has been demonstrated that one visible photon can be divided into two NIR photons by the rare earth couples Er³⁺/Yb³⁺ou Pr³⁺/Yb³⁺. However, the overall quantum yield measured with an integrating sphere is much lower than 100%. The low quantum yield is attributed to the high concentration of impurities acting as "luminescence killers" and to the charge-transfer absorption of Yb³⁺ which is located in the visible region in sulfides. The rare-earth oxysulfides are then introduced as matrix since the partial substitution of sulfur by oxygen shifts the charge transfer band of Yb³⁺ to the UV region. The oxysulfides with high purity are prepared by combustion method with subsequent sulfuration. The multiplication of photon in the NIR is confirmed in La₂O₂S doped with Pr³⁺/Yb³⁺, Er³⁺,Yb³⁺ and Tb³⁺/Yb³⁺. To the best of our knowledge, it is the first time that quantum yield higher than 100% is directly measured in La₂O₂S : Er³⁺,Yb³⁺. A core-shell structure is designed to sensitize Er3+ in the oxysulfides by Ce³⁺ in the YAG. By homogeneous precipitation with urea, the Y₂O₂S is precipitated on the fine powders of YAG : Ce³⁺. Although the desired structure is not yet obtained due to the diffusion of Er³⁺ into the YAG, further efforts on this subject seem promising to invent spectral convertors with large and intense absorption band.
