Fig. 1From: A New Up-conversion Material of Ho3+-Yb3+-Mg2+ Tri-doped TiO2 and Its Applications to Perovskite Solar CellsUp-conversion emissions of TiO2. a Ho3+-Yb3+ co-doped TiO2 (Ho:Yb:Ti = 1:x:100, molar ratio). b Ho3+-Yb3+-Mg2+ tri-doped TiO2 (Ho:Yb:Mg:Ti = 1:4:x:100, molar ratio)Back to article page