Fig. 1From: Enhanced Power Conversion Efficiency of Perovskite Solar Cells with an Up-Conversion Material of Er3+-Yb3+-Li+ Tri-doped TiO2a Up-conversion emission spectra of Er3+-Yb3+-Li+ tri-doped TiO2 (Er:Yb:Li:Ti = 0.5:10:x:100, x = 0, 10, 15, 20, and 25, molar ratio). b Schematic energy-transfer mechanisms of the Er3+ ionsBack to article page