Rare-earth-incorporated ternary Ce Cd_1- S quantum dot-sensitized solar cells

This work presents a new absorber material - rare-earth-doped ternary Ce Cd_1- S quantum dots (QDs) - for solar cells. Ce Cd_1- S QDs were synthesized by partially replacing the cation Cd in the binary sulfide CdS with Ce using a two-step solution processing process. First, Ce-S QDs were grown on a mesoporous TiO₂ electrode. Second, Cd-S QDs were grown on top of the Ce-S QDs. Post annealing transformed the Ce-S/Cd-S double layers into the ternary Ce Cd_1- S structure. The synthesized Ce Cd_1- S QDs have the same hexagonal structure as the host CdS, with an average particle size of 11.8 nm. X-ray diffraction reveals a slight lattice expansion in Ce Cd_1- S relative to CdS. The band gap E _g of Ce Cd_1- S exhibits a monotonic decrease from 2.40 to 2.24 eV with increasing Ce content x from 0 to 0.20, indicating an E _g tunable by controlling the dopant content. Ce Cd_1- S QDSCs were fabricated with a polysulfide electrolyte and CuS counter electrode. The best Ce Cd_1- S cell yields a J _sc of 8.16 mA cm^-2, a V _oc of 0.73 V, a fill factor of 62.5%, and an efficiency of 3.72% under 1 sun. The efficiency increases to 4.24% under the reduced light intensity of 0.25 sun. The efficiency of the Ce Cd_1- S cell is 25% higher than that of the host CdS cell. The improved performance is attributed to the broader absorption range resulting from Ce doping. These results suggest the potential of using Ce as a dopant in CdS to tune the E _g and improve the photovoltaic performance.