Low-Temperature\nAnnealing of CdS:In/Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> Heterojunction\nBoosting 14.5% Efficiency Kesterite\nSolar Cells

The persistent challenge in kesterite solar cells is\nthe low open-circuit\nvoltage (<i>V</i><sub>oc</sub>) and fill factor (FF) due\nto nonradiative recombination at the CdS/Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> (CZTSSe) interface. Here we demonstrate a convenient combination\nof low-temperature annealing and In doping within the buffer layer\nto establish an electrically benign high-quality CdS:In/CZTSSe heterojunction.\nThe low-temperature annealing facilitates the migration of Cu, Zn,\nand Sn impurity elements from the buffer layer to the absorber side,\nimproving lattice match and reducing detrimental defects and the conduction\nband offset (CBO) barrier involving large recombination losses. The\nIn doping boosts the donor concentration and crystallinity of the\nbuffer layer, thereby improving the electron transport and extraction\nprocesses. Consequently, the CdS:In device achieves the highest efficiency\nof 14.5% with the <i>V</i><sub>oc,deficit</sub> decreasing\nfrom 348 mV to 287 mV and the FF increasing from 66.6% to 70.3%, promising\na significant efficiency leap for CZTSSe solar cells.