Enhancement of power conversion efficiency of Ag-substituted Cu2ZnSn(S, Se)4 solar cells via tuning Cu2+/(Cu++Cu2+) percentage in precursor solution

Although trace silver (Ag) substitution for Cu in Cu2ZnSn(S, Se)4 (CZTSSe) offers benefits in improving open-circuit voltage (VOC) of CZTSSe solar cells, its further development is still hindered by the fairly low short-circuit current density (JSC) owing to the increase of bandgap (Eg) of the absorber layer. Herein, a strategy is demonstrated to mitigate the poor JSC arising from the Ag substitution by tuning the Cu2+/(Cu++Cu2+) percentage in the (Cu, Ag)2ZnSnS4 (CAZTS) precursor solution. Depth characterization indicates that the appropriate Cu2+/(Cu++Cu2+) percentage in the CAZTS precursor solution increases the photogenerated charges separation and carrier transport processes for Ag-substituted CZTSSe (CAZTSSe) solar cells. Benefiting from this strategy, the power conversion efficiency (PCE) of 9.57% is achieved when the Cu2+/(Cu++Cu2+) is 50 at.%. The main factor for the improvement of PCE is due to the increase in photogenerated current density (JL) and decrease in shunt resistance (Rsh). It is demonstrated that the increase in JL results from a widening in the depletion layer width (Wd) of the p-n junction and an increase in carrier lifetime, induced by optimization of the Cu2+/(Cu++Cu2+) percentage. While the decrease in Rsh comes from the change of reaction pathways due to the change in Cu2+/(Cu++Cu2+) percentage.