Optimization of Silver-alloying for improved photovoltaic properties of CZTSSe
Cu2ZnSn(SxSe1-x)4 (CZTSSe) photovoltaic devices are primarily limited by low open-circuit voltages, believed to be caused in part by disorder on the Cu/Zn sublattice. We show that band tailing in the absorber can be addressed by alloying Ag into CZTSSe to replace Cu. We fabricate thin films across the full alloy range of pure-Cu to pure-Ag and show that as Ag is alloyed into the CZTSe system, the gap between the PL and optical bandgap shrinks from ∼110 meV for pure CZTSe, to 0 meV for the pure Ag compound. Improved device efficiencies up to 10.2% are achieved with low Ag-ACZTSe samples. Unlike CZTSSe champion devices the current record ACZTSe device appears to be limited by the interface rather than bulk defects. To achieve optimal efficiency, the annealing conditions were modified to accommodate the significantly lower melting point of the Ag-containing compound. The range of optimal temperatures across the Cu-Ag-alloy range are shared to accelerate improved device efficiencies.