Ag-Dependent Behavior Threshold and Metastability in Wide-Gap (Ag,Cu)(In,Ga)Se₂ Solar Cells

Wide-gap, high-Ga (Ag,Cu)(In,Ga)Se-2 thin-film solar cells with a wide range of Ag contents are fabricated and characterized before and after dark storage, dark annealing at 85 degrees C, and light soaking. A 1:4 ratio of Ag to Cu enhances initial device performance significantly, with excess Ag enhancing carrier collection at the expense of open-circuit voltage and fill factor for close-stoichiometric devices. For off-stoichiometric devices, increased open-circuit voltages are offset by losses in carrier collection. Efficiency degradation after treatments is typically increased with additional Ag alloying. A second observation is a behavior threshold identified slightly below an Ag to Cu ratio of 1:1. For compositions below the threshold, the doping response to light soaking and dark annealing is similar to that exhibited by low-Ga Cu(In,Ga)Se-2. Above the threshold, light soaking reduces net doping and that dark annealing can even increase net doping. Furthermore, devices above the threshold exhibit a far greater doping responsivity than those below and display a strong dependence of initial performance and stability on group-I/group-III stoichiometry. A third observation is that all devices lose approximate to 1-2% (absolute) in efficiency after a 3 h light soak, indicating that this loss originates from the high-Ga content (1:3 In:Ga), rather than the Ag alloying.