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

Abstract

Wide‐gap, high‐Ga (Ag,Cu)(In,Ga)Se2 thin‐film solar cells with a wide range of Ag contents are fabricated and characterized before and after dark storage, dark annealing at 85 °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)Se2. 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 ≈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.