Investigating the Role of Ag and Ga Content in the Stability of Wide-Gap (Ag,Cu)(In,Ga)Se2 Thin-Film Solar Cells

The stability of thin-film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se-2 material system is evaluated over time, after dry-heat annealing and after light soaking, and the role of Ag and Ga content is explored. Ag-free CuInSe2 is relatively stable to annealing and storage, while Cu(In,Ga)Se-2 suffers a degradation of fill factor and carrier collection. High-Ga (Ag,Cu)(In,Ga)Se-2 suffers degradation of carrier collection after prolonged annealing, reducing the short-circuit current by approximate to 12%. Ga-free (Ag,Cu)InSe2 loses up to a third of open-circuit voltage and a quarter of fill factor after all treatments are applied. All samples suffer voltage losses after light soaking, with the Ga-free devices losing up to 50 mV and those containing Ga losing up to 90 mV. Ag incorporation leads to a significant reduction in doping, and a significant increase in the response of doping to treatments, with the depletion width of (Ag,Cu)(In,Ga)Se-2 samples expanding from approximate to 0.1 mu m as-grown to beyond 1.0 mu m after all treatments, compared to the Cu(In,Ga)Se-2 sample variation of approximate to 0.1-0.3 mu m. Connections between Ag content, doping instability, and performance degradation are discussed.