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

Abstract

The stability of thin-film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se2 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)Se2 suffers a degradation of fill factor and carrier collection. High-Ga (Ag,Cu)(In,Ga)Se2 suffers degradation of carrier collection after prolonged annealing, reducing the short-circuit current by ≈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)Se2 samples expanding from ≈0.1 μm as-grown to beyond 1.0 μm after all treatments, compared to the Cu(In,Ga)Se2 sample variation of ≈0.1–0.3 μm. Connections between Ag content, doping instability, and performance degradation are discussed.