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

Thin film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se2 system are fabricated and characterised using current-voltage, external quantum efficiency and capacitance-based measurements. The stability of the devices over time, after dry annealing and after lightsoaking is evaluated, and the role of Ag and Ga content is explored. Ag-free CuInSe2 and Cu(In,Ga)Se2 are observed to be stable, however high-Ag, high-Ga (Ag,Cu)(In,Ga)Se2 is observed to exhibit fluctuations in short-circuit current. High-Ag (Ag,Cu)InSe2 is instead observed to have stable current, but open-circuit voltage and fill-factor are strongly responsive. Thus, it is indicated that high fractions of Ag in the material lead to significant stability concerns, with high Ga fractions affecting the way in which degradation manifests. Discussion of probable causes and mechanisms follows.