Secondary phase formation in (Ag,Cu)(In,Ga)Se2 thin films grown by three-stage co-evaporation

(Ag,Cu)(In,Ga)Se-2 thin films deposited by a three-stage co-evaporation process have shown potential as mid and wide-bandgap absorber materials for solar cell devices. Further growth optimization requires a thorough understanding of secondary phases formed in addition to the primary chalcopyrite phase, during the group-I rich growth near the end of the second stage. In this study, we investigate how the composition ratio Ag/(Ag+Cu) controls the formation of the secondary phases containing Ag, Cu and Se. Group-I rich [composition ratio (Ag+Cu)/(In+Ga)>1] ACIGS films with varying silver content-Ag/(Ag+Cu)=0, 0.1 and 0.5-are grown until the end of the second stage of three-stage processes. Secondary phases are found to locate near film surfaces. However, they intermix with the chalcopyrite phase in the surface layer when Ag/(Ag+Cu)> 0, while they form a separate layer if no Ag is alloyed. The crystal structures of the secondary phases shifts along the Cu2Se - Ag2Se pseudo-binary tie-line as Ag/(Ag+Cu) increases. Concurrently, the film surfaces become smoother with reduced facets and voids, the accumulation of sodium is suppressed, and both the degree of In and Ga gradient and the film emissivity are reduced, all of which affect the subsequent deposition process and final solar device performance.