Phase stability in Ag-Cu-In-Ga metal precursors for (Ag,Cu)(In,Ga)Se2 thin films

The addition of Ag to Cu-Ga-In precursors for reaction to form (AgCu)(InGa)Se-2 has shown benefits including improved adhesion, greater process tolerance and potential for improved device performance. In this study, metal precursors were sputtered with a Cu-Ga/In/Ag-Ga sequence with Ag/(Cu+Ag) = 0.25 and (Ag + Cu)/(Ga+In) = 0.90. These precursor layers are shown to be unstable, with a phase evolution during storage at room temperature revealed by x-ray diffraction (XRD). This behavior was studied in samples annealed in the temperature range of 60-150 degrees C or stored for up to 90 days. XRD analyses indicated the formation of (Ag1-xCux)In-2 with Cu content of 28% and 36% for samples annealed at 100 degrees C and 150 degrees C, respectively. Energy dispersive x-ray spectroscopy and XRD analyses on selenized samples showed a uniform distribution of Ag and Cu through the films and a Ga accumulation near the back interface. Solar cells fabricated from the selenized films showed improved device performance in V-oc and FE as a result of the precursor anneal.