Reaction Rate Enhancement for Cu(In,Ga)Se₂ Absorber Materials Using Ag-Alloying

The addition of Ag to Cu-Ga-In precursors for synthesizing (Ag,Cu)(In,Ga)Se ₂ (ACIGS) thin films has shown benefits including improved adhesion, greater process tolerance, and potential for improved device performance. In this study, reaction pathways to form Cu(In,Ga)Se ₂ (CIGS) and ACIGS were studied by time-progressive reactions at 450 °C in a 5% Ar/H ₂ Se atmosphere followed by ex situ characterization. Results indicated that the addition of 25% Ag/(Ag+Cu) to the CIGS film reduces the reaction time by 50%. X-ray diffraction (XRD) analysis of CIGS films showed that the CuInSe ₂ phase initially formed after 3.5 min. The slow reaction of the stable γ-Cu ₉ (In,Ga) ₄ phase, however, required more than 20 min to complete. Importantly, the addition of Ag to the CIGS film accelerated the reaction. Energy-dispersive X-ray spectroscopy shows that Ga/(Ga+In) grading occurs in the first 10 min of the reaction. XRD analysis showed that the chalcopyrite phase fully forms after 10 min and no significant changes were observed in samples selenized from 10–45 min. Reaction pathways of Ag-alloyed films were further characterized using in situ high temperature XRD analysis. The onset temperature of Se reaction was detected at 230 °C and a AgIn ₂ phase transformation to (Ag,Cu)In ₂ occurred during the early stage of the reaction.