Bandgap gradients in (Ag,Cu)(In,Ga)Se2 thin film solar cells deposited by three-stage co-evaporation

(Ag,Cu)(In,Ga)Se₂ (ACIGS) solar cells are optimized at bandgaps greater than 1.2 eV by varying composition profile of the absorber layer using a three-stage evaporation process. Numerical modeling and cumulative process data provides insight into the process. Silver alloying CIGS changes the optimized bandgap profile by reducing carrier concentration, and reducing bandgap gradients. The minimum bandgap position is controlled by the point when the film reaches I/III stoichiometry during the second stage of the three-stage process. We achieved a 19.9% efficient solar cell with V_OC = 732 mV at a bandgap of 1.2 eV based on quantum efficiency.