Surface characterization of (AgCu)(InGa)Se2 thin films for solar cells

AgCu(InGa)Se₂ alloy absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios were deposited using multi-source elemental evaporation and analyzed by glancing incidence x-ray diffraction and energy dispersive x-ray spectroscopy. All films exhibit satellite chalcopyrite reflections in the x-ray diffraction pattern and films with 0.5 ≤ Ga <; 1 and Ag >; 0.5 have additional reflections consistent with an ordered defect phase which is limited to the near-surface region of the film. X-ray photoelectron spectroscopy results show that all films have low (Ag+Cu)/Se ratios near the surface, consistent with an ordered defect compound. Films with 0 <; w <; 1 have (Ag+Cu)/Se and (Ag+Cu)/(Ga+In) ratios at the surface close to the (AgCu)(InGa)₅Se₈ ordered defect phases. Additionally the near-surface region of (AgCu)(InGa)Se₂ films contains a higher Ag/(Ag+Cu) ratio than the bulk and the Ag(InGa)Se₂ film contains excess Ag near the surface.