A single source three-stage evaporation approach to CIGS absorber layer for thin film solar cells

A novel single source three-stage evaporation process has been developed to deposit CuIn_1-xGa_xSe₂ absorber layers well suited for thin films solar cells. By varying the electron beam current and substrate temperature for each stage, the film structure, texture, morphology, composition and composition profile can be controlled precisely. The thin films are well crystallized with compact and faceted grains, with sizes ranging from 1 to 1.5 μm and also predominantly textured along [112] direction. The elemental compositions are well controlled with Cu:(In+Ga) of 0.85:0.87 and Ga:(In+Ga) of 0.25:0.26, which is within the range for the highest efficiency CIGS solar cells. Photoluminescence and Raman spectra verify that the as-deposited thin films have desired optical properties, and secondary ion mass spectra composition depth profiling suggests that the thin films have graded bandgaps. Thin film solar cells fabricated with such films achieved a power conversion efficiency of 10.6%. This approach avoids the complexity and large area uniformity issues of co-evaporation, and therefore is promising for in-line CIGS film fabrication using the evaporation method.