Cu (In x Ga1-x )Se2 Thin Films Mechanical and Structural Properties

Abstract

Due to Cu (In x Ga1-x )Se2 (CIGS) thin films are used as an absorbing material in solar cells because their direct bandgap overlaps the red edge of the solar spectrum and they have a relatively high absorption coefficient (α~105 cm−1) [1]. The efficiency of CIGS solar cells reached 22.9% for laboratory scale devices and up to 15.7% for commercial modules [2], which is the highest conversion efficiency among all commercial polycrystalline solar cells [3]. Structural/nanomechanical properties of Cu(In x Ga1-x )Se2 films deposited on Si(100) by one-step radio frequency magnetron sputtering are discussed. The films were sputtered at different substrate temperatures (25-500 °C). X-Ray diffraction revealed dominant diffraction peak of (112). Surface morphology was examined by field emission scanning electron microscopy and atomic force microscopy. The film composition was examined by electron probe microanalysis. The Cu/(In+Ga) and Ga/(In+Ga) ratios were 0.9 and 0.3, respectively. Nanoindentation, in conjunction with the scanning electron microscopy, was conducted and revealed that the films suffered severe delamination type fracture due to poor adhesion to the Si substrate. The hardness of the films was not influenced by the substrate temperature increase except for the film grown at 500 °C. The hardness results were largely affected at depths larger than the film thickness due to poor adhesion of the film. At deep indents, the hardness results agreed closely with the Si substrate hardness.