Effects of Ga accumulation on the microstructure of Cu(In1−x,Gax)Se2 thin films during selenization

Cu(In1−x,Gax)Se2 (CIGS) thin films were prepared by the sputtering of CuInGa (CIG) alloy precursors and CuGa/In stacked precursors followed by selenization. The effects of the using CIG precursors prepared by various methods and that of use of various selenization temperatures on the microstructural characteristics of CIGS thin films were investigated and the nature of phase transformation and Ga accumulation are discussed. Observation of the cross section morphologies revealed that the CIGS thin films prepared from CIG alloy precursors show two distinct types of structure. In addition, Ga was found to accumulate in high concentrations in the films. The X-ray diffraction (XRD) patterns of the films indicated the coexistence of CuInSe2 (CIS) and CIGS phases at low selenization temperatures. However, the CIGS phase seemed to disappear when the selenization temperature was increased to 580 °C. In comparison, the films prepared from CuGa/In stacked precursors also showed Ga accumulation after selenization. Surprisingly, the distinct structure of the films improved when the selenization temperature was increased to 580 °C. The XRD patterns of the films showed that the CIS and Ga-rich CIGS phases coexisted at low selenization temperatures. When the selenization temperature was increased to 580 °C, the positions of the CIS and Ga-rich CIGS peaks seemed to mix and shift to the position of Cu(In0.7,Ga0.3)Se2.