Chemical and Electronic Structure of the i-ZnO/InxSy:Na Front Contact Interface in Cu(In,Ga)(S,Se)2 Thin-Film Solar Cells

The chemical and electronic structure of the front contact i-ZnO/In_xS_y:Na interface for Cu(In,Ga)(S,Se)₂-based thin-film solar cells is investigated using a combination of x-ray and electron spectroscopies. Upon i-ZnO sputter deposition on the In_xS_y:Na buffer layer, we find an intermixed heterojunction and the formation of InO_x and Na₂SO₄. The window layer is shown to consist of a mixture of Zn(OH)₂ and ZnO, with decreasing relative Zn(OH)₂ content for thicker window layers. Moreover, we observe diffusion of sodium to the surface of the window layer. We derive electronic surface band gaps of the i-ZnO and In_xS_y:Na layers of 3.86 ± 0.18 eV and 2.60 ± 0.18 eV, respectively, and find a largely flat conduction band alignment at the i-ZnO/In_xS_y:Na interface.