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/InxSy: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 InxSy:Na buffer layer, we find an intermixed heterojunction and the formation of InOx 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 InxSy: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/InxSy:Na interface.