Chemical and Electronic Structure of the i‐ZnO/In$_x$S$_y$ :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$_x$S$_y$: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 InxSy:Na buffer layer, we find an intermixed heterojunction and the formation of InOx and Na$_2$SO$_4$. 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/In$_x$S$_y$:Na interface.