Limitations of Near Edge X-Ray Absorption Fine Structure as a Tool for observing conduction bands in Chalcopyrite Solar Cell Heterojunctions

A non-optimized interface band alignment in a heterojunction-based solar cell can have negative effects on the current and voltage characteristics of the resulting device. To evaluate the use of Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) as a means to measure the conduction band position, Cu(In,Ga)S2 chalcopyrite thin film surfaces were investigated as these form the absorber layer in solar cells with the structure ZnO/Buffer/Cu(In,Ga)S2/Mo/Glass. The composition dependence of the structure of the conduction bands of CuInxGa1−xS2 has been revealed for x = 0, 0.67 and 1 with both hard and soft NEXAFS and the resulting changes in conduction band offset at the junction with the buffer layer discussed. A comprehensive study of the positions of the absorption edges of all elements was carried out and the development of the conduction band with Ga content was observed, also with respect to calculated densities of states.