An EXAFS Analysis of Cu$_2$SnS$_3$ for Extremely Thin Absorber Layer

We present local structure studies of Cu$_2$S and Cu$_2$SnS$_3$ composite films prepared with CVD, using extended x-ray absorption fine structure (EXAFS) technique. The EXAFS technique has the ability to probe the local environment of specific atoms, and can also give very precise ratios of elements using their fluorescence peaks. Chemical vapor deposition (CVD) deposits highly conformal films and hence is an important tool for developing nanostructured solar cells with scalability. Cu$_2$SnS$_3$ is an earth-abundant absorber that is even more cost-effective when used in an extremely thin absorber solar cell. Composite films of Cu$_2$SnS$_3$ were made using CVD layers of Cu$_2$S and Tin (IV) Sulfide (SnS$_2$) with an anneal step. Cu$_2$SnS$_3$ also has the same structure as ZnS, which allows for the formation of the quaternary Cu$_2$ZnSnS$_4$ by depositing ZnS on top of the Cu$_2$S and SnS$_2$ layers determined for Cu$_2$SnS$_3$. Stoichiometric control was established by varying the deposition times of the binary compounds and was measured using energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), and EXAFS techniques. Optical absorption results are promising for forming a photovoltaic device with copper-based ternary and quaternary materials as the absorber.