I suggest you to have a look at the following, interesting paper:
-Is It Possible To Develop Complex S–Se Graded Band Gap Profiles in Kesterite-Based Solar Cells?
Jacob Andrade-Arvizu, Víctor Izquierdo-Roca, Ignacio Becerril-Romero, Pedro Vidal-Fuentes, Robert Fonoll-Rubio, Yudania Sánchez, Marcel Placidi, Lorenzo Calvo-Barrio, Osvaldo Vigil-Galán, and Edgardo Saucedo