Is It Possible To Develop Complex S-Se Graded Band Gap Profiles in Kesterite-Based Solar Cells?

This work presents the development of a novel chalcogenization process for the fabrication of Cu2ZnSn(S,Se)4 (CZTSSe or kesterite)-based solar cells that enable the generation of sharp graded anionic compositional profiles with high S content at the top and low S content at the bottom. This is achieved through the optimization of the annealing parameters including the study of several sulfur sources with different predicted reactivities (elemental S, thiourea, SnS, and SeS2). As a result, depending on the sulfur source employed, devices with superficially localized maximum sulfur content between 50 and 20% within the charge depletion zone and between 10 and 30% toward the bulk material are obtained. This complex graded structure is confirmed and characterized by combining multiwavelength depth-resolved Raman spectroscopy measurements together with in-depth Auger electron spectroscopy and X-ray fluorescence. In addition, the devices fabricated with such graded band gap absorbers are shown to be fully functional with conversion efficiencies around 9% and with improved VOC deficit values that correlate with the presence of a gradient. These results represent one step forward toward anionic band gap grading in kesterite solar cells.