Utilizing Solvent Repulsion between Dimethylformamide and Isopropanol to Manipulate Sn Distribution for Bifacial Cu₂ZnSn(S,Se)₄ Solar Cells

Rationalizing the role of chemical interactions in the precursor solutions on the structure, morphology, and performance of thin-film Cu₂ZnSn(S,Se)₄ (CZTSSe) is key for the development of bifacial and other photovoltaic (PV) device architectures designed by scalable solution-based methods. In this study, we uncover the impact of dimethylformamide (DMF) and isopropanol (IPA) solvent mixtures on cation complexation and rheology of the precursor solution, as well as the corresponding morphology, composition, and PV performance of CZTSSe thin-film grown on fluorine-doped tin oxide (FTO). We find that increasing the proportion of IPA leads to a nonlinear increase in dynamic viscosity due to the strong repulsion between DMF and IPA, which is characterized by an interaction cohesion parameter of 3.06. The repulsive solvent interaction not only leads to complex dependence on absorber thickness and surface roughness but also on composition disorder in the annealed CZTSSe films. Systematic studies involving Raman, scanning electron microscopy, SIMS, XPS, and energy-filtered photoemission of electron microscopy show that adding 25% of IPA to DMF leads to thin films with a high degree of structure and composition homogeneity in comparison to pure DMF-based precursors. Further increasing the IPA content promotes Sn surface segregation and secondary phases, which have a clear impact on the surface electronic landscape of the absorber layer. This analysis allows for the rationalization of the device performance with the stack configuration glass/F:SnO₂/CZTSSe/CdS (50 nm)/i-ZnO (50 nm)/Al:ZnO (500 nm)/Ag (500 nm).