Utilizing\nSolvent Repulsion between Dimethylformamide\nand Isopropanol to Manipulate Sn Distribution for Bifacial Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> Solar Cells

Rationalizing the\nrole of chemical interactions in the precursor\nsolutions on the structure, morphology, and performance of thin-film\nCu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> (CZTSSe) is key for the development\nof bifacial and other photovoltaic (PV) device architectures designed\nby scalable solution-based methods. In this study, we uncover the\nimpact of dimethylformamide (DMF) and isopropanol (IPA) solvent mixtures\non cation complexation and rheology of the precursor solution, as\nwell as the corresponding morphology, composition, and PV performance\nof CZTSSe thin-film grown on fluorine-doped tin oxide (FTO). We find\nthat increasing the proportion of IPA leads to a nonlinear increase\nin dynamic viscosity due to the strong repulsion between DMF and IPA,\nwhich is characterized by an interaction cohesion parameter of 3.06.\nThe repulsive solvent interaction not only leads to complex dependence\non absorber thickness and surface roughness but also on composition\ndisorder in the annealed CZTSSe films. Systematic studies involving\nRaman, scanning electron microscopy, SIMS, XPS, and energy-filtered\nphotoemission of electron microscopy show that adding 25% of IPA to\nDMF leads to thin films with a high degree of structure and composition\nhomogeneity in comparison to pure DMF-based precursors. Further increasing\nthe IPA content promotes Sn surface segregation and secondary phases,\nwhich have a clear impact on the surface electronic landscape of the\nabsorber layer. This analysis allows for the rationalization of the\ndevice performance with the stack configuration glass/F:SnO<sub>2</sub>/CZTSSe/CdS (50 nm)/i-ZnO (50 nm)/Al:ZnO (500 nm)/Ag (500 nm).