Mn‐Mediated Phase Engineering for Efficient Cd‐Free Cu ₂ ZnSnS ₄ Solar Cells on Transparent Electrode

ABSTRACT Fluorine‐doped tin oxide (FTO) as a back electrode in Cu ₂ ZnSnS ₄ (CZTS) solar cells holds significant potential for building‐integrated photovoltaics (BIPV) and tandem cell applications. However, severe SnS ₂ secondary phase degrades device performance. Here, we report a Mn doping strategy to mitigate CZTS absorber phase purity. The results show that the formation of a large amount of SnS ₂ comes from reaction between FTO and vertical liquid SnS ₂ channel delivered S. The formation of MnS in doped film disrupts the liquid channel, which prevents back contact reaction. The mitigation of the detrimental secondary phase and incorporation of Mn into the lattice of CZTS (by occupying Zn site) greatly reduce defect concentration within absorber bulk and at heterojunction interface, leading to remarkably increased fill factor and open‐circuit voltage and an efficiency of 7.95% at doping concentration of 2.5%. Over Mn doping not only hinders grain growth but left undesirable MnS in the film, severely reducing film quality and device performance. Furthermore, wide bandgap Zn _{1‐ x} Sn _x O (ZTO) buffer layer was deposited to replace CdS, constructing a favorable “spike‐like” energy band alignment at the heterojunction with effectively suppressed interface recombination, first achieving a Cd‐free CZTS device on transparent substrate with an efficiency of 9.32%.