Structural and elemental characterization of high efficiency $Cu2ZnSnS4$ solar cells

We have carried out detailed microstructural studies of phase separation and grain boundary composition in $Cu2ZnSnS4$ based solar cells. The absorber layer was fabricated by thermal evaporation followed by post high temperature annealing on hot plate. We show that inter-reactions between the bottom molybdenum and the $Cu2ZnSnS4$⁠, besides triggering the formation of interfacial $MoSx$⁠, results in the out-diffusion of Cu from the $Cu2ZnSnS4$ layer. Phase separation of $Cu2ZnSnS4$ into ZnS and a Cu–Sn–S compound is observed at the molybdenum-$Cu2ZnSnS4$ interface, perhaps as a result of the compositional out-diffusion. Additionally, grain boundaries within the thermally evaporated absorber layer are found to be either Cu-rich or at the expected bulk composition. Such interfacial compound formation and grain boundary chemistry likely contributes to the lower than expected open circuit voltages observed for the $Cu2ZnSnS4$ devices.