Cadmium-Free Buffer Layer Engineering for Spray-Deposited Cu₂ZnSnS₄ Thin-Film Solar Cells Using ZnS

Abstract

The development of a cadmium-free buffer layer for Cu₂ZnSnS₄ (CZTS) thin-film solar cells, using spray-pyrolyzed ZnS in place of toxic CdS was reported in this study. CZTS absorber films (bandgap ~1.45 eV) were deposited by spray pyrolysis from Cu:Zn:Sn = 1:1:1 precursor, and Cd-free ZnS (bandgap ~3.6 eV) buffers were co-spray-deposited on ZnO-coated ITO substrates. The fabrication and characterisation of an ITO/ZnO/ZnS/CZTS/Ag device are described. Structural and optical analyses confirm pure kesterite CZTS (no significant Cu₂SnS₃ or ZnS secondary phases) and a transparent ZnS layer. The ZnS buffer film exhibited high visible transmittance (>90%) and a direct bandgap of ~3.6 eV. The completed solar cell achieved Voc= 0.62 V, Jsc= 18.5 mA/cm², fill factor ~0.63 and efficiency η = 7.2%. These results (which surpass many previous CZTS/ZnS devices) demonstrate that spray-deposited ZnS is a viable Cd-free buffer, with performance improvements attributed to its wide bandgap and high transparency. Key findings include the confirmation of ZnS structural properties (XRD, Raman, SEM) and the reconciliation of previously inconsistent performance values.