Effect of Na doping on structural and optical properties in Cu2ZnSnS4 thin films synthesized by thermal evaporation method

Highlights: • Na-doped Cu₂ZnSnS₄ thin films were prepared via thermal evaporation method. • Na incorporation in Cu₂ZnSnS₄ thin films improve the cristallinity and graine size. • Cu₂ZnSnS₄: Na 5% sample shows optical band gap of 1.59 eV with p-type conductivity. -- Abstract: Quaternary chalcogenide Cu₂ZnSnS₄ (CZTS) compound, a potential material for application as absorber layer in thin film solar cells, is synthesized by direct melting of the constituent elements taken in stoichiometry compositions. Alkali element Na was incorporated into CZTS thin films synthesized by thermal evaporation method, in order to further improve the structural and optical properties. X-Ray diffraction (XRD), Raman spectroscopy, Energy dispersive spectrometry and optical spectrophotometry were used to characterise the phase purity and optical properties. It showed that the diffusion of Na ions is uniform in the films after annealing. XRD analysis showed that CZTS films possess polycrystalline structure with [221] preferred orientation. Na ions incorporation in CZTS thin films could improve the cristallinity, the graine size and the absorption coefficient. For CZTS: Na 5%, optical results revealed higher absorption coefficient (>10⁵ cm⁻¹) and direct optical band gap of 1.56 eV with p-type conductivity.