Structural and optical properties of (Ag_xCu_1-x)₂ZnSnS₄ thin films synthesised via solution route

Silver (Ag) has been introduced in the quaternary compound Cu₂ZnSnS₄ synthesised via solution chemistry. Thin films of the resulting pentanary alloys (Ag_xCu_1-x)₂ZnSnS₄ (0 ≤ x ≤ 1) show remarkable change in their microstructure and electronic properties with the increasing Ag content. Going from Cu₂ZnSnS₄ (x = 0) to Ag₂ZnSnS₄ (x = 1), the grain size increased from 0.13 to 2 μm which could be attributed to the liquid assisted grain growth mechanism. The optical band gap increased from 1.5 to 2.0 eV due to the influence of d-orbitals of Ag atoms on the valence band edge. The charge carrier density decreased by two orders of magnitude with only 4 atomic % Ag (x = 0.04) incorporated in the films, which can be attributed to the reduction in the density of Cu_Zn and V_Cu point defects with Ag atoms occupying the Cu-sublattice sites. Also, with increasing Ag content the lattice parameters a and b increased from 5.42 to 5.82 Å, whereas there was negligible change in the lattice parameter c.