Modulation of the Pb/Sn ratio in Pb_1-xSn_xS thin films synthesized by chemical solution deposition

This work presents the development of a formulation for the synthesis of Pb_1-xSn_xS thin films by chemical bath deposition. The success of this formulation is due to the high miscibility between lead sulfide (PbS) and tin (II) sulfide (SnS). By adding different concentrations of a Pb²⁺ ions solution to a Sn²⁺ S²⁻ formulation, we obtain Pb_1-xSn_xS films with different Pb/Sn ratio. The presence of Pb²⁺ in those films is confirmed by X-ray energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Structural analysis by X-ray diffraction (XDR) shows a crystalline phase change from an SnS orthorhombic structure (00.00 mmol Pb²⁺) to a solid solution where the orthorhombic and cubic structures coexist (30.00 mmol Pb²⁺), and finally reaching the formation of a cubic PbS structure. Changes in the morphology of the films are observed together with the incorporation of Pb²⁺ ions in the films, reducing the size of nano-flakes and becoming a more compact layer, leading to a decrease in the film thickness from 370 nm (00.00 mmol Pb²⁺) to 96 nm (50.00 mmol Pb²⁺). The decrease of the energy band gap (Eg) of the samples from 1.92 to 1.38 eV with the increase of the Pb²⁺ concentration is also observed, obtaining wide absorption spectra from visible to near-infrared.