Low Temperature Chemical Bath Deposition of Tin Sulfide-Based Thin Films: Structural and Optical Properties

The development of earth-abundant, low-cost and chemically stable semiconductor absorbers is essential for advancing next-generation photovoltaic technologies. Among the promising candidates, copper–tin sulfide and tin sulfide based chalcogenide materials have attracted considerable attention owing to their suitable optical properties, tunable composition, and compatibility with solution-based deposition techniques. However, a comprehensive understanding of how the composition and the underlying substrate affect the structural, morphological, and optical characteristics of these thin films remains important for their rational integration into photovoltaic device architectures. The aim of this paper was to investigate copper–tin sulfide and tin sulfide thin films, prepared by chemical bath deposition on glass and ITO/glass substrates in order to show the influence of the composition and the substrate nature on their structural, morphological, and optical properties for photovoltaic applications. Thin films of copper–tin sulfide (Cu:Sn molar ratios of 1:1 and 2:1) and tin sulfide were prepared by chemical bath deposition at low temperature under identical conditions and characterized as follows: elemental composition by X-ray fluorescence (XRF), structural ordering by X-ray diffraction (XRD), morphology by atomic force microscopy (AFM), optical properties by spectroscopic ellipsometry (SE), and vibrational modes of components by Raman spectroscopy. The AFM and XRD analyses showed that the Cu-containing films are amorphous, with RMS roughness of 46.0 nm (Cu:Sn = 1:1) and up to 120.5 nm (Cu:Sn = 2:1). Without any thermal treatment, the substrate determines the tin sulfide phase: on glass, hexagonal SnS2 (berndtite) forms as nanoplates a few layers thick (crystallite size ~11–13 nm, RMS roughness 54.8 nm), whereas on ITO/glass, three-dimensional orthorhombic SnS (herzenbergite) crystallites of ~18 nm grow (RMS roughness 47.8 nm). The films show absorption coefficients of (2–5) × 104 cm−1 at 630 nm and optical band gaps between 1.87 and 2.70 eV, which make them candidates for further investigation as absorber layers in solar cells.