Influence of solvent chemistry and pre-annealing temperature on the structural optical and electrical properties of solution-processed CTS thin films

Abstract In this study, Cu 2 SnS₃ (CTS) thin films were synthesized using two different solution types: an organic solvent 2-methoxyethanol (2ME) and 2-methoxyethanol/distilled water (2-ME/DW), and the effects of solution type and pre-annealing temperature (200, 250, and 300 °C) on the structural and electrical properties of CTS films were systematically investigated. The solvent type influences precursor decomposition and crystallization behavior . Structural analysis revealed that the sample prepared with 2-ME (Type-I CTS) and pre-annealed at 200 °C (CTS-I-200) exhibited the largest crystallite size (22.52 nm) and the lowest values of microstrain (1.6 × 10 –3 ), dislocation density (1.97 × 10 –3 nm −2 ), and grain boundary density (1.14 nm −2 ), which are beneficial for photovoltaic performance due to improved charge transport and reduced recombination losses. Raman and XRD results confirmed formation of monoclinic CTS phase and indicated that the CuS phase tends to form in the bulk region rather than on the surface. Additionally, pre-annealing at 200 °C led to a more homogeneous grain distribution, with large grains integrating into the film structure. Elemental analysis showed that all samples had Cu-rich composition regardless of solution type and pre-annealing treatment. With increasing temperature, the sulfur content decreased in Type-I CTS samples but increased in aqueous-based (Type-II CTS) samples, indicating more effective sulfur retention in the presence of distilled water. The optical bandgap (Eg) varied slightly in Type-I CTS films (0.98–1.02 eV) and more significantly in Type-II CTS films (0.99–1.07 eV), influenced by Cu/Sn ratio and secondary phases like SnS 2 . All films showed p-type conductivity. The CTS-I-200 film showed 128 × 10 17 cm −3 carrier concentration and 3.96 Ω.cm resistivity values, while Type-II CTS films exhibited higher carrier concentration and lower resistivity due to the presence of Cu-rich secondary phases. Overall, it was concluded that both the solution type and pre-annealing temperature have a significant impact on the structural, optical and electrical properties of CTS thin films. Based on all the analyses performed, the CTS-I-200 sample, produced using the 2-ME solution and pre-annealed at 200 °C, was found to exhibit superior structural properties for potential solar cell applications.