Phase transformation in Cu₂SnS₃ (CTS) thin films through pre-treatment in sulfur atmosphere

In this study, Cu₂SnS₃ (CTS) thin films prepared by a two-step sulfurization process were characterized. Cu and Sn metallic layers were first deposited on glass substrates by sputtering and then annealed in-situ while in the sputtering chamber to obtain CuSn (CT) alloys. This was followed by a pre-treatment step at temperatures between 200 and 350 °C in presence of S vapors. Finally, a full sulfurization step was performed at 525 °C to obtain the desired CTS phase. CTS films were characterized using EDX, XRD, Raman spectroscopy, SEM, optical transmission and Van der Pauw methods. It was found that all CTS samples had Cu-poor chemical composition. XRD data revealed only diffraction peaks belonging to CTS structure after the full sulfurization step. Raman spectra of the samples showed that except for the CTS sample pre-treated at 250 °C (CTS-250), which displayed the tetragonal crystal system, the films were dominated by the monoclinic structure. SEM surface images showed dense and polycrystalline microstructure, CTS-200 sample exhibiting a more uniform morphology. Optical band gap values were found to be ranging from 0.92 to 1.19 eV. All samples showed p-type conductivity but the sample pre-treated at 350 °C had higher resistivity and lower carrier concentration values. Overall, the CTS layer prepared using the pre-treatment step at 200 °C exhibited more promising structural and optical properties for potential photovoltaic applications. This work demonstrated that it is possible to change the crystal structure of sulfurized CTS thin films through a pre-treatment step.