Tailoring flexible Cu₂SnS₃ thin film solar cells through controlled heating rate and sulfurization time

Cu₂SnS₃ (CTS) thin films were fabricated on flexible Mo foil substrates using a two-stage process consisting of spin coating of precursor films followed by sulfurization in a Rapid Thermal Processing (RTP) system at 525 °C. The effects of heating rate and sulfurization time on the structural, morphological, optical, electrical, and photovoltaic properties of the CTS films were systematically investigated. X-ray diffraction and Raman spectroscopy confirmed that all films predominantly crystallized in the monoclinic CTS phase, accompanied by minor tetragonal CTS and trace Cu₂Sn₃S₇ secondary phases. A phase transformation from tetragonal to monoclinic CTS was promoted at heating rates above 0.50 °C/s. Increasing the heating rate to 1.0 °C/s while maintaining a sulfurization time of 1 s significantly improved crystallinity, resulting in larger crystallite sizes, reduced lattice strain, and a denser, more uniform surface morphology. In contrast, extending the sulfurization time from 1 to 90 s degraded the structural quality by decreasing the crystallite size and increasing lattice strain and dislocation density, accompanied by non-uniform grain growth and the formation of capillary-like microstructures. The CTS films exhibited optical band gaps ranging from 1.00 to 1.09 eV, with a slight band gap narrowing at longer sulfurization times, which can be attributed to polymorphic phase evolution, defect-induced band tailing, and compositional changes. Electrical characterization revealed that all films exhibited p-type conductivity, yielding hole mobility up to 22.5 cm 2 /V·s depending on the heating rate profile. Conversely, prolonged sulfurization increased resistivity while reducing carrier concentration and drastically suppressing the mobility (down to 0.94 cm 2 /V·s) because of enhanced defect density, carrier scattering at grain boundaries, and the specific emergence of the Cu₂Sn₃S₇ secondary phase. The photovoltaic performance of flexible CTS solar cells strongly correlated with the structural and electrical quality of the absorber layers. The highest power conversion efficiency of 0.9% was achieved for the film sulfurized at a heating rate of 1.0 °C/s for 1 s, which exhibited optimal electrical properties, demonstrating that optimized sulfurization conditions promote efficient charge transport and carrier collection. Overall, a high heating rate combined with a short sulfurization time provides the optimum balance between crystallinity, morphology, and electrical properties, highlighting the potential of flexible CTS thin films as absorber layers for next-generation flexible thin film solar cells.