Cu2ZnSnSSe4 (CZTSSe) thin film solar cells, with adjustable bandgap and rich elemental content, hold promise in next-gen photovoltaics. Crystalline quality is pivotal for efficient light absorption and carrier transport. During the post-selenization process, understanding crystal growth mechanisms, and improving layer quality are essential. We explored the effects of ramp rate and annealing temperature on CZTSSe films, using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), and ultraviolet-visual spectrophotometry (UV-Vis). The optimal performance occurred at 25.25 °C/min ramp rate and 530 °C annealing. This led to smoother surfaces, higher density, and larger grains. This condition produced a single-layer structure with large grains, no secondary phases, and a 1.14 eV bandgap, making it promising for photovoltaic applications. The study has highlighted the effect of selenization conditions on the characteristics of the CZTSSe absorber layer and has provided valuable information for developing CZTSSe thin film solar cells.