Successful formation of the absorber layer is regarded as the most important step when fabricating Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells. A high-quality CZTSSe absorber layer can enhance the parameters of thin-film solar cells. We concentrated on analyzing the CZTSSe surface, which is directly related to forming the p-n junction. We made changes to the surface of the absorber layer by controlling the Cu content via the sputtering time. When the Cu content was insufficient, secondary phases and agglomerated metallic precursors appeared on top of the CZTSSe absorber layer. We analyzed the defects and secondary phases that could adversely affect the formation of the p-n junction by using energy-dispersive X-ray spectroscopy, X-ray diffractometer, and Raman spectroscopy. We also measured and calculated the electric parameters of CZTSSe thin-film solar cells to investigate the CZTSSe/CdS interface. The performance of a CZTSSe thin-film solar cell was enhanced and stabilized by optimizing the concentration of Cu in the absorber layer. Our results indicate the importance of the Cu ratio on the surface of the CZTSSe absorber layer as it affects the p-n junction in the CZTSSe thin-film solar cell. The highest photoconversion efficiency was 9.4% when the Cu component was optimized.