In this study, we investigated influence of sequential heat treatment on the structural, morphological and optical properties of Cu2ZnSn(S,Se)4 (CZTSSe) thin films. Specifically, after preparation of sputtered Cu, ZnS, and Sn stacked layers, CZTSSe thin films were synthesized by sequential heat treatment (sulfurization follow by selenization). Analyses of the processed films show that non-uniform selenium distribution through the depth of the films can be obtained by introducing a controlled amount of selenium in the second heat treatment step. The top region of the film becomes selenium rich, while the bottom region is selenium poor. This structure has higher absorption coefficients than the uniform ones. In addition, the uneven composition distribution may lead to a bandgap gradient in the film, which can reduce the photocurrent loss. Finally, numerical modeling corroborates the potential of selenium gradient films for high efficiency CZTSSe solar cells.