For parabolic grain growth, there should be a linear relationship between x2 and t. However, as shown in Fig. 2(b), this is clearly not the case and suggests that the rate constant Kp, is time dependent. The parabolic model assumes that the diffusion of selenium anions is the rate-controlling step of the reaction and the diffusivity of the CZTSSe large grain layer is assumed to be a constant. This assumption implies that the resultant large grain should be uniform, continuous and of single phase during the heat treatment period. However, analysis of the thin films at different selenization times reveals that the composition of the absorber layer is time dependent. As shown in Table 1, about 80% of sulfur is replaced by selenium in the first 5 min of the heat treatment. This increases to 94% after 40 min. As a result, the CZTSSe grain growth reaction process cannot be simply described by the parabolic mode.