Effects of the growth process on surface morphology of Cu₂(Sn_1−xGe_x)S₃ thin films

As a first step toward the realization of high-efficiency Cu₂(Sn_1-xGe_x)S₃ (CTGS) solar cells, this study investigates the effects of the growth process, focusing on the crystalline structure and morphology of CTGS films formed via different growth processes. These processes include the sulfurization of the Ge/Cu/Sn-S precursor; sulfurization of Cu₂SnS₃ (CTS) with S and germanium sulfide (GeS) vapor; and co-evaporation of Cu, Sn, Ge, and S. The CTGS films obtained by sulfurization of the Ge/Cu/Sn-S precursor consistently exhibited the largest grain sizes. However, the surface roughness of CTGS films increased with increasing x ratio. Conversely, the CTGS films obtained by the re-sulfurization of CTS films previously formed in S and GeS mixed vapor exhibited both a large grain size and flat surface roughness, rendering them suitable for forming the p-n interface in a solar cell. Therefore, the re-sulfurization of CTS films previously formed in S and GeS mixed vapor is key for achieving high-efficiency solar cells.