Formation and impact of secondary phases in Cu-poor Zn-rich Cu₂ZnSn(S_1−ySe_y)₄ (0≤y≤1) based solar cells

Cu₂ZnSn(S_1−ySe_y)₄ (CZTSSe, 0≤y≤1) based solar cells relying on earth abundant and low toxic elements are attracting a lot of research interest for photovoltaic applications. Currently, the record efficiency (12.6%) is still far away from the required value (15–18%) for future commercialization. One of the main issues concerning the performance limitation of the devices is certainly the formation of secondary phases, coupled with the off-stoichiometric growth conditions used for high efficiency solar cells. In this work, we prepared Cu-poor and Zn-rich CZTSSe absorbers and solar cells in the whole S–Se compositional range, with the aim of analyzing the formation of secondary phases and their impact on the optoelectronic properties of the devices. The results show that much less secondary phases (especially Zn(S,Se)) are prone to be present on the surface and at the back region of Se-rich CZTSSe absorbers, when compared with other types of kesterites like Cu₂ZnSnS₄, Cu₂ZnSnSe₄ and S-rich CZTSSe. CZTSSe solid solutions almost free of secondary phases on the surface and at the back region can be obtained by the appropriate control of different process parameters: thermal treatment (low Ar pressure), composition (simultaneous sulfo-selenization) and surface etching (using (NH₄)₂S solutions). Based on this absorber with very low secondary phases content, best efficiency of 6.8% for a Se-rich CZTSSe solar cell was achieved using a single-step sulfo-selenization process.