Cu2 ZnSnS4 is one of the most promising quaternary absorber materials for thin-film solar cells. Examination of the thermodynamic stability of this quaternary compound reveals that the stable chemical potential region for the formation of stoichiometric compound is small. Under these conditions, the dominant defect will be p -type CuZn antisite, which has an acceptor level deeper than the Cu vacancy. The dominant self-compensated defect pair in this quaternary compound is [CuZn- + Zn Cu+]0, which leads to the formation of various polytype structures of Cu2 ZnSnS4. We propose that to maximize the solar cell performance, growth of Cu2 ZnSnS4 under Cu-poor/Zn-rich conditions will be optimal, if the precipitation of ZnS can be avoided by kinetic barriers.