Phase stability and formation in the Cu-Zn-Sn-S system

Phases in the Cu-Zn-Sn-S-Se system such as Cu2ZnSn(S,Se)4 and Cu2SnS3 are promising solar cell absorber layer materials. The efficiencies of Cu2ZnSn(S,Se)4-based devices reached 12.6 % in early 2014 but have stabilized since then. Pure sulfide devices have reached efficiencies of around 9 % in 2016. These efficiencies were attained with little understanding of the underlying thermodynamics of the Cu-Zn-Sn-S system and the kinetics involved in phase formation. A deeper fundamental knowledge of stability in this system is seems necessary for future efficiency improvements. Furthermore, other ternary phases also represent promising materials for solar and thermoelectric applications. These ternary phases may be easier to control during processing compared to the quaternary Cu2ZnSnS4 phase. This work clarifies which phases in the Cu-Zn-Sn-S system are stable and investigates the relationships between these phases, with a focus on the Sn content of phases. It also investigates the effect of oxygen on the synthesis of Cu2ZnSnS4 in order to explain how, by reducing the Sn content of the resulting Cu2ZnSnS4 phase, the kesterite becomes more ordered