Stability of Cd 1-x Zn x O y S 1-y Quaternary Alloys Assessed with First-Principles Calculations.

One route to decreasing the absorption in CdS buffer layers in Cu(In,Ga)Se ₂ and Cu ₂ ZnSn(S,Se) ₄ thin-film photovoltaics is by alloying. Here we use first-principles calculations based on hybrid functionals to assess the energetics and stability of quaternary Cd, Zn, O, and S (Cd _1-x Zn _x O _y S _1-y ) alloys within a regular solution model. Our results identify that full miscibility of most Cd _1-x Zn _x O _y S _1-y compositions and even binaries like Zn(O,S) is outside typical photovoltaic processing conditions. The results suggest that the tendency for phase separation of the oxysulfides may drive the nucleation of other phases such as sulfates that have been increasingly observed in oxygenated CdS and ZnS.