Cation disorder regulation by microstate configurational entropy in photovoltaic absorber materials Cu₂ZnSn(S,Se)₄

Cation disorder plays a major role in the performance deterioration of Cu₂ZnSn(S,Se)₄ (CZTSSe) photovoltaic (PV) absorbers. Unfortunately the quantitative impact of cation disorder in this material is not well understood. Here, we show that changes in microstate configurational entropy (S_MCE), predicted by a combination of statistical mechanics, density functional theory, and phonon calculations, can quantitatively describe cation disorder and the associated properties in CZTSSe. For example, the predicted critical temperature of the second-order phase transition and the thermal expansion anomaly of CZTS based on S_MCE are in good agreement with experiments. We further reveal that the more separated is the same kind of cations (Cu, Zn, or Sn), the lower the energy of the corresponding microstate will be, implying the maximum entropy probability distribution of cations at high temperatures. It is suggested that the introduction of S_MCE can serve as a framework to quantitatively understand and tailor cation disorder and associated properties for synthesis of the high-quality CZTS-based solar cells.