Classification of lattice defects in the kesterite Cu₂ZnSnS ₄ and Cu₂ZnSnSe₄ earth-abundant solar cell absorbers

The kesterite-structured semiconductors Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ are drawing considerable attention recently as the active layers in earth-abundant low-cost thin-film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. Experimental identification of these defects is currently limited due to poor sample quality. Here recent theoretical research on defect formation and ionization in kesterite materials is reviewed based on new systematic calculations, and compared with the better studied chalcopyrite materials CuGaSe₂ and CuInSe₂. Four features are revealed and highlighted: (i) the strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p-type conductivity determined by the high population of acceptor Cu_Zn antisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge-compensated defect clusters such as [2Cu_Zn+Sn_Zn], [V_Cu+Zn _Cu] and [Zn_Sn+2Zn_Cu] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2Cu _Zn+Sn_Zn] clusters, especially in Cu₂ZnSnS ₄. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu₂ZnSn(S,Se)₄ cells when the S composition is high.