Classification of Lattice Defects in the Kesterite Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ Earth‐Abundant Solar Cell Absorbers

TLDR

Kesterite Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ are promising earth‑abundant thin‑film solar cell absorbers, but their quaternary composition introduces many intrinsic lattice defects that influence optical and electrical properties, and experimental identification is hampered by poor sample quality. This review surveys recent theoretical studies of defect formation and ionization in kesterite materials and contrasts them with the better‑studied chalcopyrite CuGaSe₂ and CuInSe₂. The authors employ systematic first‑principles calculations to evaluate defect formation energies and ionization levels in kesterites, enabling comparison with chalcopyrite analogues. The calculations reveal four key insights: (i) strong phase competition with secondary compounds; (ii) intrinsic p‑type conductivity governed by CuZn antisites and vacancies sensitive to Cu/(Zn+Sn) and Zn/Sn ratios; (iii) charge‑compensated defect clusters that drive non‑stoichiometry; and (iv) electron‑trapping by [2CuZn+SnZn] clusters, especially in Cu₂ZnSnS₄, explaining why Cu‑poor, Zn‑rich growth yields the highest efficiencies and indicating a sulfur‑composition‑related efficiency limit.

Abstract

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 CuZn 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 [2CuZn +SnZn ], [VCu +ZnCu ] and [ZnSn +2ZnCu ] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn +SnZn ] clusters, especially in Cu2ZnSnS4. 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.