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

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.

Native point defects in CuIn1−xGaxSe2: hybrid density functional calculations predict the origin of p- and n-type conductivity

“…10-12 and our results, but the other acceptortype defects V In/Ga and Cu In/Ga are predicted to be deep in Ref. 11,12. Similarly, the donor-type defects In/Ga Cu are reported to be deep in both Ref.…”

Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency

“…Cu 2 SnS(e) 3 , Cu 2 S(e) and SnS(e) have smaller band gaps than CZTS(e) and should be avoided by any means as their presence will decrease the open circuit voltage [3]. In agreement with this presumption, CZTS(e) based solar cells with the highest efficiencies are found to be Cu-poor ( [3,17]. Under such growth conditions ZnS(e) can be easily formed.…”

Ordering-induced direct-to-indirect band gap transition in multication semiconductor compounds

“…This is because they contain relatively earth abundant elements, are easy to grow, and have suitable direct band gap energies in the range of 1 to 1.5 eV at their ground state, [1][2][3][4][5] which are optimal for single junction solar cell applications. 3,6 However, despite the recent extensive study and rapid progress in development of this new solar cell technology, its power conversion efficiency is still lower than the more mature technologies, such as binary CdTe or ternary Cu(In 1-x Ga x )Se 2 (CIGSe) based solar cells.…”