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.

2 based technologies. Zinc-blende related structures are formed by quaternary compounds, but the complexity associated with the multi-component system introduces diffi culties in material growth, characterization, and application. First-principles electronic structure simulations, performed over the past fi ve years, that address the structural, electronic, and defect properties of this family of compounds are reviewed. Initial predictions of the bandgaps and crystal structures have recently been verifi ed experimentally. The calculations highlight the role of atomic disorder on the cation sub-lattice, as well as phase separation of Cu 2 ZnSnS 4 into ZnS and CuSnS 3 , on the material performance for light-to-electricity conversion in photovoltaic devices. Finally, the current grand challenges for materials modeling of thin-fi lm solar cells are highlighted. 401 www.MaterialsViews.com www.advenergymat.de

No abstract

Cu 2 ZnSnS 4 is one of the most promising quaternary absorber materials for thin-film solar cells. Examination of the thermodynamic stability of this quaternary compound reveals that the stable chemical potential region for the formation of stoichiometric compound is small. Under these conditions, the dominant defect will be p-type CuZn antisite, which has an acceptor level deeper than the Cu vacancy. The dominant self-compensated defect pair in this quaternary compound is [CuZn−+ZnCu+]0, which leads to the formation of various polytype structures of Cu2ZnSnS4. We propose that to maximize the solar cell performance, growth of Cu2ZnSnS4 under Cu-poor/Zn-rich conditions will be optimal, if the precipitation of ZnS can be avoided by kinetic barriers.