Ab Initio Study of Carbon Impurities in Cu$_2$ ZnSnS$_4$

Cu
₂
ZnSnS
₄
(CZTS) is a highly promising absorber material for efficient and low-cost solar cells. In a cost-effective method of production of high-quality CZTS, nanocrystals of precursor compounds, solved in an amine solution, are deposited on a substrate and sulfurized. This can generally lead to considerable residual carbon in the final samples, in the form of separate amorphous layers as observed experimentally, but also incorporated in the CZTS matrix. In this study, preferred configurations, bonding characteristics, formation energies, and ionization levels of carbon-related defects in CZTS are calculated, including interstitials, single- and double-carbon antisites, and complexes of those with intrinsic defects. For different growth conditions of carbon, the set of dominant defects is determined, and the resulting total carbon concentrations are discussed with respect to experimental values.