The suppression of Cu-related charge localized defects in Cu2ZnSnS4 thin film solar cells

Cu2ZnSnS4 (CZTS) is a promising low cost thin-film solar cell material. However, the charge localized defects greatly hinder the improvement of the solar cell efficiency, thus the identification and knowledge of the possible charge localized defects in it are extremely important. Using hybrid functional calculation, we find that CuSn and CuZn are the main charge localized defects in CZTS. In detail, our results show that CuSn is a deep level recombination center. Moreover, the growth condition of Sn determines the population of CuSn because the stable chemical potential region of ΔμSn ([− 1.74, 0] eV) is larger than that of ΔμCu ([− 0.77, 0] eV). Thus Sn-rich growth condition is proposed to suppress the CuSn. As for CuZn antisites, part is deep acceptors which will be beneficial for the efficiency of solar cell, while part forms donor-acceptor pairs with ZnCu. The CuZn+ZnCu donor-acceptor pairs will lead to large potential fluctuation in CZTS, which is a disadvantageous factor. Fortunately, such pairs can be greatly suppressed by Cd doping due to two reasons: one is that the Cd atoms prefer to substitute the Zn atomic sites leading to the reduction of the CuZn concentration, and the other is that the Cd dopant in the CZTS makes it difficult for its neighboring Zn atom be substituted by Cu atom.