First Principles Calculations of Defect Formation in In-Free Photovoltaic Semiconductors Cu₂ZnSnS₄ and Cu₂ZnSnSe₄

Abstract

To quantitatively evaluate the formation energies of Cu, Zn, Sn, and S vacancies in kesterite-type Cu 2 ZnSnS 4 (CZTS), first-principles pseudopotential calculations using plane-wave basis functions were performed. The formation energies of neutral Cu, Zn, Sn, and S vacancies were calculated as a function of the atomic chemical potentials of constituent elements. We compared the vacancy formation in the In-free photovoltaic semiconductor CZTS with those of Cu 2 ZnSnSe 4 (CZTSe) and CuInSe 2 (CIS). The obtained results were as follows. (1) Under the Cu-poor and Zn-rich condition, the formation energy of the Cu vacancy was generally smaller than those of the Zn, Sn and S vacancies in CZTS, as is the case for CZTSe. (2) The formation energies of Cu, Zn, and Sn vacancies in CZTS were larger than those in CZTSe. On the other hand, the formation energy of the S vacancy is smaller than that of the Se vacancy in CZTSe. (3) Under the Cu-poor and Zn-rich condition, the formation energies of the Cu vacancy in CZTS and CZTSe are much larger than that in CIS. These results indicate that in kesterite-type CZTS and CZTSe, the Cu vacancy is easily formed under Cu-poor, Zn-rich, and S(Se)-rich condition, but it is more difficult than that in CIS.