First-Principles Study on Cd Doping in Cu₂ZnSnS₄ and Cu₂ZnSnSe₄

Abstract

To quantitatively evaluate the substitution energies of Cd atom for Cu, Zn, or Sn atom in indium-free photovoltaic semiconductors Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe), first-principles pseudopotential calculations using plane-wave basis functions were performed. The substitution energies of Cd atom in kesterite-type CZTS and CZTSe were calculated in consideration of the atomic chemical potentials of the constituent elements of Cu, Zn, Sn, and the doping atom of Cd. During the chemical bath deposition (CBD) of the CdS layer on the CZTS or CZTSe layer, Cu, Zn, and Cd atoms dissolved in the ammonia aqueous solution and formed [Cu(NH 3 ) 2 ] + , [Zn(NH 3 ) 4 ] 2+ , and [Cd(NH 3 ) 4 ] 2+ complex ions. Therefore, the chemical potentials of Cu, Zn, and Cd atoms in [Cu(NH 3 ) 2 ] + , [Zn(NH 3 ) 4 ] 2+ , and [Cd(NH 3 ) 4 ] 2+ complex ions were calculated. We found that the substitution energies of n-type Cd Cu and charge-neutral Cd Zn in CZTS and CZTSe are smaller than that of p-type Cd Sn . The substitution energies of Cd Cu in CZTS and CZTSe are smaller than that in chalcopyrite-type CuInSe 2 (CIS). However, the substitution energies of Cd Cu , Cd Zn , and Cd Sn are positive values. The formation energy of charge-neutral Cd doping with the Cu vacancy (Cd Cu + V Cu ) pair is a negative value and greatly smaller than those of donor-type Cd Cu and neutral Cd Zn in CZTS and CZTSe. These results indicate that the charge-neutral (Cd Cu + V Cu ) vacancy pair is easily formed during the CBD of the CdS layer on the CZTS or CZTSe layer. A small amount of n-type Cd Cu and neutral Cd Zn would also be formed.