First-Principles Studies on Cd Doping in CuInSe2 and Related Compounds during Chemical Bath Deposition of CdS Buffer Layer

Abstract

First-principles calculations using plane-wave basis functions were performed to quantitatively evaluate the substitution energies of Cd atom for Cu atom and for In atoms in chalcopyrite-type CuInSe2 (CIS) and related compounds, CuGaSe2 (CGS) and CuAlSe2 (CAS). The substitution energies of Cd atoms in CIS and the related compounds were calculated in considering of the atomic chemical potentials of the constituent elements of Cu and In, and doping Cd atoms. During the chemical bath deposition (CBD) of the CdS layer on the CIGS layer, Cu and Cd atoms dissolved in the ammonia aqueous solution and formed [Cu(NH3)2]+ and [Cd(NH3)4]2+ complex ions. Therefore, the chemical potentials of Cu and Cd atoms in [Cu(NH3)2]+ and [Cd(NH3)4]2+ complex ions were calculated. We found that the substitution energy of n-type CdCu is smaller than that of p-type CdIn. The substitution energy of CdCu in CIS is smaller than those in CGS and CAS. However, the substitution energies of CdCu and CdIn in CIS are positive values. The formation energy of charge-neutral Cd doping with the Cu vacancy (CdCu+VCu) pair is a negative value and greatly smaller than those of n-type CdCu in CIS, CGS, and CAS. These results indicate that the charge-neutral (CdCu+VCu) vacancy pair is easily formed during the CBD of the CdS layer on the CIS layer and a small amount of n-type CdCu would also be formed.