Ionization potentials of (112) and (112¯) facet surfaces of CuInSe2and CuGaSe2

Abstract

The ionization potentials of the faceted and nonfaceted (110) surfaces of CuInSe${}_{2}$ (CIS) and CuGaSe${}_{2}$ (CGS), which are key components of CuIn${}_{1\ensuremath{-}x}$Ga${}_{x}$Se${}_{2}$ (CIGS) thin-film solar cells, are investigated using first-principles calculations based on a hybrid Hartree-Fock density functional theory approach. Slab models of the chalcopyrite (110) surface with both (112) and (11$\overline{2}$) facets on each surface of the slab are employed. Surface energy evaluations point out that two types of faceted surfaces with point defects, namely a combination of Cu${}_{\mathrm{In}}$ (Cu${}_{\mathrm{Ga}}$) and In${}_{\mathrm{Cu}}$ (Ga${}_{\mathrm{Cu}}$) antisites and a combination of Cu vacancies and In${}_{\mathrm{Cu}}$ (Ga${}_{\mathrm{Cu}}$) antisites, are the most stable depending on the chemical potentials. The ionization potentials are evaluated with two definitions: One highly sensitive to and the other less sensitive to localized surface states. The latter varies by 0.4 eV in CIS and 0.5 eV in CGS with the surface structure. The ionization potentials are reduced by 0.2 eV for faceted surfaces with Cu${}_{\mathrm{In}}$ (Cu${}_{\mathrm{Ga}}$) and In${}_{\mathrm{Cu}}$ (Ga${}_{\mathrm{Cu}}$) antisites when the effects of the localized surface states are considered. The values of both ionization potentials are similar between CIS and CGS with a difference of about 0.1 eV for the most stable surface structures.