Influence of copper to indium atomic ratio on the properties of Cu–In–Te based thin-film solar cells prepared by low-temperature co-evaporation

Abstract

The influence of copper to indium atomic ratio (Cu/In) on the properties of Cu–In–Te based thin films and solar cells was investigated. The films (Cu/In = 0.38–1.17) were grown on both bare and Mo-coated soda-lime glass substrates at 250 °C by single-step co-evaporation using a molecular beam epitaxy system. Highly (112)-oriented CuInTe2 films were obtained at Cu/In ratios of 0.84–0.99. However, stoichiometric and Cu-rich films showed a poor film structure with high surface roughness. The films consist of polyhedron-shaped grains, which are related to the coexistence of a Cu2−xTe phase, and significant evidence for the coexistence of the Cu2−xTe phase in the stoichiometric and Cu-rich films is presented. KCN treatment was performed for the films in order to remove the Cu2−xTe phase. The stoichiometric CuInTe2 thin films exhibited a high mobility above 50 cm2/V s at room temperature after the KCN treatment. A preliminary solar cell fabricated using a 1.4-μm-thick Cu-poor CuInTe2 thin film (Cu/In = 0.84, Eg = 0.988 eV) yielded a total-area efficiency of 2.10%. The photovoltaic performance of the cell was improved after long-term ambient aging in dark conditions.