Thin-film ZnO/CdS/CuIn1-xGaxSe2 solar cells: Anomalous physical properties of the CuIn1-xGa xSe2 absorber

Abstract

Thin-film ZnO/CdS/CuIn1-xGaxSe2 solar cells are manufactured with ∼20% solar-cell conversion efficiency. The CuIn1-xGaxSe2 (CIGS) absorber exhibits rather different physical properties compared with conventional semiconductors (e.g. Si, GaAs and ZnSe). For instance, (i) the valence-band maximum in CIGS consists of cation-d-anion-p hybridized states. (ii) Cation vacancies have low formation energies and high mobility at room temperature. (iii) The most stable surface of CuIn1-xGaxSe2 is the reconstructed, and otherwise polar, (112) surface, (iv) Solar cells with absorbers containing grain-boundaries outperform cells with crystalline absorbers. In this work, the fundamental physical properties of CIGS, like the electronic structure, the defect formation energies, as well as surface properties are discussed from a theoretical perspective.