Materials science of chalcopyrite-type and multinary solar cells compounds: crystal structure and electronic structure of CISe, CZTS, CTS, and the related compounds

Abstract

Ternary and multinary compounds used in polycrystalline thin-film solar cells are I–III–VI₂ semiconductors typified by CuInSe₂ (CISe), I₂–II–IV–VI₄ compounds such as Cu₂ZnSnS₄ (CZTS), and I₂–IV–VI₃ compounds such as Cu₂SnS₃ (CTS). Their crystal structures are chalcopyrite type (space group I42d (122)) for CISe, kesterite type (space group I4 (79)) for CZTS, and monoclinic system (space group Cc (9)) for CTS. In this review, we describe the characteristics of their crystal structures and electronic structures such as band-gap energy and energy levels of valence band maxima (VBM) and conduction band minima (CBM). The differences of electronic structures between ZnSe and CuGaSe₂, CuInSe₂ and Cu₂ZnSnSe₄, and Cu₂ZnSnS₄ and Cu₂SnS₃ will be discussed on the basis of the results of theoretical calculations and molecular orbital energy-level diagrams.