Ternary and multinary Cu-chalcogenide photovoltaic materials from CulnSe_2 to Cu_2ZnSnS_4 and other compounds

摘要

Such ternary and multinary copper chalcogenides (TMCC) as CuInSe_2 (CIS) and Cu_2ZnSnS_4 are suitable as absorber materials for thin-film solar cells. The valence band consists of Cu 3d and chalcogen p orbitals, and the conduction band consists of HI (In) res or IV (Sn) res and chalcogen p orbitals. The band structure characteristics of TMCC differ from those of conventional H-VI compounds. The valence band maximum is higher, and the bandgap is narrower than II-VI compounds. Therefore, the electrons in the valence band of TMCC are easily removed, and TMCC tends to be a p-type semiconductor. This common characteristic of TMCC is one reason that it is suitable as an absorber material in thin-film solar cells. In CIS, two kinds of chemical bond, Cu-Se and In-Se, exist. Among them, the In-Se bond is much stronger than the Cu-Se bond. This characteristic is advantageous for the "three-stage process" of high-quality CIS thin films used in solar cells. However, the characteristics of the other TMCC are not the same as those of CIS. A deposition process of high-quality TMCC thin films that exploits the characteristics of chemical bonding in TMCC should be developed.