Site occupations of Zn in AgInSe2-based chalcopyrites responsible for modified structures and significantly improved thermoelectric performance

Abstract

The band structures of AgInSe₂-based semiconductors have been calculated and the lifting of the Fermi level toward the conduction band in AgInSe₂ when Ag is replaced by Zn has been observed. This is mainly caused by the site occupation of Zn on the cation Ag site, which leads to the formation of the defect Zn_Ag¹⁺ as an active donor. While the Fermi level lowers toward the valence band when In is replaced by Zn, due to the primary formation of an acceptor Zn_In¹⁻. The ZT value reaches 0.95 ± 0.10 at ∼815 K through substituting Zn for Ag and In simultaneously. However, a higher ZT value of 1.05 ± 0.12 has been achieved by substituting an appropriate amount of Zn for Ag through largely enhancing the carrier concentration n and reducing the lattice thermal conductivity via modifying the crystal structure. Hence, we propose that when Ag is replaced by Zn in AgInSe₂ there are at least two factors i.e. the carrier concentration n and bandgap E_g that govern the electrical property, and that the enhancement in carrier concentration n seems to have a more prominent effect than the widening of bandgap E_g does.