The role of excess Sn in Cu₄Sn₇S₁₆ for modification of the band structure and a reduction in lattice thermal conductivity

Abstract

In this work, we have investigated the band structures of ternary Cu4Sn7+xS16 (x = 0–1.0) compounds with an excess of Sn, and examined their thermoelectric (TE) properties. First principles calculations reveal that the excess Sn, which exists as Sn2+ and is preferentially located at the intrinsic Cu vacancies, unpins the Fermi level (Fr) and allows Fr to enter the conduction band (CB) at x = 0.5. Accordingly, the Hall carrier concentration (nH) is enhanced by about two orders of magnitude when the x value increases from x = 0 to x = 0.5. Meanwhile, the lattice thermal conductivity (κL) is reduced significantly to 0.39 W K−1 m−1 at 893 K, which is in reasonably good agreement with the estimation using the Callaway model. As a consequence, the dimensionless TE figure of merit (ZT) of the compound Cu4Sn7+xS16 with x = 0.5 reaches 0.41 at 863 K. This value is double that of the stoichiometric Cu4Sn7S16, proving that excess Sn in Cu4Sn7S16 is beneficial for improving the TE performance

Improved thermoelectric performance of solid solution Cu4Sn7.5S16 through isoelectronic substitution of Se for S

“…Figure 3 shows the density of states (DOS) of the compounds Cu 4 Sn 7.5 S 16− x Se x ( x  = 0.5, 1.0, 1.5 and 2.0) pursuant to the first-principles, where the DOS at x  = 0 is presented for comparison 5 . When Se is incorporated into the S site (Fig.…”

“…7, where the Raman spectra of Cu 4 Sn 7 S 16 and Cu 4 Sn 7.5 S 16 5 are used for comparison. The features of the presented spectra upon Se incorporation bear an overall resemblance to those of Cu 4 Sn 7 S 16 and Cu 4 Sn 7.5 S 16 , which suggests that the molecular structures are rather similar upon Se incorporation.…”

“…For example, the TE figure of merit (ZT) of Zn-doped Cu 2 SnS 3 is 0.58 at 723 K 4 , while that of the pristine Cu 4 Sn 7 S 16 is only 0.2 at 600 K from measurement 1 . In order to improve the TE performance of ternary Cu-Sn-S compounds, we have employed some strategies, such as the band structure engineering in Cu 4 Sn 7 S 16 5 and Cu 2 SnS 3 6 , and the coordination of the Seebeck coefficient and carrier concentration in Cu 3 SnS 4 7 . By employing the above-mentioned approaches, we have achieved remarkable improvements in TE performance, obtaining a ZT value of 0.75 for Cu 3 Sn 1.2 S 4 7 and 0.4 for Cu 4 Sn 7.5 S 16 5 .…”

“…In order to improve the TE performance of ternary Cu-Sn-S compounds, we have employed some strategies, such as the band structure engineering in Cu 4 Sn 7 S 16 5 and Cu 2 SnS 3 6 , and the coordination of the Seebeck coefficient and carrier concentration in Cu 3 SnS 4 7 . By employing the above-mentioned approaches, we have achieved remarkable improvements in TE performance, obtaining a ZT value of 0.75 for Cu 3 Sn 1.2 S 4 7 and 0.4 for Cu 4 Sn 7.5 S 16 5 . Nevertheless, a great improvement of the compounds of Cu 4 Sn 7 S 16 family is still necessary.…”

“…The previous investigations revealed that when Sn resides in the framework of Cu 4 Sn 7 S 16 , extra Sn will make the Fermi level ( E f ) unpin and shift towards the conduction band, thus donating the electrons and enhancing the carrier concentration ( n H ) 5 . However, the enhancement of n H seems to cease as more Sn is added.…”