Half-filled bands from Bi-Se sigma bonds and Bi 6s “lone-pairs” induced superior thermoelectric properties of Bi/Cl codoped SnSe

The previous experimental work showed that n-type low-temperature (LT) SnSe doped with BiCl3 simultaneously exhibited larger Seebeck coefficient (S), higher electrical conductivity (sigma), and lower thermal conductivity (kappa) compared with polycrystalline LT SnSe. Here, the first-principles method was used to explore how the independent adjustment to the tangled thermoelectric (TE) transport coefficients of n-type LT SnSe was achieved and whether there exist the similar adjustment mechanisms in n-type high-temperature (HT) SnSe. Cl doping increases the electron-pockets and the total density of states (TDOS) near the Fermi level (E-F). This results in a high sigma and an obvious decrease in S. Whereas, Bi doping introduces two half-filled bands into the band gap. The two bands originate from the 6s "lone-pairs" of Bi atoms and the sigma bonds formed by Bi atoms and their nearest Se atoms. This contributes to maintaining a relative large S and a low kappa, but this cannot effectively enhance sigma. Fortunately, the coexistence of Cl ions and Bi ions can make Bi/Cl codoped LT (or HT) SnSe set a high sigma and a relatively large S in one through independent controlling the electronic structures and charge distribution near the E-F. Further, we predict that Bi/Cl co-doping is also an effective strategy to improve the TE performance of HT SnSe. (c) 2018 Published by Elsevier B.V.