Engineering band structure via the site preference of Pb2+ in the In+ site for enhanced thermoelectric performance of In6Se7

Although binary In-Se based alloys as thermoelectric (TE) candidates are of interests in recent years, little attention has been paid into In6Se7 based compounds. With substituting Pb in In6Se7, the preference of Pb2+ in the In+ site has been observed, allowing the Fermi level (Fr) shift towards the conduction band and the localized state conduction becomes dominated. Consequently, the Hall carrier concentration (nH) has been enhanced significantly with the highest nH value being about 2~3 orders of magnitude higher than that of Pb-free sample. Meanwhile, the lattice thermal conductivity (κL) tends to be reduced as nH value increases, owing to an increased phonon scattering on carriers. As a result, a significantly enhanced TE performance has been achieved with the highest TE figure of merit (ZT) of 0.4 at ~850 K. This ZT value is 27 times that of intrinsic In6Se7 (ZT=0.015 at 640 K), which proves a successful band structure engineering through site preference of Pb in In6Se7.