Structure And Thermoelectric Properties Of Doped In₆Se₇ Based Semiconductors

Thermoelectric materials can convert electricity into heat directly and vice versa. In recent years, much attention have been paid on indium selenides due to their intrinsic structural characteristics（including phases, crystal structures, and structural imperfection）.In₆Se₇ is one of the indium selenium binary semiconductor material. The unit cell is monoclinic and the space group is P21/m, in which the element In exhibits mixed valences（1+,2+ and 3+）. In this work, we identified mixed site defects（SnIn3+、Sn In+、SnIn-） upon Sn substitution for In in In₆Se₇ and observed that the Sn4+ prefers In+ to In3+ sites, thus creating Sn In3+ defect acting as an active donor; while Sn2+ creates Sn In- as an acceptor in that it is energetically favorable to In3+ sites. Due to the main contribution of donor Sn In3+, the transport properties of In₆Se₇ changes from p-type to n-type, and its electrical conductivity has also been improved. Although Sn2+ counteracts the effect of Sn4+ on the transport properties, the sample In6-xSnxSe7（x=0.1） gives the highest ZT value（0.28） at 833 K, which is about 19 times that of virgin In₆Se₇（640 K,ZT=0.015）.In addition, the thermoelectric properties of Sn-doped In₆Se₇ has beenimproved upon the substitutions of Pb with relatively bigger atomic mass for In.The highest ZT value of 0.38 at 840 K for In6-yPby Se7（x=0.5） has been reached,which is 25 times that of Pb-free In₆Se₇（640K, ZT=0.015）.