Electronic structure of defects in Mg$_2$Si and Cu$_3$SbSe$_4$ and their thermoelectric significance

Defects play an important role in the thermoelectric properties of narrow band gap semiconductors. Recently, Mg$_2$Si and its solid solutions with Mg$_2$Sn have been found to be excellent n-type thermoelectrics and have been studied extensively due to its unique feature called conduction band convergence [Liu et al., PRL 108, 166601 (2012)]. In this talk we will discuss the physics of defects in Mg$_2$Si and explore the possibilities of improving its thermoelectric properties by co-doping, using first principle calculation and supercell model. In addition, we will also discuss some of our results using the same approach on the nature of defects in another important thermoelectric system Cu$_3$SbSe$_4$ where the lone pairs of Sb control the nature of states near the band gap [Dat Do et al., J. Phys.: Condens. Matter 24, 415502 (2012)].