Thermoelectric materials: Telluride free

Energy Environ. Sci. http://doi.org/br9r (2016)

Thermoelectric materials are able to transform thermal energy into electricity and have found increasing applications in energy-harvesting devices. Bismuth telluride (Bi₂Te₃) is a commercialized thermoelectric material, but the rarity of the Te element hinders its large-scale use. Te-free Bi₂Se₃ has been proposed as an alternative thermoelectric material but its energy conversion, often characterized by the figure of merit, ZT, remains inefficient, largely because of its low band degeneracy in the conduction band minimum. Now, Wenqing Zhang, Jihui Yang and colleagues in USA and China report on how electronic and phononic properties can be improved in iodine-doped (Bi,Sb)₂Se₃: the maximum ZT can reach 1.0 at 800 K, which is three times that of Bi₂Se₃ and comparable to that of Bi₂Te₃.

Starting with the Te-free (Bi,Sb)₂Se₃ system, the researchers identified a significant phase transition in (Bi,Sb)₂Se₃ due to the Sb alloying, which exerts profound effects on the physical properties of the system. The band degeneracy at the conduction band minimum is doubled, thereby improving the electron transport. Furthermore, substantial softening of the chemical bonds, along with the phase transition, significantly lowers phonon velocity and increases bonding anharmonicity, which leads to favourable phonon transport features for thermoelectrics. The researchers also showed that the iodine doping at the Se sites increases the carrier concentration, further contributing to the much improved electrical conductivity and thus the conversion efficiency.