Improvement in thermoelectric properties of Bi-Mg co-doped SnTe via band engineering and nanostructuring

In the present work, the effects of Bi-Mg co-doping on the thermoelectric properties of SnTe materials are investigated. Pristine SnTe and Sn0.96Bi0.02Mg0.02Te materials are synthesized using a facile solvothermal technique. Calculated crystallite size and grain size decrease after Bi-Mg co-doping. In Sn0.96Bi0.02Mg0.02Te various lattice defects, lattice strain, stacking fault and dislocation densities are higher as compared to pristine SnTe. Minimum lattice thermal conductivity is 0.21 W/mK in Sn0.96Bi0.02Mg0.02Te and 1.13 W/mK in pristine SnTe at 584 K. Lower lattice thermal conductivity in Bi-Mg co-doped SnTe may be due to more phonon scattering at interfaces and lattice defects. A decrease in carrier concentration following doping may lead to a decrease in electrical conductivity in Sn0.96Bi0.02Mg0.02Te as compared to SnTe. Maximum S in Sn0.96Bi0.02Mg0.02Te is 72 mu V/K and in pure SnTe is 66 mu V/K at 584 K. The increase in S may be due to band engineering via Bi-Mg co-doping as (i) Bi generates resonant energy states at the Fermi level of SnTe (ii) Mg doping modifies the band structure of SnTe via converging valence bands and widening the band gap. Maximum 'figure of merit' zT = 0.21 in Sn0.96Bi0.02Mg0.02Te is 91% higher than pristine SnTe at 584 K. Material quality factor (B = BET/kappa(L)) indicates that maximum zT > 1 can be achieved until S = 246 mu V/K in Sn0.96Bi0.02Mg0.02Te. The electronic contribution of quality factor falls after doping which demonstrates that the increase in zT is caused by a decrease in lattice thermal conductivity. The thermoelectric efficiency and figure of merit 'ZT' of thermoelectric devices composed of Sn0.96Bi0.02Mg0.02Te are almost double those of pristine SnTe. Thus experimental findings reveal that Bi-Mg co-doped SnTe can be a potential thermoelectric material.