Microstructure modulation responsible for the improvement in thermoelectric property of a wide-gap AgIn ₅Se ₈ semiconductor

Here we employed microstructure modulation (MM) to tailor the thermoelectric property of a wide-gap ternary AgIn ₅Se ₈ compound. The results reveal that the MM was realized through proper heat treatments. A gradual microstructure change was observed from twinning to nanoparticle-dominated structure as the annealing time increases to 100 days at 180 °C. This change also becomes more evident as both the lattice thermal conductivity and Seebeck coefficient increase. Neither twinning nor nanoparticle-dominated structure achieves the best thermoelectric performance. In contrast, what is particularly important is the hybrid structure that involves submicro-patches and -domains. The compound with this hybrid structure, which was obtained by being annealed for 60 days, achieves the maximum thermoelectric figure of merit ZT of 0.72 at 863 K. It is not only higher by a factor of 5 than the compound unannealed, but it is also by a factor of 1.3 higher than that annealed for 100 days. These findings seem to challenge the well-known nanostructuring engineering that aims at improving the thermoelectric performance via the reduction in lattice thermal conductivity.