Optimization of sodium hydroxide for securing high thermoelectric performance in polycrystalline Sn1-xSe via anisotropy and vacancy synergy

The morphology and composition are two key factors to determine the thermoelectric performance of aqueously synthesized tin selenide (SnSe) crystals; however, their controlling is still under exploring. In this study, we report a high figure-of-merit (ZT) of similar to 1.5 at 823 K in p-type polycrystalline Sn1 - xSe resulted from a synergy of morphology control and vacancy optimization, realized by carefully tuning the sodium hydroxide (NaOH) concentration during solvothermal synthesis. After a comprehensive investigation on various NaOH concentrations, it was found that an optimized NaOH amount of 10 mL with a concentration of 10 mol L(-1)can simultaneously achieve a large average crystal size and a high Sn vacancy concentration of similar to 2.5%. The large microplate-like crystals lead to a considerable anisotropy in the sintered pellets, and the high Sn vacancy level contributes to an optimum hole concentration to the level of similar to 2.3 x 10(19) cm(-3), and in turn a high power factor of similar to 7.4 mu W cm(-1)K(-2)at 823 K, measured along the direction perpendicular to the sintering pressure. In addition, a low thermal conductivity of similar to 0.41 W m(-1)K(-1)is achieved by effective phonon scattering at localized crystal imperfections including lattice distortions, grain boundaries, and vacancy domains, as observed by detailed structural characterizations. Furthermore, a competitive compressive strength of similar to 52.1 MPa can be achieved along the direction of high thermoelectric performance, indicating a mechanically robust feature. This study provides a new avenue in achieving high thermoelectric performance in SnSe-based thermoelectric materials.