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)of1.5 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 of2.5%.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 of2.3×10^19 cm^−3,and in turn a high power factor of7.4μW cm^−1 K^−2 at 823 K,measured along the direction perpendicular to the sintering pressure.In addition,a low thermal conductivity of0.41 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 of52.1 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.