Surface Chemistry and Band Engineering in AgSbSe2: Toward High Thermoelectric Performance

AgSbSe2 is a promisingthermoelectric (TE) p-type material for applicationsin the middle-temperature range.AgSbSe2 is characterized by relatively low thermal conductivitiesand high Seebeck coefficients, but its main limitation is moderateelectrical conductivity. Herein, we detail an efficient and scalablehot-injection synthesis route to produce AgSbSe2 nanocrystals(NCs). To increase the carrier concentration and improve the electricalconductivity, these NCs are doped with Sn2+ on Sb3+ sites. Upon processing, the Sn2+ chemical state is conservedusing a reducing NaBH4 solution to displace the organicligand and anneal the material under a forming gas flow. The TE propertiesof the dense materials obtained from the consolidation of the NCsusing a hot pressing are then characterized. The presence of Sn2+ ions replacing Sb3+ significantly increases thecharge carrier concentration and, consequently, the electrical conductivity.Opportunely, the measured Seebeck coefficient varied within a smallrange upon Sn doping. The excellent performance obtained when Sn2+ ions are prevented from oxidation is rationalized by modelingthe system. Calculated band structures disclosed that Sn doping inducesconvergence of the AgSbSe2 valence bands, accounting foran enhanced electronic effective mass. The dramatically enhanced carriertransport leads to a maximized power factor for AgSb0.98Sn0.02Se2 of 0.63 mW m(-1) K-2 at 640 K. Thermally, phonon scattering is significantlyenhanced in the NC-based materials, yielding an ultralow thermal conductivityof 0.3 W mK(-1) at 666 K. Overall, a record-high figureof merit (zT) is obtained at 666 K for AgSb0.98Sn0.02Se2 at zT = 1.37, wellabove the values obtained for undoped AgSbSe2, at zT = 0.58 and state-of-art Pb- and Te-free materials, whichmakes AgSb0.98Sn0.02Se2 an excellent p-type candidate for medium-temperature TE applications.