Surface Chemistry and Band Engineering in AgSbSe 2 : Toward High Thermoelectric Performance.

AgSbSe 2 is a promising thermoelectric (TE) p -type material for applications in the middle-temperature range. AgSbSe 2 is characterized by relatively low thermal conductivities and high Seebeck coefficients, but its main limitation is moderate electrical conductivity. Herein, we detail an efficient and scalable hot-injection synthesis route to produce AgSbSe 2 nanocrystals (NCs). To increase the carrier concentration and improve the electrical conductivity, these NCs are doped with Sn 2+ on Sb 3+ sites. Upon processing, the Sn 2+ chemical state is conserved using a reducing NaBH 4 solution to displace the organic ligand and anneal the material under a forming gas flow. The TE properties of the dense materials obtained from the consolidation of the NCs using a hot pressing are then characterized. The presence of Sn 2+ ions replacing Sb 3+ significantly increases the charge carrier concentration and, consequently, the electrical conductivity. Opportunely, the measured Seebeck coefficient varied within a small range upon Sn doping. The excellent performance obtained when Sn 2+ ions are prevented from oxidation is rationalized by modeling the system. Calculated band structures disclosed that Sn doping induces convergence of the AgSbSe 2 valence bands, accounting for an enhanced electronic effective mass. The dramatically enhanced carrier transport leads to a maximized power factor for AgSb 0.98 Sn 0.02 Se 2 of 0.63 mW m -1 K -2 at 640 K. Thermally, phonon scattering is significantly enhanced in the NC-based materials, yielding an ultralow thermal conductivity of 0.3 W mK -1 at 666 K. Overall, a record-high figure of merit ( zT ) is obtained at 666 K for AgSb 0.98 Sn 0.02 Se 2 at zT = 1.37, well above the values obtained for undoped AgSbSe 2 , at zT = 0.58 and state-of-art Pb- and Te-free materials, which makes AgSb 0.98 Sn 0.02 Se 2 an excellent p -type candidate for medium-temperature TE applications.