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

Abstract

AgSbSe₂ is a promising thermoelectric (TE) <i>p</i>-type material for applications in the middle-temperature range. AgSbSe₂ 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 AgSbSe₂ nanocrystals (NCs). To increase the carrier concentration and improve the electrical conductivity, these NCs are doped with Sn²⁺ on Sb³⁺ sites. Upon processing, the Sn²⁺ chemical state is conserved using a reducing NaBH₄ 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 Sn²⁺ ions replacing Sb³⁺ 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 Sn²⁺ ions are prevented from oxidation is rationalized by modeling the system. Calculated band structures disclosed that Sn doping induces convergence of the AgSbSe₂ valence bands, accounting for an enhanced electronic effective mass. The dramatically enhanced carrier transport leads to a maximized power factor for AgSb_0.98Sn_0.02Se₂ 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 (<i>zT</i>) is obtained at 666 K for AgSb_0.98Sn_0.02Se₂ at <i>zT</i> = 1.37, well above the values obtained for undoped AgSbSe₂, at <i>zT</i> = 0.58 and state-of-art Pb- and Te-free materials, which makes AgSb_0.98Sn_0.02Se₂ an excellent <i>p</i>-type candidate for medium-temperature TE applications.