Intrinsic Off-Centering and Light Conduction Band Structure Lead to High Thermoelectric Performance in N-Type Diamondoid AgInSe₂

Historically, the rocksalt crystal structure and its variants have long dominated the field of advanced thermoelectrics. Developing new structural thermoelectric materials is an interesting topic for the thermoelectric community. In this work, an n-type diamondoid compound, AgInSe₂, was identified with extremely low thermal conductivity and very high carrier mobility. The intrinsic Ag off-centering behavior combined with the densely distributed twin boundaries and switched grain orientations significantly disrupts phonon transport and leads to the extremely low thermal conductivity of ∼0.19 W m^-1 K^-1 at 850 K in the Ag_0.98In_1.02Se₂-0.03CdSe material. On the other hand, the light conduction band structure of AgInSe₂ leads to a small density-of-state effective mass of 0.1 <i>m</i>_e and very high electron mobility of 700 cm² V^-1 s^-1 at room temperature. Combining this with the significant increase in carrier concentration induced by CdSe alloying, a record-high maximum <i>ZT</i> of 1.4 at 850 K and an average <i>ZT</i> of 0.8 are achieved in n-type Ag_0.98In_1.02Se₂-0.03CdSe, which can be comparable to many typical rocksalt n-type thermoelectrics. Moreover, a maximum output power of ∼10 mW and an energy conversion efficiency of ∼5% were demonstrated in the AgInSe₂-based single-leg device, highlighting the application potential of this novel n-type diamondoid thermoelectric.