Realizing High ZT_ave in N-Type Diamondoid AgInSe₂ Thermoelectrics via Lone-Pair Electrons and Isomorphous Alloy

Recently, diamondoid compounds have attracted significant attention in thermoelectrics due to their unique transport properties, with ZT beyond 1.6 reported in several p-type systems. In contrast, n-type diamondoid compounds remain largely unexplored. This work systematically investigates the transport properties of the novel n-type diamondoid material AgInSe₂, enhancing its thermoelectric performance through Ga doping and CdSe alloying. Additionally, its power generation potential is assessed using a single-leg device. These findings show that intrinsic AgInSe₂ possesses a light conduction band with a low density-of-state effective mass of 0.13 m₀, leading to a high electron mobility of ∼650 cm² V^-1s^-1 at room temperature. Furthermore, Ga is found to exist in dual oxidation states of Ga⁺ and Ga³⁺ in Ag_1-xGa_xInSe₂. The incorporation of Ga³⁺ effectively increases the carrier concentration and electrical conductivity, while Ga⁺ introduces lone-pair electrons that enhance lattice anharmonicity. This synergistic modulation of electronic and phonon transport leads to a 274% improvement in ZT, reaching 0.74 at 873 K for Ag_0.98Ga_0.02InSe₂. Further alloying CdSe into Ag_0.98Ga_0.02InSe₂ leads to partial substitution of Cd at the Ag sublattice, significantly increasing the carrier concentration and power factor. Simultaneously, CdSe incorporation induces dislocation arrays that intensify phonon scattering and further reduce thermal conductivity. These combined effects yield a maximum ZT of ≈1.2 and a decent average ZT_ave of 0.55 in Ag_0.98Ga_0.02InSe₂-0.03CdSe.