Deciphering Cationic/Anionic Substitutions Decouple Phononic and Electronic Transport, Enabling High Thermoelectricity in Heterostructures

Despite exhibiting low carrier mobility, the 1T-phase of two-dimensional (2D) transition metal dichalcogenides (TMDs) are proposed as a promising candidate for thermoelectric applications, owing to their tunable phononic and electronic transport properties. These materials can be further functionalized by strategically substituting cationic and anionic species within the layers, forming bilayer heterostructures (BHS). In this context, we employ first-principles-based density functional theory calculations combined with solving the Boltzmann transport equation to systematically investigate how cationic (ZrSe2/SnSe2), anionic (ZrSe2/ZrS2), and dual-site (ZrSe2/SnS2) substitutions in bilayer ZrSe2 affect the carrier transport. Our results show that cationic and dual-site-substituted BHS yield an ultralow lattice thermal conductivity (κl) of 0.71 W m–1 K–1 and 0.79 W m–1 K–1, respectively, at room temperature. Such a low κl of cationic and dual-site-substituted BHS primarily stems from the strong phonon softening and concerted rattling inducing antibonding states, particularly at the low energy region, which significantly contributes to the suppression of phonon transport. Furthermore, to resolve the inconsistency in the conventional deformation potential theory formalism, we incorporate the Fröhlich interaction, enabling a more comprehensive and accurate evaluation of carrier mobility. Interestingly, anionic and dual-site-substituted BHS demonstrate superior electronic transport compared to cation-substituted BHS. However, cation-substituted BHS produces unconventional band convergence in the valence band, leading to a superior p-type Seebeck coefficient. As a result, at 700 K, the cation-substituted BHS achieves maximum ZT values of 2.69 and 1.20 for p-type and n-type carriers, respectively, while the anion-substituted BHS attains a peak p-type ZT of 0.76. Overall, the present work highlights that strategic cationic/anionic substitutions can effectively decouple phononic and electronic transport, where cationic substitution favors ultralow κl and anionic substitution benefits the electrical transport, offering valuable guidelines for designing high-efficiency thermoelectric materials.