Interface-Mismatch-Induced High Thermoelectricity in MSe–TlSe Superlattices

Engineering superlattices with enhanced intrinsic in-plane transport and high thermoelectric performance involves exploring new quantum phenomena that emerge from their individual sublayers and corresponding interfaces, which play crucial roles in modern semiconductor technology. Herein, we report a first-principles formalism along with Boltzmann transport theory to study the structural, mechanical, electronic, and thermoelectric properties of MSe (M = Ga, In) layer stacked on TlSe, forming MSe–TlSe superlattices. Lattice dynamics confirm that TlSe layers exclusively dominate the low-frequency region, inducing flat acoustic modes that are absent in pristine GaSe and InSe but intrinsic to TlSe, thereby enhancing phonon scattering and lowering lattice thermal conductivity (κl). Electronic structure analysis exhibits TlSe sublayer-induced valley degeneracy and strong unconventional band convergence originating from MSe sublayer in the conduction band regime, which enhances the density of states near the Fermi level and promotes favorable Seebeck coefficients as well as improved thermoelectric performance. Bonding and charge density analyses indicate that the MSe sublayer combined with TlSe introduces a conductive network that enhances carrier transport relative to pristine bulk systems. To address the inconsistencies present in the conventional DPT formalism, we incorporate the Fröhlich interaction, allowing for a more comprehensive and accurate assessment of carrier mobility. Quantitatively, GaSe–TlSe behaves as an n-type material with ZT values of 1.09 (electrons) and 0.79 (holes) at 600 K, while InSe–TlSe is p-type with superior ZT values of 2.45 (holes) and 0.74 (electrons) at 600 K. Overall, these results demonstrate how the TlSe layer suppresses phonon transport, while the MSe layer establishes a conductive network for efficient electronic transport, offering a rational pathway toward next-generation thermoelectric quantum materials.