Enhanced Thermoelectric Performance of InSnTe Monolayer via Spatially Independent Electron and Phonon Transport Induced by Dual-Element Substitution

Thermoelectric materials enable direct heat-to-electricity conversion, but their efficiency is limited by intrinsic trade-offs between electrical conductivity (σ) and thermal conductivity (κ). Multilayer architectures present a transformative solution by segregating carrier and phonon transport pathways into functionally specialized sublayers, thereby enabling unprecedented electrical and thermal transport decoupling. Here, we report a novel two-dimensional (2D) quinary chalcogenide GaInPbSnTe₂, engineered via Ga and Pb codoping of the parent InSnTe sextuple layer, which achieves synergistic optimization of thermoelectric performance through a unique dual-channel transport model induced by isoelectronic substitution in the sublayers. It induces spatial separation of charge and heat transport channels: carriers are confined to the covalently bonded Z₁Z₂ layer (middle PbSn layer) with ultralow effective masses, enabling high mobility and single-layer dominated electrical conduction. Meanwhile, phonon transport is dominated by the MX layers (outer In(Ga)Te layer), where lattice anharmonicity is enhanced due to softened interatomic bonds manifested by reduced interatomic force constants, atomic mass difference, and elevated Grüneisen parameters. These effects synergistically suppress the room temperature lattice thermal conductivity (κ_L) from 9.0 W/mK in InSnTe to 1.17 W/mK for the substituted system by shortening phonon lifetimes and reducing group velocities. Electronic structure calculations show that GaInPbSnTe₂ exhibits a direct bandgap (0.22 eV) with multivalley features, contrasting with InSnTe's single transport energy valley. This, combined with the dual-channel transport design, enhances the power factor while minimizing κ_L. As a result, n-type GaInPbSnTe₂ achieved a peak ZT of 1.91 at 500 K, far exceeding the pristine InSnTe (ZT ≈ 0.1). This work demonstrates that Ga and Pb dual-element substitution is pivotal for breaking thermoelectric trade-offs in low dimensional layered materials: it not only modulates the electronic structure to boost σ via localized charge transport in the ZZ layer but also induces strong anharmonicity in the heat transport dominated MX layers to suppress κ_L. Our work establishes isoelectronic codoping as a typical example that enables atomic-scale electrical and thermal decoupling through precisely engineered transport architectures, advancing the development of high-efficiency two-dimensional thermoelectric systems.