Tuning Band Degeneracy and Lattice Thermal Conductivity in CuInTe ₂ via Strain and Nanostructuring

Lattice strain and nanostructuring are effective, composition-preserving approaches for tuning coupled electronic and phonon transport in thermoelectric materials. To assess the efficacy of these approaches, we performed a systematic analysis of a chalcopyrite semiconductor using first-principles calculations, Boltzmann transport theory, and anharmonic phonon analysis. CuInTe2, a widely studied chalcopyrite with established experimental benchmarks, is investigated as a promising lead-free thermoelectric material. We focus on enhancing thermoelectric performance at room temperature while also considering performance at high temperatures. The results reveal that moderate tensile strain induces an enhancement in the figure of merit, ZT, by simultaneously improving the power factor and suppressing the lattice thermal conductivity. The power factor enhancement originates from strain-induced band convergence near the Fermi level, while the reduction in lattice thermal conductivity arises from reduced acoustic phonon group velocities and enhanced anharmonic phonon scattering. At 3% hydrostatic tensile strain, the room-temperature ZT increases by approximately a factor of five, extending the functional relevance of CuInTe2 beyond its traditional high-temperature regime. Enhancement is also observed at elevated temperatures, with the ZT at 900 K increasing by nearly a factor of two. In addition to strain engineering, we examine the role of nanostructuring through grain-size reduction. Nanostructuring further suppresses lattice thermal conductivity and enhances the thermoelectric figure of merit at elevated temperatures; however, its impact at room temperature is smaller due to the concurrent reduction in the power factor. These findings demonstrate that controlled lattice strain effectively enhances room-temperature thermoelectric performance, while nanostructuring serves as a complementary route to optimize high-temperature performance in lead-free chalcopyrite materials.