CoSb*

We observed the dramatic improvement in the thermoelectric performance of TiCoSb by introducing three-dimensional (3D) modulation doping and synergistic band engineering in the composites of the form (1 – f)A + fB, where A and B refer to the phases Ti_1–xNb_xCoSb and Nb_0.8+δCoSb, respectively, and f is the volume fraction of phase B. We show that the electrical conductivity and Seebeck coefficient of these composites increase simultaneously due to modulation doping, giving rise to colossal power factor (PF) enhancement from 0.3 μW cm^–1 K^–2 (TiCoSb) to 18 μW cm^–1 K^–2 (x = f ≈ 0.05) at 300 K and exceeding 25 μW cm^–1 K^–2 over a broad temperature range (T > 600 K). Due to the Ti–Nb point mass fluctuation in phase A, high concentration of defects in phase B, and interfacial phonon scattering between A and B, these composites also exhibit very low lattice thermal conductivity (κ_L), resulting in a high zT of 0.81 near 970 K. The simulation of κ_L using the Klemens model successfully describes the significant reduction of κ_L for these composites, observed experimentally. Our ab initio DFT calculations show that Ti_1–xNb_xCoSb exhibits band convergence as x increases, which contributes to improving the charge transport. Thus, benefiting from the synergistic effect of band convergence and 3D modulation doping, a high zT is obtained. Using multiple concepts togather can be a new approach to enhance heat to electricity conversion efficiency.