## 2 Materials and Methods  
Ingots of Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{x}\) (\(x\) = 0, 0.003, 0.01, 0.1) and Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{x}\) (\(x\) = 0, 0.005, 0.01, 0.1) were prepared by conventional melting and quenching techniques using high-purity elemental Bi (99.999%, shots, 5N Plus, Montreal, QC, Canada), Sb (99.999%, shots, 5N Plus), Te (99.999%, pieces, 5N Plus), Nb (99.99%, powder, Sigma Aldrich, St. Louis, MO, USA), and Ni (99.9%, powder, Sigma Aldrich), as starting materials. Stoichiometric mixtures were loaded into a quartz tube (15 mm in diameter). The tube was vacuum-sealed under \(10^{-4}\) torr, and the contents were melted in a box furnace at 1423 K for 20 h, and then quenched to room temperature. The ingots were grounded using a high-energy ball mill (8000D, SPEX, Mettuchen, NJ, USA) for 5 min. Subsequently, melt spinning of the prepared ingots (except for Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{0.1}\) and Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{0.1}\)) was performed, under a copper wheel rotation of 3600 rpm, and the molten solution was shot at 0.03 MPa under Ar. The ribbons were grounded in an agate mortar. Then, disk-shaped polycrystalline bulk samples (10 mm in diameter and 10 mm thickness) were prepared by spark plasma sintering (SPS) at 430 \({}^{\circ}\)C for 5 min, under 50 MPa in vacuum.  
X-ray diffraction (XRD, D8 Discover, Bruker, Billerica, MA, USA) was performed for phase analysis. The temperature-dependent \(S\) and \(\sigma\) parameters were measured within a temperature range from room temperature to 480 K on the samples, perpendicular to the direction of the applied pressure (ZEM-3 Advanced-RIKO, Yokohama, Japan). The nominal temperature gradients were given as 10, 20, and 30 K. The experimental error margin for \(S\) and \(\sigma\) are estimated as \(\pm\)5% and \(\pm\)3%, respectively. The carrier concentrations were determined by van der Pauw measurements under a magnetic field of 0.5 T (AHT-55T5, Eopia, Anyang, Korea) in the same direction; the input current can be regulated between 1 nA and 20 mA. The van der Pauw method typically provides the carrier concentration with relative error within 5%. The \(\kappa\) values of the samples were calculated from the expression \(\kappa\) = \(\rho_{\rm s}\)-C\({}_{\rm P}\cdot\lambda\), considering their theoretical density (\(\rho_{\rm s}\)), heat capacity (C\({}_{\rm P}\)), and thermal diffusivity (\(\lambda\)) measured across the same direction (perpendicular to the pressing direction), so the \(zT\) can be determined. The C\({}_{\rm P}\) values from 300 K to 400 K were measured by using a physical property measurement system (PPMS, Quantum Design, San Diego, CA, USA), and 121.6 J mol\({}^{-1}\) K\({}^{-1}\) were used, which were estimated from Dulong-Petit fitting. The densities of all samples were measured as being higher than 98.5%, from the sintered samples of cylinder shape and 10 mm diameter with \(\sim\)12 mm height, by the Archimedes method, and the \(\rho_{8}\) for Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) of 6.88 g/cm\({}^{3}\) was used for samples with small addition (\(x\leq 0.01\)). The \(\lambda\) were measured using a laser flash installation (Laser Flash Analysis, LFA 467, Netzsch, Selb, Germany). The experimental error margin for \(\lambda\) is estimated as \(\pm\)3%.