## 1 Introduction  
Thermoelectric (TE) technology is widely used for cooling and energy harvesting applications, since it can directly convert thermal energy into electricity (and vice versa) by using semiconducting TE materials. Bi\({}_{2}\)Te\({}_{3}\)-based alloys (\(p\)-type Sb-substituted and \(n\)-type Se-substituted Bi\({}_{2}\)Te\({}_{3}\) compounds) with narrow bandgap (\(\sim\)130 meV) semiconductor characteristics [1] are the only commercialized TE materials for solid-state cooling and low-temperature power generation. One of the most critical factors to determine the efficiency of TE systems is the performance of the TE materials, which can be denoted by a dimensionless figure of merit, \(zT\) ( = \(S^{2}\sigma T/\kappa\), where S is the Seebeck coefficient, \(\sigma\) is the electrical conductivity, and \(\kappa\) is the thermal conductivity at a given absolute temperature \(T\)). Another issue in Bi\({}_{2}\)Te\({}_{3}\)-based TE materials is their low mechanical reliability when fabricated as ingots by using a directional solidification method, such as zone melting; this because the 00\(l\) planes held together by weak van der Waals interactions are highly aligned along the growth direction [2]. Thus, many researches have been focusing on the development of polycrystalline materials with enhanced \(zT\) due to their intrinsically high mechanical strength compared to that of ingots.  
Recently, significantly enhanced \(zT\)s have been obtained via nanostructuring approaches [3; 4; 5]; however, a \(zT\) enhancement by a facile and straightforward compositional tuning is always favorable. For \(p\)-type Sb-substituted Bi\({}_{2}\)Te\({}_{3}\) alloys (Bi-Sb-Te), doping Ag, Cu, or Pb at Bi/Sb-sites has been effective to improve \(zT\), especially at higher temperatures, mainly due to the suppression of bipolar thermal conduction [6; 7; 8]. Substitutional doping also provides a chance to enhance \(zT\) due to the enlarged density-of-states (DOS) by band engineering (e.g., band flattening, band convergence, and resonant state formation) [9; 10; 11; 12] and/or intensified phonon scattering by the formation of point defects [13]. A higher maximum \(zT\) (\(zT_{\text{max}}\)), over 1.2 times that of commercial ingot (\(zT_{\text{max}}\)\(\sim\)1.0), has been observed in \(p\)-type Bi-Sb-Te by compositional tuning. However, the \(zT_{\text{max}}\) of \(n\)-type Se-substituted Bi\({}_{2}\)Te\({}_{3}\) compounds (Bi-Te-Se) remained about 1.0 [14].  
In the present study, Cu-introduced Bi\({}_{2}\)Te\({}_{2.7}\)Se\({}_{0.3}\) (Cu\({}_{0.01}\)Bi\({}_{2}\)Te\({}_{2.7}\)Se\({}_{0.3}\)) is selected as a pristine compound due to its highly reproducible TE properties as well as high \(zT\); furthermore, polycrystalline bulks of Hf-doped Cu\({}_{0.01}\)Bi\({}_{2}\)Te\({}_{2.7}\)Se\({}_{0.3}\) are prepared by using a conventional melt-solidification process and a subsequent spark plasma sintering (SPS) technique. The electronic and thermal transport parameters are estimated to elucidate the Hf-doping effect (at Bi-sites) on the TE transport properties of \(n\)-type Bi-Te-Se alloys. The power factor is improved due to the enlarged DOS effective mass, and lattice thermal conductivity (\(x_{\text{lat}}\) = \(\kappa\)-\(x_{\text{ele}}\), where \(x_{\text{ele}}\) is the electronic contribution to the thermal conduction) is reduced simultaneously. Resultantly, an enhanced \(zT\) of 0.83 at 320 K is obtained in 3.75 at.% Hf-doped Cu\({}_{0.01}\)Bi\({}_{2}\)Te\({}_{2.7}\)Se\({}_{0.3}\).