Enhancing thermoelectric performance in hierarchically structured BiCuSeO by increasing bond covalency and weakening carrier-phonon coupling

BiCuSeO oxyselenides are promising thermoelectric materials at intermediate temperatures, primarily due to their ultralow lattice thermal conductivity (κ_L) and high Seebeck coefficient. The intrinsically low carrier mobility in these materials, normally below ∼20 cm² V^-1 s^-1 at 300 K, however, largely limits further improvements of their thermoelectric properties. In this study, by introducing less electronegative Te into the conductive Cu-Se layers, we demonstrate that the enhanced chemical bond covalency results in smaller effective mass and thus improved carrier mobility, through the weakening of carrier-phonon coupling. The improved carrier mobility by Te-doping largely retains the electrical conductivity values and thus high power factors even with decreased carrier concentrations. Meanwhile, the hierarchical structural features including dual point defects, nanoinclusions, grain boundaries, etc., originating from the nonequilibrium self-propagating high-temperature synthesis (SHS) processes, further reduce κ_L close to the amorphous limit. Ultimately, a maximum ZT value of ∼1.2 at 873 K is achieved in Bi_0.96Pb_0.04CuSe_0.95Te_0.05O, ∼35% improvement as compared with that of Te-free Bi_0.96Pb_0.04CuSeO and ∼2.4 times higher than that of the pristine sample. Furthermore, our study elucidates that weakening of carrier-phonon coupling through regulating chemical bonding within the conductive functionalities can be an effective avenue for further improving the thermoelectric performance of BiCuSeO.