Suppressing bipolar effect to broadening the optimum range of thermoelectric performance for p-type bismuth telluride–based alloys via calcium doping

Bi2Te3-based alloys are well-known thermoelectric materials near room temperature. However, the strong intrinsic excitations above 400 K seriously deteriorate its thermoelectric performance, which limits their applications as thermoelectric power generators. To conquer this problem, we report herein Ca-doped Bi0.5Sb1.5Te3 alloys prepared by spark plasma sintering that combines intrinsic point defect and multiscale microstructure engineering. Ca doping suppresses the detrimental bipolar effect at elevated temperatures by increasing the hole concentrations. Furthermore, by detailed electron microscopy investigations, combined with theoretical analysis on phonon transports, we propose that ultralow thermal conductivity is attributed to the strong phonon scatterings in a wide frequency range via tuning multiscale microstructures, which include nanoprecipitates, dislocations, and stacking faults caused by Ca doping. As a result, a peak ZT value of 1.3 at 400 K was obtained, with a state-of-the-art average ZT value of 1.21 between 300 and 500 KK in Bi0.48Ca0.02Sb1.5Te3. These results demonstrate the efficacy of the multiple synergies that can also be applied to optimize other thermoelectric materials. Owing to the large radius of Ca and huge difference in electronegativity between Ca and Bi atoms, the Ca has been proved to be occupied into Bi sites and improve hole concentration. As a result, the bipolar effect is suppressed and the optimum range of thermoelectric performance is broadened. A high average ZT of 1.21 is obtained, which is beneficial for practical applications. [Display omitted] •A new strategy to suppress bipolar effect by effectively improving hole concentration via a tiny amount of Calcium doping.•The dislocations, nano-precipitates, and stacking faults significantly decrease lattice thermal conductivity.•A broadened optimum range and a very high average ZT value of 1.21 are reached in p-type Bi0.48Ca0.02Sb1.5Te3 sample.