HUANG Peishan ¹, LIU Chengyan ², CHEN Junliang ¹, ZHOU Jianhua ¹, ZHANG Zhixiang ³,

LIANG Huiting ¹, SI Luohong ¹, MAO Yongning ², KE Shanjun ⁴

(1. School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, Guangxi, China;

2. Department of Energy and Power Engineering, Hunan University of Humanities, Science and Technology, Loudi 417099, Hunan, China; 3. College of Optoelectronic Engineering, Guilin University of Electronic Technology, Guilin 541004,

Guangxi, China; 4. Guangxi Ou Shennuo Ceramics Co., Ltd., Wuzhou 543300, Guangxi, China)

Extended abstract:

[Background and purposes] Thermoelectric conversion technology enables direct transformation between electrical and thermal energy. Thermoelectric power generation based on the Seebeck effect and thermoelectric cooling based on the Peltier effect both hold immense application potential in their respective fields. Bismuth telluride-based thermoelectric materials rank among the most outstanding thermoelectric materials near room temperature and are currently the only commercially available thermoelectric materials. They are primarily used for cooling and power generation near room temperature, in applications such as wearable devices and localized precision cooling. Bismuth telluride has attracted significant attention due to its excellent electrical transport properties and low lattice thermal conductivity. For polycrystalline n-type bismuth telluride, fragmentation and mechanical deformation during fabrication often induce donor-like effects, leading to uncontrolled increases in carrier concentration. This shifts carriers away from the optimal concentration range, hindering improvements in thermoelectric performance. Moreover, the maximum ZT value typically appears at higher temperatures, which does not align with the practical application scenarios of current bismuth telluride-based thermoelectric materials. It is well known that doping serves as an effective strategy for regulating carrier concentration and optimizing the operating temperature of bismuth telluride-based thermoelectric materials. In this study, Se/S co-doped n-type bismuth telluride was prepared, whose structure and thermoelectric properties were thoroughly analyzed to gain insights into the underlying causes of optimal operating temperature variations. Finally, Se/S co-doping simultaneously enhanced the thermoelectric performance of n-type bismuth telluride, while regulating its optimal operating temperature.

[Methods] A series of Bi₂Te_3−xSe_x (x=0, 0.25, 0.5, 0.75, 1) and Bi₂Te₂Se_1−yS_y (y=0, 0.3, 0.4, 0.5) compounds were prepared via vacuum melting, combined with discharge plasma sintering. Following the experimental design, raw materials were weighed in an argon-atmosphere glove box and vacuum-sealed into quartz tubes. Quartz tubes containing mixed powders were placed in a box furnace at 1073 K and held for 10 h. After cooling, the resulting ingots were ground to eliminate large particles. The powder was then placed into 12.7 mm diameter graphite molds and subjected to discharge plasma sintering at 50 MPa, 723 K, for 20 min. Phase composition of the samples was characterized by using a SmartLab X-ray diffractometer. Microstructure and elemental distribution of the samples were characterized by using a JAM-7610F scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). Electrical transport properties of the samples were measured by using a thermoelectric material testing system (CTA-3). Thermal diffusivity of the samples was measured by using a laser thermal analyzer (LFA 467 HT). Sample density was determined by using the Archimedes method. Carrier concentration at room temperature was measured by using a Hall effect measurement system (Lakeshore 8404) via the van der Burg method.

[Results] XRD patterns of the samples reveal diffraction peaksmatching to the PDF reference cards, with a shift toward higher angles, confirming the solid solution of Se and S elements into the bismuth telluride lattice. SEM images display the characteristic layered crystal structure of bismuth telluride, while EDS maps indicate uniform elemental distribution without enrichment or the formation of secondary phases. Regarding thermoelectric properties, both Se doping alone and combined Se-S doping increase the formation energy of anti-site defects. This suppresses the donor-like effect, optimizes carrier concentration and simultaneously affects the material's electrical conductivity and Seebeck coefficient. However, it also introduces lattice distortion, reducing the material’s lattice thermal conductivity and consequently lowering overall thermal conductivity. While Se doping alone elevated the ZT value of n-type bismuth telluride from 0.31 to 0.69, the resulting bandgap widening suppressed the bipolar effect, pushing the temperature corresponding to the maximum ZT value to 523 K. Co-doping with Se and S further suppresses the quasi-donor effect, significantly reducing carrier concentration and electrical conductivity, while inhibiting electron thermal conductivity. This results in a substantial decrease in overall thermal conductivity, increasing the ZT value from 0.69 to 0.72. Concurrently, the reduced carrier concentration advances the onset of the bipolar effect, lowering the temperature corresponding to the maximum ZT value to 373 K.

[Conclusions] For polycrystalline n-type bismuth telluride, optimizing electrical and thermal transport properties by adjusting selenium doping concentration increased the ZT value from 0.31 to 0.69. Co-doping with Se and S substantially reduced the material’s overall thermal conductivity, resulting in a ZT value increase from 0.69 to 0.72 and shifting the optimal operating temperature from 523 K to 373 K. It is demonstrated that, beyond the conventional focus on enhancing ZT values, co-doping strategies offer an effective method for regulating the optimum operating temperature of n-type bismuth telluride. These findings make polycrystalline n-type bismuth telluride more suitable for power generation and refrigeration near room temperature.

Key words: n-type bismuth telluride; co-doping; carrier concentration; bipolar effect; working temperature