Development of high-performance multidimensional bismuth telluride-based thermoelectric materials

Thermoelectric materials enable the direct conversion between heat and electricity through the Seebeck effect. The efficiency of energy conversion is governed by a dimensionless figure of merit (zT) as: zT = S2σT/κ, where S, σ, κ, and T is the Seebeck coefficient, electrical conductivity, thermal conductivity, and operating temperature, respectively. By fabricating thermoelectric materials into pellets or thin films, they can be facilely integrated with wearable electronics to harvest human body heat at the low temperature, acting as the energy-autonomous, maintenance-free and emission-free power sources. To this end, developing high-performance multi-dimensional low-temperature thermoelectric materials is of vital significance. So far, the best low-temperature (200-400 K) thermoelectric materials are bismuth telluride (Bi2Te3) and its compounds, where promising zT can be achieved from enhanced power factor (S2σ) and suppressed lattice thermal conductivity (κl). With the combination of several strategies including point defect, texture and nanostructure engineering, zT of Bi2Te3-based thermoelectric pellet has been improved up to 1.5. On the other hand, studies on Bi2Te3-based thermoelectric thin films mainly focus on the optimizations of inorganic film crystallinity and defects, and interface engineering in inorganic/organic hybrids, showing record zT up to 0.89. In order to further boost zT of Bi2Te3-based thermoelectric pellets and thin films, there are at least four issues deserving to be further explored. (1) Carrier concentration (n) of Bi2Te3 pellets still deviates from the predicted optimal value, leading relatively low S2σ. (2) Further κ reduction of Bi2Te3 pellets is desired, considering current studies have pushed κ close to predicted amorphous limit. (3) Interfacial carrier transports between Bi2Te3-based inorganic fillers and conductive polymers are impeded by high interfacial contact resistances, leading poor σ in Bi2Te3-based inorganic/organic thermoelectric films. (4) Low crystallinity of conductive polymers and poor interfacial carrier transports lead poor carrier mobility (μ) in Bi2Te3-based inorganic/organic thermoelectric films. Given aforementioned four issues, corresponding studies have been conducted in this PhD project, which are summarized in the following. i. Nanostructured n-type Bi2Te3 pellet was fabricated through solvothermal synthesis followed by spark plasma sintering (SPS), where n was modulated by non-equilibrium fast reaction to approach the optimal value calculated by single parabolic band (SPB) model. Te vacancies were found to be effectively suppressed, leading reduced n from pristinely ~1 × 1020 to ~6 × 1019 cm-3 and generating a decent S2σ of 12.84 μW cm-1 K-2 at 320 K. Meanwhile, the decreased electronic thermal conductivity (κe) due to deteriorated σ enabled a very low κ of 0.48 W m-1 K-1, which ultimately secured a promising zT of ~1.1 at 420 K and an outstanding average zT of ~1 from 320 to 470 K. (Chemical Engineering Journal, 2019, 391: 123513). ii. Nanostructured n-type Bi2Te3 pellet with porous structure was fabricated using solvothermal synthesis and SPS techinique. Homogeneously distributed pores and dense grain boundaries were successfully introduced into the Bi2Te3 matrix, causing strong phonon scatterings. As a result, an ultralow κl of < 0.1 W m-1 K-1 was achieved. With the well-maintained decent S2σ of 10.57 μW cm-1 K-2, a promising zT value of 0.97 was secured at 420 K. (ACS Applied Materials & Interfaces, 2019, 11: 31237-31244). iii. Interfacial carrier transports in Bi0.5Sb1.5Te3/PEDOT:PSS flexible thermoelectric films were optimized by coating Bi0.5Sb1.5Te3 fillers with highly conductive CuTe layer, using facile electroless plating method. Highly conductive CuTe layer can render carriers to travel within CuTe layers or through Bi0.5Sb1.5Te3fillers, rather than being scattered. Consequently, interfacial contact resistances were significantly reduced. Meanwhile, highly crystallized conductive polymer PEDOT:PSS with microstructure of lamella stacking was prepared by DMSO-H2SO4 double treatments, where insulating PSS was effectively depleted. Optimized interfacial carrier transports and highly crystallized PEDOT:PSS synergistically contributed to significant boost of μ from pristinely ~0.77 to ~18.82 cm2 V-1 s-1, resulting in outstanding σ of ~2300 S cm-1 and record-high room-temperature S2σ of 312 μW m-1 K-2 in Bi0.5Sb1.5Te3/PEDOT:PSS flexible thermoelectric films. Accordingly, a home-made flexible thermoelectric device was fabricated using as-prepared composites, generating a promising open-circuit thermovoltage of ~7.7 mV with the human wrist as the thermal source. (Chemical Engineering Journal, 2020, 397: 125360