N-type flexible Bi2Se3 nanosheets/SWCNTs composite films with improved thermoelectric performance for low-grade waste-heat harvesting

Due to the limited thermoelectric performance of polymers and intrinsic rigidness of inorganic materials, it is difficult to obtain n-type thermoelectric materials and devices with low-cost, excellent flexibility, and high performance. In this work, we have successfully synthesized n-type flexible free-standing thermoelectric films by a low-cost solvothermal method, which is composed of Bi2Se3 nanosheets and single-wall carbon nanotubes (SWCNTs). Bi2Se3 nanosheets with excellent crystallinity and rhombohedral morphology were grown on SWCNTs. The optimized Bi2Se3/SWCNTs composite films exhibited a high electrical conductivity of 292.7 S cm−1 and a Seebeck coefficient of − 42.4 μV K−1 at room temperature, eventually achieving an improved power factor of 52.7 μW m−1 K−2. Furthermore, the composite films exhibited excellent flexibility due to the interweaving of Bi2Se3 nanosheets with SWCNTs, where the thermoelectric performance remained almost unchanged after 5000 bending cycles around a 3.5 mm radius rod. A thermoelectric device, composed of the optimized films, was constructed to further show the practicality, which generated a voltage and a power density of 11.5 mV and 86.2 μW cm−2 at a temperature gradient of 58 K, respectively. This work can provide support for the preparation of other n-type free-standing thermoelectric films with both high thermoelectric performance and excellent flexibility.