(Invited) Effective Strategies to Improve Powerfactor of Flexible Thermoelectric Composites
Authors: Moon Jaeyun 
Published: February 2020

Recent developments in wearable electronics and Internet of Things (IoT) have stimulated the research interests in light-weight, flexible, renewable and sustainable energy-harvesting sources. Considering these needs, flexible thermoelectric devices have been getting attention because of simplicity of the device and abundance of available heat energy sources. In addition, from the manufacturing point of view, various cost-effective printing techniques, such as screen printing, spraying and direct-writing, can be employed to create flexible thermoelectric devices while most commercially available thermoelectric devices are fabricated by costly and inefficient inorganic material processing methods. On the other hand, many of the advances in improving thermoelectric properties over a couple of last decades were achieved by utilizing unique phenomena in nanoscale materials. Thermoelectric nano-scaled materials allow us to manipulate heat and carrier transport to decouple interdependent parameters, thus to enhance the thermoelectric performance. Based on these backgrounds, a combination of nano-engineering of thermoelectric materials and printing manufacturing processes are expected to enable the development of highly-performing and low-cost flexible thermoelectric devices. In this research, n-type thermoelectric printing inks which are one of critical required components of the printed thermoelectric flexible devices were investigated in terms of synthesis methods, post-treatments, thermoelectric properties and mechanical flexibility.
The n-type thermoelectric inks consisting of Bi2Te3 nanowires (NWs) and carbon nanotubes (CNTs) were synthesized by two different methods, ex-situ and in-situ syntheses, to understand the influences of interface between NWs and CNTs on the thermoelectric power factor. Different post-treatment processes (i.e., thermal treatment and mechanical pressing) were employed to improve the thermoelectric power factor of the vacuum-filtered Bi2Te3 NWs - CNTs thermoelectric films. First of all, the composition of CNTs in the composites was varied in the range 10 – 20 vol% and the corresponding electrical conductivity and Seebeck coefficient were measured to identify the correlation between the composition and two parameters. Then, in the same manner, the thermoelectric power factor of the composites was assessed after the annealing process in an inert environment, and the subsequent mechanical pressing, in order to to see how those processes affect the ex-situ and in-situ synthesized films and consequently their thermoelectric properties. The composites films prepared with different conditions including CNTs composition and different post-treatments were characterized using X-ray Diffraction Spectroscopy (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and X-ray Photoelectron Spectroscopy (XPS) to reveal the relationship among the post treatment processes, the microstructure of materials, and the thermoelectric properties. We found that the thermal treatment process is significantly effective to improve stoichiometry and crystallinity of Bi2Te3 NWs, leading to enhance Seebeck coefficient and electrical conductivity, simultaneously. As a result, the thermoelectric power factor of the ex-situ composites and the in-situ films after the post-treatments was drastically increased to be 225.9 μW m−1K−2 and 0.74 mW m−1K−2, respectively. In particular, the considerable enhancement observed in the in-situ synthesized sample is attributed to the phase homogenization of Bi2Te3 inorganic nanowires as well as the enhanced interfaces between NWs and CNTs both qualitatively and quantitatively, through a combination of three approaches, (i) in-situ synthesis (ii) annealing and (iii) mechanical pressing. The flexibility and the conductivity change over the repeated bending cycle were examined to show the excellent mechanical robustness and flexibility of the films. We expect that the findings from this study can be applied to other flexible thermoelectric materials systems to enhance thermoelectric properties.