Research on PEDOT:PSS as a thermoelectric material

2025/01/17

Nagoya Institute of Technology
Associate Professor　Naoki Kishi

Abstract

PEDOT:PSS, a conductive polymer material, has attracted attention in recent years as an organic thermoelectric material. We are researching additive materials that enhance the thermoelectric properties of PEDOT:PSS. Here, we report on the effects of adding surfactants on the thermoelectric properties of PEDOT:PSS.

Main text

There are many places around us that are warmer (hotter) than the surrounding area, such as the manufacturing equipment in factories and the back of home appliances in living spaces. Most of the thermal energy in these places is released as waste heat into the surrounding area without being used. Thermoelectric power generation is a way of making use of this unused waste heat. Thermoelectric power generation is a method of generating power that directly obtains power from the temperature difference formed in thermoelectric materials based on the Seebeck effect.
Figure 1 shows an example of the structure of a thermoelectric device. It has a structure in which p-type and n-type thermoelectric materials are connected in series, and one side is in contact with a heat source, causing a temperature difference to occur between the two sides. In this device structure, a temperature difference is formed in the thermoelectric material, and a thermoelectric voltage in the direction shown in Figure 1 is generated by the Seebeck effect. By connecting a load to this device and forming a circuit, an electric current flows and can be used for power generation.

Figure 1. Example of a thermoelectric device structure

The thermoelectric properties of thermoelectric materials are generally expressed using the figure of merit ZT.

ZT = S²σT κ

S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, temperature, and thermal conductivity, respectively. In order to improve thermoelectric properties, it is necessary to increase the Seebeck coefficient and electrical conductivity and decrease the thermal conductivity.
In recent years, organic thermoelectric materials have been attracting attention. Because organic thermoelectric materials are flexible (Figure 2), they are expected to be used to generate electricity from heat sources with curved surfaces. The thermoelectric properties of conductive polymers such as polythiophene, polypyrrole, and polyaniline have been reported. Among these, PEDOT:PSS (Figure 3), a thiophene-based conductive polymer, is known to exhibit high thermoelectric properties among organic thermoelectric materials.

A typical way to improve the thermoelectric properties of PEDOT:PSS is to add a high-boiling-point solvent. Here, we report on the effect of the addition of surfactants on thermoelectric conversion characteristics as the result of our research [1-3]. Figure 2 shows the dependence of the ZT (T = 300 K) of PEDOT:PSS on the concentration of the surfactant. It can be seen that the ZT value depends on the concentration of the surfactant, and that it reaches its maximum value at a concentration of around 1 wt%. The improvement in ZT is mainly due to the improvement in the electrical conductivity of PEDOT:PSS. In addition, the results of X-ray diffraction showed that the addition of surfactants improved the crystal quality of PEDOT:PSS. This improvement in crystal quality contributed to the improvement in the electrical conductivity and ZT of PEDOT:PSS.

Figure 4. Dependence of the ZT of PEDOT:PSS on the concentration of surfactant

References
[1] Enhancement of thermoelectric properties of PEDOT:PSS thin films by addition of anionic surfactants, Naoki Kishi, Yuya Kondo, Hiroki Kunieda, Satoshi Hibi, Yuma Sawada, Journal of Materials Science: Materials in Electronics, 29 (2018) 4030.
[2] Role of anionic surfactant addition in improving thermoelectric properties of PEDOT:PSS free‑standing films, Shafayat Hossain, Yuya Yamamoto, Shogo Baba, Shohei Sakai, Naoki Kishi, Journal of Polymer Research, 31 (2024) 233.
[3] Tuning the mechanical and thermoelectric properties of self-standing stretchable PEDOT:PSS/SDBS films, Shafayat Hossain, Yuya Yamamoto, Naoki Kishi, Journal of Applied Polymer Science, 141 (2024) e55713.