As interest in green energy is increasing, there have been many studies on thermoelectric generator (TEG) that convert waste heat into electricity. To evaluate the TE characteristics, the figure of merit ZT value is used, which is defined as ZT = σS2T / k. Where σ is conductivity, S is seebeck coefficient and k is thermal conductivity. σS2 is expressed by a power factor. The higher the ZT value, the higher the power factor and the lower the k value, the better the TE material. Traditionally, bismuth telluride-based inorganic materials have been studied because of their high ZT values. However, wearable TEG using body heat has been actively researched meaning that solid and bulk inorganic materials are not suitable. Therefore, conductive polymer is studied recently which is suitable for wearable devices because of its flexibility, transparency, low cost fabrication and solution process.
Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT : PSS) is one of the most promising organic materials because it shows the best performance and stability compared to other organic materials. PEDOT: PSS is a mixture of PEDOT and PSS. PEDOT : PSS is suitable for the solution process because it is dissolved in water. PEDOT : PSS is basically thin film through spin coating. Because it has low thermal conductivity but the power factor is lower than that of inorganic materials, there have been many studies to improve the power factor of PEDOT : PSS. In many researches, conductivity can be greatly increased by adding solvent PEDOT PSS. Most of polar organic solvents with high boiling point like DMSO, EG, PEG, MeOH and H2SO4 are used as solvents. Recently, Kim et al. obtained high TE property of ZT = 0.42 by adding DMSO into PEDOT: PSS. However, these studies did not consider any change in mechanical properties with solvents addition. To find the optimal wearable TEG material, both thermoelectric and mechanical properties should be considered simultaneously.
We compared the mechanical properties of PEDOT PSS with various solvents and measured the thermoelectric properties according to strain. According to this study, the H2SO4 treatment method, which is known to achieve high conductivity, is not suitable for wearable TEG materials because the PEDOT: PSS is too brittle and the flexibility is very low. On the other hand, a solvent such as PEG had highly flexible PEDOT: PSS thin film with favorable thermoelectric properties.