Vertically Aligned Se Nanowires-PEDOT:PSS Hybrid Composites on Flexible PDMS Substrate and Their Thermoelectric Properties
Authors: Lee Kang Yeol , Lim Jae-Hong 
Published: December 2020

The best thermoelectric materials should have low thermal conductivity (κ) and high Seebeck coefficient (S) and electrical conductivity (σ). However, the three parameters, which constitute ZT (ZT = S2σT/κ), are interdependent on each other. Advances in high-ZT thermoelectric materials have been mostly achieved by optimizing material composition and modifying the nanostructure of inorganic materials. However, the inorganic thermoelectric materials are generally less abundant, expensive, and toxic. Recently, researches have been focused on polymer-based thermoelectric materials in order to overcome these limitations of inorganic materials. In particular, conducting polymers have attracted great attention as promising thermoelectric materials because of their unique advantages such as low thermal conductivity, flexibility, low cost, and low toxicity. However, the typical ZT value of the conducting polymer-based materials are on the order of 10−2 ~ 10−3 and are much lower than those of inorganic materials. It is essential to enhance the Seebeck coefficient without decreasing the electrical conductivity. The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most promising materials. Water soluble PEDOT:PSS is already widely used for thermoelectrics as a base material for composition between inorganic and organic materials. It can be also modified by a secondary doping according to adding high boiling point cosolvents to increase its conductivity more than 1000 S/cm. Among the cosolvents, ethylene glycol (EG), which is miscible with water and has a boiling point of 189 °C, is one of the most commonly used cosolvents for this purpose.
Interestingly, the one-dimensionally aligned inorganic structures can also allow for more effective thermal and electrical energy transfer as forming the hybrid microstructures with the conducting PEDOT:PSS, because the number of inorganic-organic interface grain boundaries that cause energy loss is lower than that in the case of randomly dispersed structures. Moreover, the aligned structures transfer the produced energy to the neighboring structures more effectively. These characteristics of the aligned structures can prevent the cancellation of charge carrier movements among the core materials and can produce the relative enhancement of the energy generation.
In this study, we have demonstrated the developed thermoelectric properties of ethylene glycol (EG)-doped poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS)/Se nanowires hybrid composites film on the flexible PDMS substrate (PEDOT:PSS-Se/PDMS). The amount of EG added to the PEDOT:PSS solutions was optimized by measuring the σ , using a conventional four-probe method. As the EG concentration increased from 0 to 10%, σ increased to ~ 1000 S/cm. We have demonstrated in tuning the barrier energy difference of between PEDOT:PSS and Se via the EG doping. The S can be maximized up to 121.6 μV/K through optimization of PEDOT:PSS-Se/PDMS barrier energy according to so-called energy filtering effect. The power factor of the fabricated EG 8%-doped PEDOT:PSS-Se/PDMS composite film was 1.59 mW/mK2 at 30 °C. The electrical conductivity can be also enhanced simultaneously owing to the EG-induced π
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π stacking among PEDOT chains. This work provides a smart approach to design and modulate the thermoelectric properties of conducting polymer/inorganic nanostructure composites. This study also shows that flexible thermoelectric devices based on cheap conducting polymers have great potential in wearable electronics.

Figure 1