Thermal energy harvesting from low-temperature heat sources, such as the human body, is a promising area of research that is currently attracting lots of attention from scientists and engineers.

In particular, it is thermoelectric generators that use conductive polymers that are looking to be the most suitable candidates for thermal energy harvesting thanks to their lightweight, flexible profiles and how easy they are to fabricate.

Optimizing Conductive Polymers

However, conducting polymers exhibit a relatively low thermoelectric performance in contrast to that of commercial products, so optimizing these polymers is essential for the design of high-performance thermoelectric devices.

Now, a team from Nagoya University is said to have significantly improved the conductivity of a plastic polymer by untwisting the chains of atoms found within it. According to the Nagoya team’s research, led by applied physicist Hisaaki Tanaka, their work could help accelerate the development of wearable power sources for use in consumer Internet of Things (IoT) devices.

Conductive Polymers

Conductive polymers, which are made from thin films, feature highly disordered structures that are formed of crystalline and non-crystalline parts. This makes it difficult for researchers to gain an understanding of their properties and thus makes it difficult to find ways to optimize thermoelectric performance.

In thermoelectric devices, one of the key performance metrics is the power factor, P = S2σ. Here, S is the Seebeck coefficient and σ is the electrical conductivity.

Due to the nature of polymers’ disordered films, most conductive polymers will exhibit no maxima of power factor upon carrier doping. In other words, the power factor continuously increases with increasing electrical conductivity for higher doping levels. In their research, the team demonstrates that the empirical relationship between the Seebeck coefficient and electrical conductivity (S-σ) can actually be modified through controlled carrier doping.

An illustration of the doping technique used to form linkages (red) between PBTTT's crystalline parts (blue rectangles). Image used courtesy of Takenobu Group

Doping Polymers to Improve Conductivity

The team used poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT) as a target polymer since it exhibits the highest conductivity of 1300 S/cm among solution-processable semicrystalline polymers via chemical carrier doping.

Working in conjunction with Japanese researchers, the team set out to understand the thermoelectric properties of PBTTT by doping it with a thin ion electrolyte gel. This gel is only able to penetrate (dope) the polymer once a specific voltage is applied.

Recharging Wearables Through Heat Harvesting

Once doped, the team used a variety of measurement techniques to paint a picture of the electronic and structural changes it underwent as a result of doping. Their results show that, when doped with the electrolyte gel, the polymer’s structure changes from being highly twisted to planar. They also found that doping created links between the polymer’s crystalline parts, improving conductivity.

According to the researchers, it is the formation of these interconnective conductive links that is what determines the polymer’s maximum thermoelectric performance.

They are hopeful that with further optimizations and a change in fabrication conditions, thin-film polymers could finally begin to find applications in consumer devices, such as wearables, that are able to recharge themselves through heat harvesting.