Elastic conducting polymer composites in thermoelectric modules

The rapid growth of wearables has created a demand for lightweight, elastic and conformal energy harvesting and storage devices. The conducting polymer poly(3,4-ethylenedioxythiophene) has shown great promise for thermoelectric generators, however, the thick layers of pristine poly(3,4-ethylenedioxythiophene) required for effective energy harvesting are too hard and brittle for seamless integration into wearables. Poly(3,4-ethylenedioxythiophene)-elastomer composites have been developed to improve its mechanical properties, although so far without simultaneously achieving softness, high electrical conductivity, and stretchability. Here we report an aqueously processed poly(3,4-ethylenedioxythiophene)-polyurethane-ionic liquid composite, which combines high conductivity (>140 S cm−1) with superior stretchability (>600%), elasticity, and low Young’s modulus (<7 MPa). The outstanding performance of this organic nanocomposite is the result of favorable percolation networks on the nano- and micro-scale and the plasticizing effect of the ionic liquid. The elastic thermoelectric material is implemented in the first reported intrinsically stretchable organic thermoelectric module.

In order to explain the correlation between dc and S shown in Fig. 2c & 2f, we propose the following hypothesis: In good approximation, the transport of charge carriers in a hopping regime between localized states is governed by the density of state (DOS). The Fermi distribution dictates the filling of the DOS; which also determines a level of energy called the transport energy E* 1 , dominating the percolation transport because of its highest hopping probability. The Seebeck coefficient (S) can be described as 2,3 : , where q is the charge of the carrier, T the temperature, EF is the Fermi Level.

4
The density of percolation paths in PEDOT:PSS is strongly affected by the morphology of the film as known by the addition of high boiling point solvents and ionic liquids that promotes the demixing of the excess of insulating PSS and promotes the creation of 3D percolation paths 4,5 ; or by the addition of insulating polymers breaking the percolation paths 6 . Sometime small amounts of those additives lead to a drastic change in transport properties. For instance, the same absorption spectrum is observed for films with an electrical conductivity varying by 3 orders of magnitude 7 , thus indicating that the concentration of PEDOT chains is barely affected, but their connectivity is much different and governed by microscopic morphology. Hence, we can thus speak about PEDOT chains with good connectivity and bad connectivity. PEDOT chains with good connectivity can form nanocrystals where short range order dominates the transport (blue domains in Supplementary   Fig. 2). While PEDOT chains with bad connectivity are in an amorphous phase without even local order (green domains). The electrostatic potential profile on those two different types of PEDOT chains is different 8 and it is thus expected to affect their electronic energy levels. We propose that their DOS are thus also different. As a result, our hypothesis is to consider a DOS for PEDOT:PSS systems composed two DOS peaks. One DOS peak representing the electronic levels for the connected PEDOT chains and another DOS peak representing the electronic bipolaronic levels of the disconnected PEDOT chains. In this hypothesis, we thus propose that upon the removal of ionic liquid or the addition of insulating WPU polymer, the ratio of connected vs. disconnected PEDOT chains decreases and the intensity ratio of those two DOS peaks differs, which leads to an increase of S as proposed recently for polymer blends 9 . , while RG/R0 for Poisson's ratio 0.5 is calculated from the equation:

Supplementary
where v is the tensile strain which is the same as dl/l0. at 100 °C led to a significantly different morphology in conjunction with a severe deterioration in mechanical property (i.e. elongation at break decreased down to 27%). Based on haze in the image and poor mechanical property observed for the composite without ionic liquids, we assume that the phase separation between different components and the migration of ionic liquids to the surface may occur at this temperature.

Supplementary Tables
Supplementary Table 1| Various PEDOT-or PEDOT:PSS-elastomer composites. Compositions, fabrication procedures, and their performances as elastic conductors were compared.

Supplementary Table 2| Quantitative analysis of AFM height images. Lateral cluster diameter
and surface roughness of CP-WPU composite films with and without a different type of ILs or DMSO. The cluster diameter was averaged from the five biggest clusters shown in Fig. 5b, while the surface roughness was determined from the RMS values taken from three 20×20 m 2 height images for each sample.

Sample
Lateral cluster diameter (m) RMS surface roughness (nm)