Thermoelectric generators can be used to directly convert heat into electricity. The performance of thermoelectric (TE) materials is determined by the figure of merit ZT (ZT=(σS^2*T)/k, with σ as the electrical conductivity, S as the Seebeck coefficient, k as the thermal conductivity, and σS^2 as the Power Factor). An efficient TE material requires large Power Factor and low thermal conductivity. Semiconductors are typical TE materials. Despite of their high Power Factors, it is still challenging to reduce their thermal conductivities lower than 1 W/(mK). Furthermore, semiconductors are rigid and contain rare earth elements, e.g. tellurium.
The intrinsic low thermal conductivity of polymers (0.1-0.7 W/(mK)) promotes their usage as potential TE materials for room temperature applications. Compared to semiconductors, polymers are cheap, flexible and can be brought easily into different shapes. Polymeric materials can be fabricated by low cost production techniques such as solution or melt processing methods. Compared to solution processing, melt processing of polymers avoids solvents and can be easily scaled up to currently industrial level technology.
In our study, polymer composites with an industrially widely used polymer, namely polypropylene (PP), as the matrix were prepared by melt processing. Single walled carbon nanotubes (CNTs) were applied to construct an electrical conducting network. For the selected CNTs, electrical percolation for charge transport occurs already at 0.2 wt% CNTs. However, the charge carriers injected by CNT addition are detrimental to the Seebeck coefficient, leading to an optimized Power Factor at 4 wt% CNTs. On the other hand, the addition of additive, ionic liquid (IL), together with CNTs, simultaneously increases σ and S compared to composites filled with only CNTs. The highest obtained Seebeck coefficient of such melt prepared composites with mixed filler system is about 63 µV/K, much larger than that common solution processed conjugated polymers, e.g. PEDOT:PSS (~ 20 µV/K). The effectivity of this mixed filler system of CNTs and IL inside PP matrix is confirmed to be dependent on the CNT types, e.g. the addition of IL resulted in 10 times higher Power Factor for one selected CNT type. In addition, it is intended to develop both n-type and p-type composites from the same base material for better compatibility regarding device fabrication.