The thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials have attracted attention recently because of their remarkable electrical conductivity, power factor, and figure of merit. In this review, we summarize recent efforts toward improving the thermoelectric properties of PEDOT-based materials. We also discuss thermoelectric measurement techniques and several unsolved problems with the PEDOT system such as the effect of water absorption from the air and the anisotropic thermoelectric properties. In the last part, we describe our work on improving the power output of thermoelectric modules by using PEDOT, and we outline the potential applications of polymer thermoelectric generators.

Screen-printed polymer thermoelectric devices provided sufficient power to illuminate light-emitting diodes.

X-ray charge densities of Al-based icosahedral quasicrystalline approximant crystals $\alpha -\mathrm{AlReSi},$ $\alpha -\mathrm{AlMnSi},$ and ${\mathrm{Al}}_{12}\mathrm{Re}$ were observed by a combination of the maximum entropy method with the Rietveld method. We successfully obtained the clear images of interatomic covalent bonds between Al and transition metals (Mn, Re) and those in the Al (or Si) icosahedron in Mackay icosahedral clusters of both $\alpha -\mathrm{AlReSi}$ and $\alpha -\mathrm{AlMnSi}$ approximant crystals. The bonding nature of the three kinds of glue atom sites connecting Mackay icosahedral clusters was also clarified. This covalent bonding nature should strongly relate with the enhancement of the electron density-of-states pseudogap near the Fermi level. In addition, the interatomic covalent bonds of $\alpha -\mathrm{AlReSi}$ are stronger than those of $\alpha -\mathrm{AlMnSi}.$ This fact leads to the low effective carrier density of $\alpha -\mathrm{AlReSi}$ in comparison with that of $\alpha -\mathrm{AlMnSi}.$ Unlike the covalent bonding nature of an icosahedron in $\alpha -\mathrm{AlReSi}$ and $\alpha -\mathrm{AlMnSi}$ crystals, the Al icosahedron with an Re center atom exhibits no Al-Al interatomic covalent bonds in the ${\mathrm{Al}}_{12}\mathrm{Re}$ crystal. The tendency for metallic-covalent bonding conversion in the Al icosahedron, which is related to the atom site occupancy of the icosahedral cluster center, is also strongly supported.

Measuring the in-plane thermal conductivity of organic thermoelectric materials is challenging but is critically important. Here, a method to study the in-plane thermal conductivity of free-standing films (via the use of commercial equipment) based on temperature wave analysis is explored in depth. This subject method required a free-standing thin film with a thickness larger than 10 μm and an area larger than 1 cm², which are not difficult to obtain for most solution-processable organic thermoelectric materials. We evaluated thermal conductivities and anisotropic ratios for various types of samples including insulating polymers, undoped semiconducting polymers, doped conducting polymers, and one-dimensional carbon fiber bulky papers. This approach facilitated a rapid screening of in-plane thermal conductivities for various organic thermoelectric materials.