  Low-dimensional materials, in which the electronic and transport properties
are drastically modified in comparison to those of three-dimensional bulk
materials, yield a key class of thermoelectric materials with high conversion
efficiency. Among such materials, the organic compounds may serve peculiar
properties owing to their unique molecular-based low-dimensional structures
with highly anisotropic molecular orbitals. Here we present the thermoelectric
transport properties of the quasi-one-dimensional dimer-Mott insulator
$\beta'$-(BEDT-TTF)$_2$ICl$_2$, where BEDT-TTF stands for
bis(ethylenedithio)-tetrathiafulvalene. We find that the thermopower exhibits
typical activation-type temperature variation expected for insulators but its
absolute value is anomalously large compared to the expected value from the
activation-type temperature dependence of the electrical resistivity.
Successively, the Jonker-plot analysis, in which the thermopower is usually
scaled by the logarithm of the resistivity, shows an unusual relation among
such transport quantities. We discuss a role of the low dimensionality for the
enhanced thermopower along with recent observations of such a large thermopower
in several low-dimensional materials.
