  Confined electrons in low dimensions host desirable material functions for
downscaled electronics as well as advanced energy technologies.
Thermoelectricity is a most fascinating example, since the dimensionality
modifies the electron density of states dramatically, leading to enhanced
thermopower as experimentally examined in artificial two-dimensional (2D)
structures. However, it is still an open question whether such an enhanced
thermopower in low dimensions is realized in layered materials with strong 2D
characters such as cuprates. Here, we report unusual enhancement of the
thermopower in the layered organic compound $\alpha$-(BEDT-TTF)$_2$I$_3$, where
BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the
slope in the Jonker plot (thermopower $S$ vs. logarithm of electrical
conductivity $\log\sigma$) for $\alpha$-(BEDT-TTF)$_2$I$_3$ is significantly
larger than that of conventional semiconductors. Moreover, the large slope is
also seen in the related layered salt, demonstrating the impact of the
2D-confined carriers in the layered organics on thermoelectricity.
