Thermoelectric (TE) devices, which can directly convert a temperature gradient to electricity, have been studied for several decades with inorganic semiconductors such as Bi2Te3. Although the performance in terms of the figure-of-merit, ZT, has been improved sufficiently for being used in energy harvesting, the practical utilization of the TE devices are limited to a few applications due to the high cost of materials and a bulky form of the devices. To overcome those disadvantages of inorganic TE devices, organic TE materials have attracted attentions for the next-generation TE devices. The organic TE materials can be synthesized easily with low cost, and using these materials TE devices can be fabricated with various shapes and high flexibility. Moreover, the performance of organic TE devices comparable to the inorganic TE devices has been reported recently. However, studies on the characterization of the output power and module design are still far apart from the main research topics in organic TE devices, which are important for practical use of the TE devices. In this work, we characterized and compared the output power characteristics of various organic TE devices based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The Seebeck coefficient of the PEDOT:PSS thin films were measured to be about 14–17 μV/K depending on the device conditions. We also analyzed the internal electrical properties of the devices by the impedance spectroscopy. We believe that the results are meaningful to develop the organic TE devices and to practically utilize them. In the presentation, we will show the detailed results and analysis of the PEDOT:PSS TE devices.