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    Organic Thermoelectrics


◆ Overview : Organic Thermoelectrics

  As the global energy demands surge and the related environmental issues become severe, the thermoelectric (TE) conversion, which transforms heat directly into electricity and vice versa, has received an increasing attention for the clean and renewable power generation. Recently, organic materials are considered for TE applications due to their mechanical flexibility, low processing cost, material abundance and non-toxicity. For a highly-efficient TE generation, it is required to develop both materials that possess high TE figure-of-merit ZT values and the device architecture that efficiently harnesses the heat generated from various sources.

The list below are the recent works of our research group:
 I. Controlling Polymer-Dopant Interaction for High TE Performance
 II. Heat-Sink-Free TE Generator


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I. Controlling Polymer-Dopant Interaction for High TE Performance


  The energy conversion efficiency of a TE generator is governed by the TE figure-of-merit ZT=S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. In order to realize high σ for an organic material, doping process is essentially required. However, increasing the carrier concentration by doping typically reduces S. This tradeoff has been a serious obstacle to the further development of TE materials. Here, systematic control of the electrostatic interaction between a conducting polymer and a dopant induces a positive deviation from this TE tradeoff relation so that the electrical conductivity and the Seebeck coefficient simultaneously increase. Upon reduction of the electrostatic interaction, substantial changes in the film morphology, chain conformation, and crystalline ordering are observed, all of which critically affect the TE charge transport. As a result, the electrostatic interaction control is found to be an effective strategy to enhance the power factor, overcoming the tradeoff between TE parameters. Adapting this strategy to poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate results in a remarkable power factor (=700.2 μW m−1 K−2 ) and figure of merit ZT (=0.25).

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II. Heat-Sink-Free TE Generator

  Recent advances in TE figure-of-merit of organic TE material reaches that of bulk inorganic TE material. However, organic TE material has not been fully functionalized to the TE generator, which shows low power output, operates in a lateral heat flow, and requires an additional heat sink. We present herein a flexible organic TE generator by introducing an optimized solution process, the chevron device structure, and a foam medium. Optimized poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) film involves a distinctive film morphology, achieving a high power factor (642 μW m-1 K-2) and a low sheet resistance (< 10 Ω sq-1). The chevron-structured TE generator, integrating 24 PEDOT:PSS patterns, was highly flexible and yielded a remarkable TE output (~ 1 μW at ΔT = 17.5 K) in a vertical heat flow. In addition, internal thermal transport through the device was systematically controlled by the inserted-medium, which enables the heat-sink-free operation of the TE generator retaining 70 % of its maximum voltage output in the absence of an additional heat sink.

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Written by OTEG Team ( Daegun Kim, Duckhyun Ju, Jiwoo Min, Seonghyeon Kim, Jimin Kim )
Edited by Kwangwoo Cho ( 2019.11.11 )

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