Conjugated polymers (CPs) have attracted interest as promising thermoelectric(TE) materials due to mechanical flexibility, solution-processed, and simple manufacturing process compared to inorganic materials. However, TE performance of CPs is still lower than that of inorganic materials. At present, researchers have carried out several strategies to achieve higher charge transport properties, such as polymer backbone engineering, polymer side-chain engineering, controlling polymer alignment, or optimizing doping conditions In this study, we demonstrate the correlation among TE properties, charge transport behaviors, and side-chain effects using newly synthesized four semi-crystalline CPs, including alkyl side-chain attached poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2 yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and oligo ethylene glycol side-chain attached PPDT2FBTs with different side-chain lengths. We adopted an electrochemical doping system, which could continuously and reversibly control the doping level of CPs., via electrolyte-gated TE measurement system using 1-propyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (PMIM+-TFSI-). By comparing the Seebeck coefficient and electrical conductivity relation based on the Kang-Snyder charge transport model, we identified the variation of charge transport properties by side-chain effects. Also, we demonstrated that TE properties were varied by the energetic disorder related to the status of the density of states(DOS) through in-situ experimental measurements. DOS depends on the energetic disorder and has been considered a critical factor for TE properties because the narrow width and higher slope indicate an enhanced Seebeck coefficient. Furthermore, p- to the n-type transition of the major carriers, indicated by the sigh transition of Seebeck coefficient, were interpreted in respect of the energy difference between Fermi energy level and transport edge with the variation of DOS during the electrochemical doping process. Finally, our analysis provides strategies for side-chain engineered CPs to investigate enhanced TE properties based on charge transport behaviors.

In Situ Investigation of Charge Generation, Transport, and Thermoelectric Properties of Side-Chain-Engineered Semicrystalline p-Type Polymers during Electrochemical Doping

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