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Sequential vapor doping is a vital process in controlling the electronic transport properties of semiconducting polymers relevant to opto-electronic and thermoelectric applications. Here, we employed an in situ conductivity method to determine the temporal electronic conductivity ( σ ) profile when vapor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) doping poly(3-hexylthiophene) (P3HT) thin films held at a different temperatures. The temporal profile of σ first showed a fast exponential increase, followed by a brief linear increase until reaching a σ max , and followed by a slow decay in σ . The σ profile were correlated to structural changes through a combination UV-vis-NIR spectroscopy, X-ray scattering, and Raman spectroscopy. We find that the timing for σ max , and subsequent drop in σ of P3HT:F4TCNQ thin films corresponds to the evolution of doping in the crystalline (ordered) and amorphous (disordered) domains. Specifically, Raman spectroscopy resonant at 785 nm highlighted that the crystalline domains reached their saturated doping level near σ max and subsequent smaller level of doping occurred in regions in the disordered domains. Overall, this study emphasizes the importance of granular understanding of σ and the corresponding structural changes in the crystalline and amorphous domains.
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Ionic Dopant‐Induced Ordering Enhances the Thermoelectric Properties of a Polythiophene‐Based Block Copolymer
Abstract Conjugated polymer‐based block copolymers (CP‐BCPs) are an unexplored class of materials for organic thermoelectrics. Herein, the authors report on the electronic conductivity (σ) and Seebeck coefficient (α) of a newly synthesized CP‐BCP, poly(3‐hexylthiophene)‐block‐poly (oligo‐oxyethylene methacrylate) (P3HT‐b‐POEM), upon solution co‐processing with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and subsequently vapor‐doping with a molecular dopant, 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ). It is found that the addition of the hydrophilic block POEM greatly enhances the processability of P3HT, enabling homogeneous solution‐mixing with LiTFSI. Notably, interactions between P3HT‐b‐POEM with ionic species significantly improve molecular order and unexpectedly cause electrical oxidizing doping of P3HT block both in solution and solid‐states, a phenomenon that has not been previously observed in Li‐salt containing P3HT. Vapor doping of P3HT‐b‐POEM‐LiTFSI thin films with F4TCNQ further enhances σ and yields a thermoelectric power factorPF=α2σ of 13.0 µW m−1 K−2, which is more than 20 times higher than salt‐free P3HT‐b‐POEM sample. Through modeling thermoelectric behaviors of P3HT‐b‐POEM with the Kang‐Snyder transport model, the improvement inPFis attributed to higher electronic charge mobility originating from the enhanced molecular ordering of P3HT. The results demonstrate that solution co‐processing CP‐BCPs with a salt is a powerful method to control structure and performance of organic thermoelectric materials.
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- PAR ID:
- 10447444
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Functional Materials
- Volume:
- 31
- Issue:
- 51
- ISSN:
- 1616-301X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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