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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Santa Barbara Electronic Theses and Dissertations (2021)

The properties of organic semiconductors make them well-suited for certain applications in electronic devices or energy conversion. Because of their low inherent conductivity, organic semiconductors need to be doped to be used in many of these applications. The limited set of small molecules commonly used for organic semiconductor doping do not always have the desired combination of properties for some applications, such as high ionization energy or air stability. In addition to removing or adding charges, these small molecule dopants can significantly alter the semiconducting film structure through Coulombic interactions. These challenges suggest a need to explore new dopants and doping mechanisms and to better understand the effects of doping on the electronic structure and morphology of organic semiconductors. We used a combination of experimental methods and DFT calculations to explore new doping mechanisms based on Lewis acid-base pair chemistry. Combining UV-Vis-NIR spectroscopy, electrical transport measurements, and grazing incidence wide-angle X-ray scattering, we connected changes in the electronic structure to changes in morphology and transport in both amorphous and semicrystalline semiconductor systems. Our results suggest ways to control the electronic structure and morphology of organic semiconductors, as well as providing a starting point to further apply Lewis pair chemistry to doping.

    Cover page: Electronic Structure, Chemical Mechanism, and Morphology in Doping Organic Semiconductors
    • Article
    • Peer Reviewed
    UC Santa Barbara Previously Published Works (2018)
    Doping of thin films of semiconducting polymers provides control of their electrical conductivity and thermopower. The electrical conductivity of semiconducting polymers rises nonlinearly with the carrier concentration, and there is a lack of understanding of the detailed factors that lead to this behavior. We report a study of the morphological effects of doping on the electrical conductivity of poly(3-hexylthiophene) (P3HT) thin films doped with small molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Resonant soft X-ray scattering shows that the morphology of films of P3HT is not strongly changed by infiltration of F4TCNQ from the vapor phase. We show that the local ordering of P3HT, the texture and form factor of crystallites, and the long-range connectivity of crystalline domains contribute to the electrical conductivity in thin films. The thermopower of films of P3HT doped with F4TCNQ from the vapor phase is not strongly enhanced relative to films doped from solution, but the electrical conductivity is significantly higher, improving the thermoelectric power factor.
      Cover page: Thermoelectric Properties of Poly(3-hexylthiophene) (P3HT) Doped with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) by Vapor-Phase Infiltration
      • Article
      • Peer Reviewed
      UC Santa Barbara Previously Published Works (2017)
      The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with F n TCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.
        Cover page: Morphology controls the thermoelectric power factor of a doped semiconducting polymer
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