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Fundamental Studies on Unique Doping Mechanisms in Organic Semiconductors

  • Author(s): Cao, David Xi
  • Advisor(s): Nguyen, Thuc-Quyen
  • et al.

A world that runs on electronic devices is here to stay. The widespread use of electronic devices in our everyday life has ensured their staying power, while also driving new innovation towards improving everything from the power output of batteries, the computing power of CPUs, and the mechanical flexibility of our devices. Most of our current electronics are built in large part using silicon thanks to our robust ability to create precisely doped p- and n-type silicon wafers that allow for the creation of p-n junctions that underlie the heart of modern electronics. One drawback however to silicon-based devices is their relative mechanical inflexibility. Given recent trends towards wearable electronics and flexible sensors that can be used for biological applications, development of conductive, flexible organic semiconductors can fill a market and technological need. In addition, organic semiconductors have the potential to be lower in cost and take less energy to fabricate than their silicon counterparts.

Since the 1990s discovery of polymers that could be made conductive through doping, there has been a lot of work in the field to make organic electronics a reality. The field has spent a lot of its time and energy focused on using and understanding the dopant F4TCNQ to create doped organic semiconductors over the past decade, which operates on the principle of integer charge transfer in most circumstances. However, limits to F4TCNQ doping have driven research into novel dopants and understanding their doping mechanism to open new avenues for creating doped polymers.

This work tackles some of these new dopants and their doping mechanism, specifically Bronsted and Lewis acids. While they have both been used previously for doping polymers, little attention has been paid to them over the past decade while the field has chased dopants with increasingly deeper electron affinities for use in integer charge transfer doping. We bring these dopants back to light and provide an in-depth look into the doping mechanism for Bronsted acid doping. By using a wide slate of techniques including XPS, FTIR, UV-vis-nIR, AFM, TGA-MS, XRR and electrical measurements, we provide an estimation of the doping efficiency along with a complete picture of the Bronsted-acid doping mechanism for the conjugated polyelectrolyte PCPDTBT-SO3K, and the importance of sulfonate for these self-doping conjugated polyelectrolytes. We also touch on Lewis acid doping, and how Lewis acid strength does not correlate to increased doping efficiency or stability of these dopants by using impedance spectroscopy, EPR, UV-vis-nIR, XPS and electrical measurements. Lastly, we make use of p-doped OSCs and cAFM to show how de-doping on the microscale can be used as a novel way to image the efficacy of bacterial electron transfer to the substrate which can lead to a better understanding of how to improve the efficiency of bacteria driven fuel cells.

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