UC Santa Barbara

This dissertation is concerned with the fabrication and characterization of organic electrochemical transistors (OECTs) made from polyelectrolytes based on the material PCPDTBT-SO3-Na (CPE-Na). OECTs are electrolyte-gated transistors that allow the electrolyte to permeate into the bulk of the film and directly changes the doping state of the bulk of the semiconducting layer. The relationship between the molecular structure of the polyelectrolytes and the material properties are explored in the lens of optimizing device characteristics in OECTs. The first chapter of this thesis focuses on the development of OECTs made using PCPDTBT-SO3-K (CPE-K) as the active layer. While the large majority of OECT research still utilizes poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the active layer, a few new OECT materials have been recently reported. CPE-K has been used in the past for applications such as DNA-detection, thermoelectric devices and as interlayers in organic photovoltaic devices. Conjugated polyelectrolytes (CPEs) represent a promising and unique class of materials which are characterized by their conjugated carbon backbone and pendant ionic chains. Their water solubility in addition to their electronic and ionic conductivity makes them an ideal candidate for OECT applications. Using various spectroscopic and electric characterization techniques, CPE-K was fully characterized so that it could be accurately compared to other OECT materials reported in the literature. In fact, CPE-K is among the short list of high-performance OECT materials. The techniques used in this work was used to shed light on the operational mechanism of CPE-based OECTs. In addition, the effective use of interdigitated contacts was used to increase the transconductance of OECT devices.The second portion of this thesis expands on the work of CPE OECTs, by exploring their structure-property-relationship. A series of CPE-Na polymers with varying sidechain lengths were synthesized by collaborator Luana Llanes from the lab of Professor Guillermo Bazan. A series of CPE-Na with increasing alkyl chain distances (2-5 methylene units) were synthesized. The purpose of synthesizing these materials was to explore the effect of varying the distance of the pendant charged sulfonate on the side-chain. CPE-Na is a self-doped polymer, due to the negative charge on the sulfonate stabilizing the positively charged polaron. It was hypothesized that reducing the side-chain length would result in easier doping of the CPE due to the proximity of the sulfonate. To our surprise, the opposite trend was observed. By using a wide range of characterization techniques, the structure-property-relationship between the sidechain length and ease of doping of the CPE backbone was explored. The next chapter of the thesis switches topics to organic field-effect transistors (OFETs). Due to convenience and low leakage current, many OFETs use silicon dioxide as the gate dielectric material. As demonstrated by Dr. Hung Phan, trap sites at the dielectric-semiconductor interface result charge trapping. This charge trapping results in unwanted artifacts such as hysteresis, double-slope nonideality characteristics, and reduction in current when the device is continuously biased. The fluoropolymer P(VDF-HFP) was used as a dielectric material. We found that the device transfer characteristics are heavily dependent on the processing conditions and choosing the proper grade of P(VDF-HFP). We presented the correct choice of processing conditions and material grade results in ideal transfer characteristics such as high on-off ratio, low threshold voltage, low hysteresis and high current stability. The final section of the thesis discusses a project to explore the structure-property relationship of a group of 12 different n-type non-fullerene acceptor small molecules. Non-fullerene acceptors (NFAs) have recently become an area of interest to researchers aiming to develop high performance organic solar cells due to their promising electronic properties and greater tunability relative to fullerene-based acceptors. Despite the success of NFAs in organic solar cells, there have only been a few studies looking at the electron mobility of these materials. A collection of 12 NFAs with a variety of core groups and sidechain configurations were tested as the active layer in an organic field-effect transistor (OFET) to measure the mobility of horizontal charge transport.