Doping and photoluminescence of poly(phenylene vinylene)s and polythiophenes in electrochemical devices and sensors

With the discovery of conduction in conjugated polymer polyacetylene, emerged a class of materials with vast applicative potential, and scientific descriptions integrating the theories of semiconductor physics and organic chemistry disciplines. The additional discovery of electroluminescence in poly(para phyenylene vinylene) (PPV) furthered the possibilities of these amorphous plastics in the design of organic optoelectronic devices. Though decades of re search have fueled the use of conjugated polymers in applications such as light-emitting diodes (PLED), light-emitting electrochemical cells (LEC), actuators, electrochromic devices (ECD), transistors, solar cells and sensors, fundamental mechanisms concerning the optical and electrical nature of the materials are still uncertain.; In this thesis, I present several studies designed to elucidate relationships between the doping and optical properties of conjugated polymers as they used in electrochemical devices and sensors. In Chapter one, I provide an introduction to the semiconducting and optical traits of organic polymers, specifically PPVs and polythiophenes, as well as an introduction to surface enhanced optical phenomena. In Chapter two, I elaborate on the experimental processes and models used throughout.; Doping introduces structural changes in polymer chains, altering their physical and optical properties. Electrochemical doping of conjugated polymers, significant to the operation of devices such as LECs and polymer actuators, is not fully understood. In Chapter three, I use cyclic voltammetry as a technique for understanding electrochemical doping in poly[2 methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and interpret the results in terms of the formation of fundamental quasi-particles that are interrelated to changes in the absorption and photoluminescence of the material. An ECD is a simple alternative device structures to the LEC for studying solid-state p- and n-type doping. In Chapter four I describe the operation of a solid-state ECD made from MEH-PPV and characterize its behavior with changes in dopant concentration, dopant type, and film thickness.; Chemical and electrochemical doping in electroactive polymers cause changes in the absorption profile and drastically reduce the luminescence efficiency; consequently, because of growing demand for low-cost, sensitive, processable materials for chemical and biological sensing, the luminescence sensitivity of conjugated polymers to molecular quenching is an attractive characteristic. In Chapter five, I present the design of a chemically robust platform for examining chemical doping in polythiophenes. I report the photoluminescence sensitivity, and consequently the induced conductivity, of grafted polythiophenes when exposed to oxidants iodine, ferric chloride and methyl viologen.; Furthermore, the ability to enhance the luminescence quantum efficiency of a conjugated polymer without chemically modifying the structure would benefit all light-emitting, photovoltaic and optically-based sensing applications. In exploration of this concept, Chapter six describes currently unexplored, experiments on surface enhanced photoluminescence in MEH-PPV films using thermally evaporated silver nanoislands. In these, I find enhancement possible but more complicated than similar processes on small molecule dyes as a result of highly efficient luminescence quenching and thin film interference. I analyze the complications and propose studies to further probe the mechanisms involved.