Nonlinear Optical and Photovoltaic Studies of Specific Nonconjugated Conductive Polymers and Metallic Nanoparticles

Abstract

In this research, the nonlinear optical properties of specific nonconjugated conductive polymers and gold nanoparticles in transparent dielectric media have been investigated. Photovoltaic devices utilizing nonconjugated conductive polymers have also been studied. Nonconjugated conductive polymers are polymers with at least one double bond in the repeat. 1,4-polyisoprene (cis and trans), styrene butadiene rubber, and poly(β-pinene) are readily available examples of nonconjugated conductive polymers that have been investigated. Nonconjugated conductive polymers exhibit increases of many orders of magnitude in electrical conductivity upon doping with electron acceptors such as iodine. This change in conductivity results from charge-transfer from isolated double bonds in the polymer to the dopant. Exceptionally large optical nonlinearities have been reported for nonconjugated conductive polymers since they form sub-nanometer size metallic domains (quantum dots) upon doping. Nonconjugated conductive polymers have been shown to have many potential applications in nonlinear optics, electro-optics, and photovoltaics. The quadratic electro-optic effect and electroabsorption have been investigated in several nonconjugated conductive polymers including cis-1,4-polyisoprene, trans-polyisoprene, styrene butadiene rubber, and polyethylene terephthalate. The effects of iodine doping on polyethylene terephthalate have also been investigated using spectroscopy. Metallic nanoparticles in a dielectric medium have also been shown to have high magnitude nonlinear optical susceptibilities. Large nonlinear susceptibilities have been reported near the surface plasmon resonance frequencies in the materials. These effects have been attributed to dielectric confinement of charges within the metal nanoparticles and were predicted theoretically to be related to the size of the related charge-system. The nonlinear optical properties of gold nanoparticles in a dielectric medium (glass) and Iodine-doped nonconjugated conductive polymers have been investigated. These studies used the field-induced birefringence method to measure the Kerr effect in these materials. Measurements of Kerr coefficients for the materials investigated have been performed. The magnitudes of the Kerr coefficients for the gold nanoparticle samples have been compared and used to verify theoretical models on the relationship between particle diameter and third-order optical susceptibility. Nonlinear absorption (electroabsorption) has also been measured in these materials. These measurements were made using applied electric fields to change the absorption of the material. The results have been used to gain theoretical understanding of optical nonlinearities of metallic nanoparticles down to the sub-nanometer dimensions. The nonlinearity has been shown to increase as 1/𝑑^3 where d is the diameter of the nanoparticle (quantum dot). Newer nonconjugated conductive polymers such as polyethylene terephthalate have also been studied. Photovoltaic devices utilizing Iodine doped nonconjugated conductive polymers have been constructed and evaluated. These devices were fabricated in a similar way to dye-sensitized solar cells with the polymer used as the absorbing layer. Incident light on the cell excites electrons in the nonconjugated polymer which are then transferred through an electrolyte to a thin layer of titanium dioxide. The created devices were exposed to light and their open-circuit voltages and short-circuit currents were measured. These results have been compared for the various nonconjugated polymers. The stability of the devices has also been investigated. The cells typically show degradation in the photocurrent output over time. Attempts were made to determine the cause of the rapid reduction in output and to find a method of extending the lifetime of the fabricated cells. These studies showed that sealing cells in order to reduce the loss of the liquid electrolyte can extend the lifetime of the cells.