From Small Molecules to Polymers: Linear and Nonlinear Optical Properties of Organic Conjugated Systems for Solar Application

The world is seeing rapid population growth and with this growth, energy demands have soared. Traditionally, fossil fuels have been used to meet these demands, but extraction of fossil fuels is detrimental to the environment, burning of fossil fuels contributes significantly to adding pollutants to the environment, and have directly resulted in climate change. This has spurred the public and scientists to search for energy sources that are more environmentally friendly and are renewable. The sun is the largest energy source in the solar system. If materials and devices can be designed to collect even a fraction of the sun's energy, humanity's energy needs will be me many times over. One of the most promising light harvesting materials is organic light harvesting materials because of their ease of processing, lightweight, high absorption capabilities, and flexibility.;In this thesis, the optical properties of conjugated organic light harvesting materials were investigated. In order to improve the efficiency of these devices, the fundamental optical properties need to be understood. In the first study, the optical properties of light harvesting donor-acceptor polymers with the same donor monomer with different strength acceptors were investigated. Stronger acceptors donor-acceptor polymers had enhanced charge transfer characteristics, lower quantum yields, and shorter fluorescence lifetimes compared to weaker acceptor polymers. In the second study, the effect of donor conjugation length on the optical properties of donor-acceptor light harvesting polymers was investigated with ultrafast spectroscopic techniques. Polymers with longer donor conjugation lengths had higher extinction coefficients, higher charge transfer characteristics, and transient absorption experiments revealed additional species in the excited state. Computations studies related the electronic structures to the optical properties of the investigated polymers. Two new experiments were designed and installed for unprecedented wavelength selection and unprecedented long timescale investigations. An overview of a new nanosecond system that I have installed and two-photon absorption experiment that I have designed is presented. The experimental procedure and design is described. Preliminary results are presented demonstrating the success of this new system and experimental design. A new nanosecond transient experiment was installed. The experimental procedure for the nanosecond transient experiment is described. Preliminary results are presented demonstrating the successful implementation and operation of this new system.