Thermopower in Organic Molecular Solids

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Non-Technical AbstractOrganic semiconductors are carbon-based materials, similar to dyes, that conduct electricity. They can be dissolved in solvents and printed using inkjet-printing on flexible substrates for use in low-cost electronics such as displays and sensors. Light emitting diodes based on organic semiconductors are already commercially available in mobile phones. With support from the Solid State and Materials Chemistry Program in the Division of Materials Research, this research project expands their use through the study of their fundamental physical properties such as their electrical conductivity, and explores possibilities for converting excess thermal energy to electrical power. The project has significant impact on the future scientific workforce because graduate student researchers carrying out the research gain significant training in multidisciplinary materials science and engineering. The research team also performs outreach to the public through educational activities in local K-12 schools.Technical AbstractA key challenge in molecular organic electronic materials is the control of their electronic properties through solid state ordering. Electrical doping of organic semiconductors is important for electrode layers in organic light emitting diodes and solar cells and in emerging thermoelectric applications. The research in this project examines how the crystalline order in solution processed organic semiconductors, both in neat films and in blends with acceptors, affects their electrical conductivity and electronic structure. The intrinsic properties of electrical conduction are compared in samples doped by molecular acceptors in model bilayer structures and in the bulk. Thermopower measurements and advanced x-ray structural characterization are carried out to determine how the microstructure and electronic structure of organic molecular solids evolve upon electrical doping. Gated thermopower measurements are used to determine the electronic structure of undoped materials to gain insight into their electronic structure. This research provides insight into new means to control the electrical conductivity of organic semiconductors in order to determine the limits of the electrical properties of these important materials systems.