Scanned-probe characterization of degradation and charge generation in organic semiconductors

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Prof. John Marohn and his research team examine two performance-determining processes in organic semiconductors by using electric force microscopy to acquire time-resolved and wavelength-resolved images of surface potential and capacitance. In the first study, the research team fabricates organic thin-film transistors (from a series of n-channel organic semiconductor molecules using different materials for the transistor's dielectric layer) and uses time-resolved, spatially resolved, and wavelength-resolved measurements of the rate of charge trap formation and decay to reveal charge-trapping mechanisms and, potentially, the internal electronic energy levels of the charge-trapping species. In a second study, these researchers prepare solar-cell films (by spinning polymer blends and polymer:molecule blends onto substrates with patterned electrodes having varying, well controlled work functions) and study local charge generation kinetics in these films as a function of spatial position and illumination wavelength to obtain a better microscopic understanding of the dependence of charge generation efficiency on background charge density in organic solar cell films.Energy plays a central role in the U.S. economy. For environmental and security reasons, it is expected that solar power generation will become a crucially important component of the U.S. energy portfolio of the future. It remains a challenge to devise a solar cell that can be manufactured in sufficiently large quantities at reasonable cost. Since plastics can already be made in large quantities, it makes sense to understand how to make better solar cells from plastics. In order to accelerate the manufacturing of improved plastic solar cells, this research investigates the details of two key processes - how plastic solar-cell materials degrade under operation and how they turn light into electricity. The students and postdoctoral scientist mentored during the course of this research are needed for the U.S. to retain and improve its lead in the nanotechnology-based energy-generation and energy-storage industries.