Scanned-probe Characterization of Charge Generation, Recombination, and Motion in Organic Semiconductors

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Nontechnical Abstract:Sunlight represents a limitless source of clean energy. How can we provide everyone with a cheap way to capture and use the sun's energy to run their appliances and vehicles? Rooftop silicon-based photovoltaic cells already exist for converting sunlight into electricity; however, these cells are expensive to manufacture and install. This project's goal is to study solar cells made from plastics and organic molecules, which could allow solar cells to be as thin and inexpensive as a coat of paint. Plastic/organic solar-cell materials have been studied for a long time, but how these materials convert light into electricity at the molecular level remains poorly understood. To better understand these materials we are developing a microscope capable of watching charge move at the molecular level on the timescale of one nanosecond (one billionth of a second) or faster. Such a microscope would give us a new way to study molecular-level processes ranging from photosynthesis to industrially important chemical reactions taking place at a metal surface. The researchers funded by this project are using their knowledge of scientific instrumentation and computer programming to develop experiments introducing middle and high school students to topics ranging from the physics of music to the principles of electrochemistry.Technical Abstract:The goal of this project is to test molecular design rules thought to govern how light is converted into electricity in thin-film solar cells made from electron-accepting and electron-donating molecules, so-called donor-acceptor blends. Localized measurements of charge recombination dynamics and charge mobility are carried out on donor-acceptor blends prepared from molecules with systematically varied energy levels and background charge density to support or refute theories describing how charge is generated from sunlight in these blends. To enable these studies, scanned-probe-microscope measurements are developed capable of determining, with nanosecond temporal resolution and nanometer spatial resolution, the free charge yield and subsequent recombination dynamics in a thin film of a donor-acceptor blend following the application of a nanosecond-duration pulse of light. A separate set of scanned-probe measurements are developed for inferring the local charge mobility from measurements of near-surface electric field fluctuations. The tools created during this project are expected to ultimately have broad applications in studying industrially important processes like photocatalysis and electrocatalysis and in naturally important processes like photosynthesis. The researchers funded by this research are using their knowledge of scientific instrumentation and computer programming to develop experiments introducing middle and high school students to topics ranging from the physics of music to the principles of electrochemistry.