In-situ and Operando Studies of Metastable and Transient States of Organic Semiconductor Thin Films

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Nontechnical Description: Organic semiconductors are designed at the molecular level to exhibit useful properties, and they promise the realization of a new generation of electronic circuits and solar cells. This research activity utilizes highly controllable methods of depositing ordered crystalline thin films from liquid solutions bearing small organic molecules. Experiments are being performed to understand ways that molecules organize from a disordered solution state into highly ordered crystalline solids. These molecular arrangements can also be sensitively tuned by stretching or compressing thin films, which may dramatically affect their properties. The experiments utilize polarized optical microscopy and X-ray scattering - two techniques which can help determine molecular ordering in the films. Electronic measurements are used to determine how easily electrons move in these materials. Calculations are also being performed to test theoretical predictions of the link between the molecular arrangement and electron motion. The societal impact of the research is amplified by innovative community outreach activities, including lectures and demonstrations designed to engage the public in a discussion of science and applications of energy, light, and nano-materials. This project provides graduate student training in a highly interdisciplinary area, and it introduces undergraduate students to X-ray science and thin film deposition technology.Technical Description: The rectangular hollow pen writer method can produce highly oriented films with controllable grain size that can exceed one centimeter, while in comparison, vapor deposited films typically have randomly oriented nanometer-scale grains. This project involves studies of the relationships between process parameters and film properties, such as grain size and orientation, strain, and defect density. Recent work suggests that many small molecule materials pass through a precursor stage during crystallization from solution, and this state can be unambiguously observed through a combination of optical and X-ray studies. Polarized optical video microscopy is sensitive to the nucleation and development of grain structure, while real-time optical spectroscopy is highly sensitive to local ordering, such as aggregation and molecular packing geometries. Real-time synchrotron X-ray diffraction is highly complementary because it is sensitive to long-range order, allowing time-resolved studies with millisecond resolution. Carrier transport in organic field effect transistors will also be studied as a function of applied strain by elastic stretching of thin films and substrates. The results will be related to ab-initio studies of the effect of strain on the electronic band structure and carrier transport properties. This research may lead to new environmentally responsible manufacturing methods based on roll-to-roll processes. These methods will underpin new technologies for low-cost, flexible electronics and solar cells.