Quantum Coherence Effects on Charge Generation in Organic Semiconductors

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A fundamental understanding of energy and charge transfer processes in organic semiconductors is critical to the development of versatile next-generation photonic and electronic technologies, such as light-emitting diodes, photovoltaics, and sensors. Coherence is a phenomenon resulting from the quantum nature of semiconductors that plays an important role in determining energy and charge migration pathways within these materials and thus influences device performance. However, the role of coherence in charge transfer remains underexplored. This project is investigating how coherences involving molecular vibrations assist charge generation following light absorption by organic semiconductors. The graduate student participants in this project are being educated using an integrated approach that trains them to be problem solvers. The students are being taught to prepare complex materials and at the same time are learning the physical techniques necessary to answer the questions addressed by their research. This approach is very effective in developing the kind of intellectual flexibility necessary to meet the rapidly changing roles of scientists in society. This project also engages the broader community through outreach to elementary schools through to the collegiate level to educate and inspire the next generation of scientists.Organic semiconductors suitable for photonic device applications must be designed for ultrafast photo-driven electron-hole pair formation and subsequent rapid charge transport over long distances. Many such materials are based on electron donor-acceptor molecular components, such that optimizing the performance of organic semiconductors requires a fundamental understanding of the microscopic aspects of charge transfer in donor-acceptor systems at the quantum mechanical level. The primary goal of this project is to investigate whether vibrational and/or vibronic coherences assist symmetry-breaking charge generation following photoexcitation of organic semiconductors. The secondary goals of the project are to investigate 1) whether higher-order polygonal structures in solids, i.e. tilings and tessellations, lead to coherences that assist charge separation in organic semiconductors, and 2) whether excitonic coherences resulting from energy transfer processes in organic semiconductors influence subsequent charge transfer within them. While charge transfer is inherently quantum mechanical, many of the salient features of electron transfer kinetics have been treated classically. However, these treatments largely ignore the role of quantum coherences between states as they are typically very short lived (~10-100 fs for electronic states, up to several ps for vibrational/vibronic states) and as such are assumed to have decayed prior to electron transfer. However, with increasingly advanced spectroscopic techniques and improvements in time resolution, it is now possible to probe these processes on the requisite timescales and observe these coherences and their effects directly, which will aid in the design of high-performance organic semiconductors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.