Exciton Dissociation Dynamics at Organic-Organic and Organic-Inorganic Semiconductor Heterojunctions

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Technical: This project aims to investigate fundamental mechanisms governing exciton dissociation at organic-organic or organic-inorganic hybrid semiconductor interfaces. These include energetic driving force, electronic coupling strength, and nuclear reorganization energy in interfacial electron transfer, the potential roles of geminate pair formation, and charge-recombination as competing dynamic channels. Femtosecond time-resolved two-photon photoemission spectroscopy (TR-2PPE) will be used to probe exciton dissociation dynamics at well-ordered organic-organic or organic-inorganic interfaces. The use of single crystal substrates and highly ordered crystalline thin films minimizes heterogeneity problems in nanostructured materials; examples of model systems include oligothiophene and pentacene on single crystal C60 and ZnO surfaces. A TR-2PPE experiment will allow one to directly follow the electron in time, energy, and momentum spaces as it is excited in the form of an exciton in the donor material, as it is en route to the acceptor, as it forms a geminate pair across the donor/acceptor interface, and as it relaxes in the acceptor. The long-term objective is to establish fundamental principles and design rules for exciton dissociation and charge separation at organic heterojunctions for solar cells.Non-technical: The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to fill an important knowledge gap in the development of organic solar cells. Developing solar energy technologies that are economically competitive is one of the most important scientific and technological challenges in this century. From an educational perspective, the project will provide an inspiring opportunity for participating students to work on scientifically challenging and socially responsible research. The PI has established a track record and will continue not only in educating graduate students, but also in motivating and involving undergraduates and high school students in research. The PI will also develop a freshman seminar course which focuses on solar energy and the environment challenges.