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Femtosecond Time-Resolved Spectroscopy Studies of Interfacial Charge Separation Processes

McCleese, Christopher

Abstract Details

2017, Doctor of Philosophy, Case Western Reserve University, Chemistry.
My work has been focused on relating a materials structural and electronic properties to its optical properties. Specifically, methyl ammonium lead halide perovskites, a new class of hybrid organic-inorganic semiconductor for optoelectronic applications such as photovoltaic, light emitting diodes, and lasers. For photovoltaic applications, perovskites are typically coated on either a planar or mesoporous titanium dioxide electron acceptor layer. Using femtosecond transient absorption spectroscopy, I have investigated the effects of these different substrate architectures on the electron injection lifetimes from the perovskite to titanium dioxide. Perovskite that was crystallized on mesoporous titanium dioxide more readily formed PbI2, which acted as a passivation layer and prolonged the excited state lifetimes of the perovskite. The addition of a chloride precursor has been shown to increase solar cell device efficiencies. To better understand the root of this enhancement, I have also investigated how a combination of the titanium dioxide structure and concentration of the chloride precursor affects the electron injection efficiencies from perovskite to titanium dioxide. For mesoporous architectures, no major changes in the injection efficiencies were observed because of the large perovskite/titanium dioxide interfacial area which provides near optimal electron injection efficiency in the absence of chloride. For planar architectures, the addition of chloride precursor led to increased perovskite coverage, better electronic coupling, and increased electron injection efficiencies from perovskite to titanium dioxide. Currently, the electronic structure of perovskites has been an issue of debate. The most recent theory shows that the conduction and valence band of perovskite is split due to the high spin-orbit coupling of the heavy Pb atoms and the break in inversion symmetry due to the randomly oriented methylammonium cations and atom displacement within in the crystal lattice. The lowest energy state has been predicted to be a dark spin triplet state under typical linear polarized one-photon excitation. However, I show that by using two-photon excitation, this dark state becomes an optically allowed transition. Population of this lowest lying state leads to excited state lifetimes an order of magnitude longer compared to one-photon excitation. Lastly, I investigate a series of structurally symmetric bacteriochlorin dyads to model the naturally occurring light absorber in bacteria. Typically, photosynthesis is modeled using porphyrins dyads, however, the work here shows that the electronic and optical properties of bacteriochlorins are quite different from porphyrins. Upon photoexcitation, the electronic symmetry of the bacteriochlorin dyads is broken and intramolecular charge transfer is observed. Overall, this work provides insight to the structural, electronic, and optical properties of materials for solar energy conversion.
Clemens Burda (Advisor)
Carlos Crespo-Hernandez (Committee Chair)
Emily Pentzer (Committee Member)
Thomas Gray (Committee Member)
Liming Dai (Committee Member)
171 p.

Recommended Citations

Citations

  • McCleese, C. (2017). Femtosecond Time-Resolved Spectroscopy Studies of Interfacial Charge Separation Processes [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case149182204612303

    APA Style (7th edition)

  • McCleese, Christopher. Femtosecond Time-Resolved Spectroscopy Studies of Interfacial Charge Separation Processes . 2017. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case149182204612303.

    MLA Style (8th edition)

  • McCleese, Christopher. "Femtosecond Time-Resolved Spectroscopy Studies of Interfacial Charge Separation Processes ." Doctoral dissertation, Case Western Reserve University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=case149182204612303

    Chicago Manual of Style (17th edition)