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(2022)ThesisSinglet fission is a photo-physical process that generates two triplet excitons from one singlet exciton and can potentially enhance efficiency in photovoltaic systems. The combination of photovoltaics and singlet fission is a novel field for solar energy conversion when there is much interest in renewable, non-destructive, and continuously available energy sources. Singlet fission can also overcome thermalization losses in photovoltaics, which happens in traditional cells when the incident photon energy is higher than the silicon bandgap energy, using a carrier multiplication mechanism. This thesis will design, construct, and characterize photovoltaic devices incorporating singlet fission materials to study singlet fission in practical application. The research focuses on materials characterization, spin dynamics, and electron transfers between acene and the semiconductor layer in Au/TiO2 ballistic cells, and the incorporation of singlet fission layers on silicon-based cell structures. In detail, a set of investigations was developed and summarized by implementing singlet fission materials into a state-of-the-art ballistic photovoltaic device and silicon-based solar cell. The studies demonstrate proof of concept and rationally explain the process. The first part of the thesis investigates thin films of pentacene, TIPS-pentacene, and tetracene via crystallinity, morphology, absorption, and thickness characterization. Additionally, Au and TiO2 layers in Schottky device structures were optimized to achieve the best performance for energy transfer from an applied dye layer (merbromin). The drop-casted dye layer influences the device performance by increasing short-circuit current and open-circuit voltage, demonstrating the ability of charge transfer between the device and the applied film. This device structure provides a test bed for studying charge and energy transfer from singlet fission films. The latter part of the thesis describes several investigations to understand singlet fission in a thin film using this architecture. Magneto-photoconductivity measurements were primarily used to observe the spin dynamics via photoconductivity under an external magnetic field. Control experiments with bare Au/TiO2 devices showed no observable magneto-photoconductivity signal. In contrast, devices with pentacene and tetracene singlet fission layers showed a strong magnetoconductivity effect caused by ballistic electron transfer from the singlet fission layer into the TiO2 n-type semiconductor through an ultra-thin gold layer inserted between the layers. A qualitatively different behavior is seen between the pentacene and tetracene, which reveals that the energy alignment plays a crucial part in the charge transfer between the singlet fission layer and the device. The last section investigates the application of pentacene and tetracene evaporated thin-films as sensitizer layers to a silicon-based solar cell. The optimized Si cell structure with the annealing treatment improved the cell's performance by increasing short-circuit current and open-circuit voltage. The deposition of pentacene and tetracene as sensitizer layers into the device showed some results but posed several challenges that need to be addressed. As the current-voltage and external quantum efficiency measurements were taken, it was observed that material interfaces need to be designed to fully achieve the singlet fission of the acene layer into the Si devices.
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(2024)ThesisPhonons are quasiparticles that are relatively uncontrollable for applications compared to electrons and photons. As a fundamental excitation unit for lattice vibrations, phonons play a key role in defining the thermal, electrical, and optical properties of materials. Phonon engineering has the potential to manipulate the heat flow, control thermal noise in quantum information devices, and efficient energy harvesting. Superlattices, a stack of alternating layers of dissimilar materials, have been instrumental in confining and tailoring the propagation of acoustic phonons. In this work, a variety of GaAs/AlAs based superlattices, acoustic distributed Bragg reflectors (ADBR) and phonon cavities have been designed to confine acoustic phonons in the frequency range of 100-150 GHz. Phonon cavities exhibit parabolic and quartic acoustic phonon potentials along the direction of growth. These one-dimensional phonon potentials have been realized by adiabatically changing the local phonon dispersion. A systematic control of acoustic phonon confinement provides an excellent platform to investigate and mimic different kinds of solid state phenomena, for example acoustic phonons in a phonon potential is an analogue of electrons in an electric field. The designed devices have been grown via molecular beam epitaxy and characterized for growth quality using continuous wave photoluminescence spectroscopy. For phonon dynamics, ultrafast vibrational spectroscopy (UVS) has been employed, which is a nondestructive, repeatable, and highly efficient characterization technique. The measured phononic response extracted from differential reflectivity agrees with the calculated reflectivity, showing the accuracy and robustness of the methods presented in this thesis. In addition, phononically engineered devices have been characterized using wavelength and time resolved photoluminescence spectroscopy to study the impact of phononic manipulation on the energy loss rate of photoexcited carriers. The results indicate that the energy loss rate improves as the acoustic phonon confinement increases. A path to workable applications in heat management, energy harvesting, and transduction in the THz frequency range can be made possible by the practical insights gained through phonon engineering using superlattices, which can contribute to the advancement of the field of phononics and provide a robust platform to investigate the fundamental interactions of elementary particles.