Ultrafast Photophysical Dynamics of Structural and Electronic Behavior in Hybrid and Nanostructured Semiconductors

Panuganti, Shobhana

doi:https://doi.org/10.21985/n2-4z23-v377

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Ultrafast Photophysical Dynamics of Structural and Electronic Behavior in Hybrid and Nanostructured Semiconductors

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Renewable energy technology, more so than ever before, is critical to the survival of humanity. For decades, concentrated efforts into designing and developing such novel devices resulted in the innovation of solar-driven photovoltaics that were competitive with nonrenewable alternatives. This thesis explores the dynamic behavior of alternative material candidates that exhibit exceptional light-absorbing properties due to their unique hybrid and nanostructured compositions. Many previous studies of the materials discussed herein have focused on gradual improvements to specific aspects of device architectures, though much remains elusive regarding their fundamental electronic responses that would accelerate material design by guiding synthesis for certain properties.The influence of the distance on rates of energy transfer between donor and acceptor states within an extended nanostructured solid, such as two-dimensional perovskite quantum wells, was investigated via transient absorption and time-resolved emission studies. Importantly, specific distances were targeted with high precision throughout the bulk material through synthetic, compositional modifications. Empirically measured, correlated kinetics of electronic behavior and subsequently calculated rates of energy transfer between states aligned well with computational models of such systems, pointing to the reliability of this test system for explorations of other fundamental photophysical phenomena. In colloidal studies of the lifetimes of spin-coherence in biexcitons generated by cross-polarized incident laser pulses, several variables were carefully manipulated with respect to the structure and morphology of the nanoparticles. Results indicated that lifetimes of coherence increased slightly with increasing nanoplatelet thickness but were significantly prolonged by the presence of core-shell morphology regardless of heterojunction identities and band alignment, pointing to surface passivation as a principal factor in dictating spin-coherence lifetimes. Transient X-ray diffraction of bulk hybrid perovskites, including thin films of methylammonium lead iodide, revealed non-equilibrium phase transitions that had not been previously observed or accessed under experimental conditions. These findings demonstrated the dissonance between the steady-state and in situ properties of materials, especially under operating conditions for devices, further highlighting the need for ultrafast structural characterization. In particular, the extent of thermal and electronic effects following photoexcitation was clarified, with electronic reorganization playing a dominant role in the observed structural dynamics.

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