UC Merced

Nanostructured materials are at the forefront of device and materials physics research due to their novel and favorable characteristics that offer the potential of surpassing previously achieved device efficiencies. This dissertation explores such materials, with specific focus on their interfacial properties, which could be implemented in semiconductor devices, including photovoltaics and highly responsive sensors. The first project in the dissertation focuses on an investigation of the potential of hybrid organic-inorganic Perovskite thin films as luminescent solar concentrators. Luminescent solar concentrators were first explored as an alternative to solar cells in the 1970s, and typically consist of high refractive index substrates doped with fluorescent materials, used to absorb incident sunlight and down-convert solar radiation which is subsequently collected by a solar cell attached to the edge of the luminescent solar concentrator. Commonly implemented materials have encompassed fluorescent dyes and quantum dots, but have not yet exceeded the device efficiencies of solar cells, and these fluorophores suffer from limited stability, narrow absorption spectra and low quantum yield. Therefore, hybrid Perovskite thin films possess a range of characteristics that render the material suitable for luminescent solar concentrators: it demonstrates a broad absorption spectrum, high refractive index of 2.5 which is significantly higher than typically used glass or polymer, and quantum yield reaching 80%, the combination of which is promising for efficient confinement of re-emitted light. In this work, hybrid Perovskite thin films are successfully implemented as luminescent solar concentrators, with optical efficiency reaching 29%, whereas the devices remain operational after 7 weeks of storage in ambient conditions underling the stability of optical properties of hybrid Perovskite thin films. Furthermore, the study extends to an investigation of different Perovskite compositions to identify the optimum precursor and thickness for high device performance, reaching optical efficiencies of 34.7% for 150-300 nm thick samples. 3D Monte Carlo simulations have demonstrated the scalability of these devices up to 100 cm in length, further establishing hybrid Perovskite thin films as successful candidates for optoelectronic devices.

The second project in the dissertation investigates the interaction of hybrid Perovskite thin films with ZnO substrates of varying degrees of surface roughness, as these are often used in Perovskite solar cells. The goal of this study is to complete a systematic investigation of the effects of the ZnO substrate on the excitonic properties of the hybrid Perovskite thin films, as most of the current research focuses on device studies and performance, whereas understanding of the fundamental processes occurring at the interface and how these can be tuned by tailoring the interface morphology is somewhat limited. The work performed here has produced some surprising results. While single crystalline ZnO substrates appear to perform best as electron extraction layers and are thus suited for implementation in a solar cell, substrates of higher surface roughness, such nanostructures and microstructures, have the opposite effect and result in confinement of electron-hole pairs within Perovskite grains. This conclusion is reached through electron microscopy studies correlated with charge transfer properties, probed via temperature, power, and time-resolved photoluminescence (PL). ZnO single crystal/hybrid Perovskite thin film interfaces show quenched PL intensity and reduced recombination lifetime as compared to a control sample of hybrid Perovskite thin film on glass, both indicative of efficient electron transfer between the two materials. On the other hand, microstructured ZnO/hybrid Perovskite thin film interfaces show a mild enhancement of the PL signal at room temperature, whereas nanostructured ZnO/hybrid Perovskite thin film interfaces demonstrate a 30 thousand-fold enhancement of the PL while significantly reducing the recombination lifetime. These results vary with temperature, indicate that with increasing substrate roughness, the Perovskite grain size is reduced and some grain separation is observed, further supporting the result that excitons are confined within individual grains which is deleterious to both electron transport and electron transfer. As a result, this work provides evidence of tuning of Perovskite excitonic properties using the substrate and can be leveraged for various types of applications depending on the end-goal.

The third project in this dissertation is a study of the enhancement of magneto-optical properties of hematite nanowires using excitation of localized surface plasmon resonance of Au nanoparticles (NPs). Magneto-plasmonics is a recent area of research which combines the properties of magnetism and plasmons to produce devices of superior performance. The basic premise is to leverage the increased local electric field of plasmonic NPs to enhance the magneto-optical response of ferromagnetic materials, or to use the magnetization of the ferromagnetic material to produce a narrower plasmon resonance that would be more sensitive to changes in the surrounding medium. While significant portion of magneto-plasmonic research has been focused on ferromagnetic metals, metal oxides offer some benefits over metals, such as superior stability and lower optical losses. Among iron oxides, hematite is the most abundant material and is low cost, while a breadth of techniques is available to fabricate various types of nanostructures. This enables the preparation of nanowires which offer the possibility of enhanced magneto-optical properties compared to a hematite film due to shape and magnetic anisotropy. As a result, this work shows that Kerr rotation can be observed using hematite nanowires, which was previously thought unlikely due to their weak ferromagnetism at room temperature, and decorating the nanowires with AuNPs results in enhanced Kerr rotation when the optical excitation is spectrally matched to the localized surface plasmon resonance (LSPR) of the AuNPs. Some dependence on the loading of AuNPs is observed, as lower loading of AuNPs is beneficial for enhancing the real part of Kerr rotation – changing the angle of polarization the reflected light – whereas higher loading has a greater effect on the imaginary part of Kerr rotation – the ellipticity of reflected light. The outcomes of this work are important in demonstrating that hematite can be implemented in sensing applications when combined with plasmonic nanoparticles to improve performance.