UC Santa Cruz

With greenhouse gasses and temperatures rising, the search for sustainable materials and energy production has never been more important. Solar irradiation provides far more energy than the world consumes, making solar panels an ideal renewable energy source. Silicon is readily available and has become the standard in solar cells, however, maximum efficiencies are limited, and current manufacturing technologies are not sustainable. By exploring new, sustainable materials and applications to improve efficiencies we can improve power outputs and create cleaner technologies. In this thesis, we work toward improving performance in new materials and applications by investigating the impact of the local structure in nanocrystals and in multi-dye luminescent solar concentrators (LSCs).First, we explore the role of n-type dopants in germanium quantum dots (QDs). Synthesis of Bi- and Sb-doped Ge QDs, 0 - 1.5 mol %, was confirmed by powder X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Local structure and disorder was found by ex- tended X-ray absorption fine structure (EXAFS). Optical properties and disorder were characterized by photothermal deflection spectroscopy (PDS). Electrical behavior was determined by fabricating thin-film devices and testing current under an applied voltage. Bi-doped Ge QDs resulted in Bi sitting at the surface of the particle. Increasing the amount of dopant resulted in increased disorder, however, a post-synthesis ligand exchange restored order in 1.5 mol % QDs. Devices showed an increase in conductivity with increasing Bi content, and all showed increased conductivity under light. Doping with Sb created a different result; Sb atoms were incorporated into the core of the host lattice, however, induced a neighboring vacancy, and were still present at grain boundaries. Increasing dopant concentration led to a decrease in the core to grain boundary content ratio, implying a limit to the solubility of Sb in the Ge QDs. The Sb-vacancy defect resulted in high levels of disorder in the QDs and ultimately led to p-type behavior, contrary to what was expected. This showed promise for photovoltaic applications, as both hole and electron transport is needed in a successful device; however, doped devices were outperformed by pristine devices. Further work is needed to fully understand charge transport in these materials and to optimize them for potential application. Secondly, we look into the effects of aggregation in multi-dye LSCs. By deconvoluting absorption and fluorescence spectroscopic, we determine quantum yields, energy transfer, and molecular separation in blended fluorophore systems cast in polymethyl-methacrylate. Three UV dyes are chosen and are each blended with a near unity dye currently used in commercial LSCs, LR305. At low concentration, where large particle separation is expected to result in only radiative-energy transfer, we find non-radiative effects; we see this again at high concentration, with the effect of Förster Resonance Energy Transfer (FRET) stronger than predicted. This implies that aggregation of dye molecules is occurring, increasing the role of FRET. By estimating the enhancement to existing panels based on fluorescence spectroscopies, we determine that this has beneficial effects, resulting in unexpected increases in efficiency from low yield samples. Though more studies on why the aggregation occurs must be done to ensure viability, this shows promise for increasing efficiencies in commercial greenhouse LSCs.