Browsing by Subject "Quantum dot"
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Item Assembly and characterization of quantum-dot solar cells(2009-09) Leschkies, Kurtis SiegfriedEnvironmentally clean renewable energy resources such as solar energy have gained significant attention due to a continual increase in worldwide energy demand. A variety of technologies have been developed to harness solar energy. For example, photovoltaic (or solar) cells based on silicon wafers can convert solar energy directly into electricity with high efficiency, however they are expensive to manufacture, and thus unattractive for widespread use. As the need for low-cost, solar-derived energy becomes more dire, strategies are underway to identify materials and photovoltaic device architectures that are inexpensive yet efficient compared to traditional silicon solar cells. Nanotechnology enables novel approaches to solar-to-electric energy conversion that may provide both high efficiencies and simpler manufacturing methods. For example, nanometer-size semiconductor crystallites, or semiconductor quantum dots (QDs), can be used as photoactive materials in solar cells to potentially achieve a maximum theoretical power conversion efficiency which exceeds that of current mainstay solar technology at a much lower cost. However, the novel concepts of quantum dot solar cells and their energy conversion designs are still very much in their infancy, as a general understanding of their assembly and operation is limited. This thesis introduces various innovative and novel solar cell architectures based on semiconductor QDs and provides a fundamental understanding of the operating principles that govern the performance of these solar cells. Such effort may lead to the advancement of current nanotechnology-based solar power technologies and perhaps new initiatives in nextgeneration solar energy conversion devices. We assemble QD-based solar cells by depositing photoactive QDs directly onto thin ZnO films or ZnO nanowires. In one scheme, we combine CdSe QDs and singlecrystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell (QDSSC). An array of ZnO nanowires was grown vertically from a fluorine-doped-tinoxide conducting substrate and decorated with an ensemble of CdSe QDs, capped with mercaptopropionic acid. When illuminated with visible light, the CdSe QDs absorb photons and inject electrons into the ZnO nanowires. The morphology of the nanowires then provided these photoinjected electrons with a direct and efficient electrical pathway to the photoanode. When using a liquid electrolyte as the hole transport medium, our quantum-dot-sensitized nanowire solar cells exhibited short-circuit current densities up to 2.1 mA/cm2 and open-circuit voltages between 0.6–0.65 V when illuminated with 100 mW/cm2 of simulated AM1.5 light. Our QDSSCs also demonstrated internal quantum efficiencies as high as 50–60%, comparable to those reported for dye-sensitized solar cells made using similar nanowires. We found that the overall power conversion efficiency of these QDSSCs is largely limited by the surface area of the nanowires available for QD adsorption. Unfortunately, the QDs used to make these devices corrode in the presence of the liquid electrolyte and QDSSC performance degrades after several hours. Consequently, further improvements on the efficiency and stability of these QDSSCs required development of an optimal hole transport medium and a transition away from the liquid electrolyte. Towards improving the reliability of semiconductor QDs in solar cells, we developed a new type of all-solid-based solar cell based on heterojunctions between PbSe QDs and thin ZnO films. We found that the photovoltage obtained in these devices depends on QD size and increases linearly with the QD effective bandgap energy. Thus, these solar cells resemble traditional photovoltaic devices based on a semiconductor– semiconductor heterojunction but with the important difference that the bandgap energy of one of the semiconductors, and consequently the cell’s photovoltage, can be varied by changing the size of the QDs. Under simulated 100 mW/cm2 AM1.5 illumination, these QD-based solar cells exhibit short-circuit current densities as high as 15 mA/cm2 and open-circuit voltages up to 0.45 V, larger than that achieved with solar cells based on junctions between PbSe QDs and metal films. Moreover, we found that incident-photonto- current-conversion efficiency in these solar cells can be increased by replacing the ZnO films with a vertically-oriented array of single crystal ZnO nanowires, separated by distances comparable to the exciton diffusion length, and infiltrating this array with colloidal PbSe QDs. In this scheme, photogenerated excitons can encounter a donor– acceptor junction before they recombine. Thus, we were able to construct solar cells with thick QD absorber layers that were still capable of efficiently extracting charge despite short exciton or charge carrier diffusion lengths. When illuminated with the AM1.5 spectrum, these nanowire-based quantum-dot solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin film ZnO devices constructed with the same amount of QDs. Supporting experiments using field-effect transistors made from the PbSe QDs as well as the sensitivity of these transistors to nitrogen and oxygen gas show that the solar cells described above are unlikely to be operating like traditional p–n heterojunction solar cells. All data, including significant improvements in both photocurrent and power conversion efficiency with increasing nanowire length, suggest that these photovoltaic devices operate as excitonic solar cells.Item Electronic and Optical Properties of Quantum Dots: Metal-Insulator Transitions and Ultrafast Spectroscopy(2020-05) Robinson, ZacharyIn this thesis I will discuss crossing the metal-insulator transition in ZnO nanocrystal networks as well as the synthesis, electronic states and optical properties of novel infrared emitting CdSe/HgS/CdS QDs. To observe the metal-insulator transition we use a photonic sintering process to selectively increase both the inter-nanocrystal facet radius and the free electron density. This, combined with atomic layer deposition to infill the film, enables us to clear cross the metal-insulator transition. Second, I discuss the synthesis of high quality CdSe/HgS/CdS QDs. The HgS interlayer creates a 'well' for the electrons while holes are delocalized throughout the CdSe/HgS structure. This quasi-type-II system exhibits tunable emission in discrete steps as a function of the HgS interlayer thickness, which we can control with atomic level precision. We investigate the multi-excitonic properties of these dots, and also show their utility for use as near infrared single photon emitters.Item Surface Engineering of Colloidal Group IV Nanocrystals for Optoelectronics(2014-06) Wheeler, LanceColloidal nanocrystals (NCs), often synonymous with"quantum dots," represent a burgeoning class of next-generation optoelectronic materials. The promise of NCs is twofold: (i) Their optical properties are tunable and offer unique opportunities for enhanced energy conversion due to quantum confinement effects. (ii) The NCs can be processed into thin films using cost-efficient roll-to-roll printing techniques for large-scale integration into devices. Taken together, these two attributes enable a new platform for optoelectronic technology where energy-efficient devices can be produced at low costs. There is an array of research efforts to produce NC-based optoelectronic devices such as photovoltaic cells, light emitting devices, and photodetectors. Much of the recent progress in this direction hinges on the ability to manipulate the NC surface. Conventional solution synthesis yields NCs with ligands bound to metal surface atoms through a labile acid-base complex. The electrically-insulating native ligands are thus routinely exchanged to produce conductive NC arrays for devices integration. Just as surface manipulation has launched metal-based NCs to the forefront of optoelectronic technology, it is the inability to do so with the covalent surface of group IV (germanium and silicon) NCs that has greatly hindered progress. The motivation of this research is to bridge the gap between group IV and metal-based NCs in order to establish an abundant, non-toxic alternative to NCs that contain toxic lead or cadmium. The bridge is built by developing new Si NC surface chemistries, understanding how they interact with molecules, and applying chemical and physical models to uncover the mechanism of NC colloidal stability. The research begins by developing nonthermal plasma synthesis of Si NCs from a new precursor, silicon tetrachloride. This work builds on previous studies on chlorine-terminated germanium NCs synthesized from germanium tetrachloride, which were observed to form stable colloids without covalent ligand attachment. Synthesis from silicon tetrachloride offers the same flexibility for tuning size and crystallinity as typical silane synthesis but yields a new chlorinated surface chemistry. Si-Cl surface groups of the NCs are shown to be crucial for achieving the same colloidal stability observed in Ge NCs. It was determined spectroscopically the polarized Si-Cl surface bond renders the surface Si atoms Lewis acidic and capable of hypervalent interactions with Lewis basic molecules. The NCs were thus dispersible in select Lewis basic solvents. Interestingly, these interactions are also shown to be responsible for a reversible "surface doping" effect, which was also explored spectroscopically and by electrical characterization of a thin film device. The notion of a Lewis acidic surface gave rise to the development of a more robust Si NC surface chemistry. In this work, plasma synthesis that includes diborane is applied. The resulting Si NC surface is then terminated by a classic Lewis acid, boron, which is demonstrated to be an even more versatile chemistry than the Si-Cl surface. These NCs are also used as a model system for uncovering the mechanism of colloidal stability due to these surface interactions with solvent molecules. It is found that conventional theory cannot account for the stability observed, and a simple alternative model is developed. In light of this model, we are able to demonstrate stable Si NC colloids in media that spans hexane to water. The thesis concludes with a peripheral effort on Ge NCs, a material lacking in maturity even to Si NCs. In this work, the NC surface is modified to enhance the optical properties of the material as opposed to the ability to process the NCs into films from solution. Size-tunable band gap emission is demonstrated for the first time in gas-phase synthesized Ge NCs by applying Grignard chemistry to the Ge-Cl surface groups. The emission is narrower than any previous report, and emission near the bulk band gap of Ge is attained for the first time.