Author: Luman Qu
Publisher:
Published: 2018
Total Pages:
ISBN-13: 9780438290501
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Functionalized nanostructures play a central role of ever-increasing importance in renewable energy applications and researches. There are many forms of nanostructures, most notably of which are nanoparticles (NP) and nanowires (NW). The former have great promise for optoelectronic and photovoltaic applications, while the latter can be used in sensor and energy-harvesting devices. For NPs, one major application is next-generation solar cells. Progress has been rapid in increasing the efficiency of energy conversion. However, extraction of the photo-generated charge carriers remains challenging. One key task is to greatly improve the charge carrier mobilities in NP solids, so that photo-generated electron/hole pair can be collected before recombining. The first crucial step to achieve this goal is to understand the fundamental underlying physics governing the transport. To study the transport properties in NP, we have developed the Hierarchical Nanoparticle Transport Simulator, or HiNTS. Details of theories and implementations of HiNTS are presented in this dissertation. We used HiNTS in various transport studies in NP solids, and reported three of them in this dissertation. First, we used HiNTS to simulate the metal-insulator transition (MIT) in NP films. Electrons transfer between neighboring NPs via activated hopping when the NP energies differ by more than an overlap energy, but transfer by a non-activated quantum delocalization, if the NP energies are closer than the overlap energy. As the overlap energy increases, emerging percolating clusters support a metallic transport across the entire film. We simulated the evolution of the temperature-dependent electron mobility. We analyzed our data in terms of two candidate models of the MIT: (a) as a Quantum Critical Transition, signaled by an effective gap going to zero; and (b) as a Quantum Percolation Transition, where a sample-spanning metallic percolation path is formed as the fraction of the hopping bonds in the transport paths is going to zero. We found that the Quantum Percolation Transition theory provides a better description of the MIT. We also observed an anomalously low gap region next to the MIT. We discussed the relevance of our results in the light of recent experimental measurements. Second, we analyzed charge transport in glassy binary NP films composed of large and small PbSe NPs. In films with small fractions of large NPs (LNPs), the LNPs act as traps for mobile charge carriers and the carrier mobility decreases with increasing LNP fraction f[subscript LNP]. For f[subscript LNP] above the percolation threshold f[subscript P], the LNPs form sample-spanning percolation networks that facilitate carrier transport. The increasing density of these percolation networks leads to a gradual recovery of the mobility as f[subscript LNP] approaches 1. Measurements of field-effect transistors made from mixtures of 6.5 nm and 5.1 nm PbSe NPs show a deep mobility minimum at f[subscript LNP] ~ 0.2. We used HiNTS to help explain the experimental results and elucidate the percolation physics of binary NP films. We explored the impact of ligand length, electron density, site energy disorder, charging energy, and temperature on the position (f[subscript LNP]) and depth of the mobility minimum. The simulation results can be understood in terms of a renormalized trap model, but the simulations fail to account for the weak temperature dependence of the mobility minimum observed in experiment unless mid-gap traps are assumed to play a key role in charge transport. Heat maps of electron residence times constructed from the simulations help to visualize transport within the percolation networks. The close comparison of experiment and simulations presented in this study is a promising systematic approach to unmasking the factors that control charge transport in NP films. Third, we used HiNTS to study the commensuration effects in Nanoparticle FETs (NP-FETs). For the case when the two NP layers closest to the gate are active for transport, our results include the following. (1) We observed the emergence of commensuration effects when the electron filling factors in both NP layers reached integer values. These commensuration effects were profound as they reduced the mobility all the way to zero. (2) We identified and characterized different classes of commensuration effects for different parameter regions. (3) We studied these commensuration effects in a four-dimensional parameter space, as a function of the on-site charging energy E[subscript C], the gate voltage V[subscript G], the disorder D, and the temperature k[subscript B]T. We explored the regions, or dynamical phases, in the parameter space characterized by the distinct commensuration effects. All three NP-related projects greatly advance our understanding of transport mechanism in NP solids, which is crucial in unlocking the full potential of NP in optoelectronic and photovoltaic applications. The rest of the dissertation switches focus to another two emerging nanostructures for renewable energy applications: First, we explored the potential use of nanowires for energy harvesting purposes. We have demonstrated the feasibility of using ZnO nanowires to harvest both mechanical and low-quality thermal energy in simple, scalable devices. These devices were fabricated on kapton films and used ZnO nanowires with the same growth direction to assure alignment of the piezoelectric potentials of all of the wires. Mechanical harvesting from these devices was demonstrated using a periodic application of force, modeling the motion of the human body. Tapping the device from the top of the device with a wood stick, for example yielded an Open Circuit Voltage (OCV) of 0.2-4 V, which is in an ideal range for device applications. To demonstrate thermal harvesting from low quality heat sources, a commercially available Nitinol (Ni-Ti alloy) foil was attached to the nanowire piezoelectric device to create a compound thermoelectric. When bent at room temperature and then heated to 50 ̊C, the Nitinol foil was restored to its original flat shape, which yielded an output voltage of nearly 1V from the ZnO nanowire device. In both cases, optimization of the nanowire device from materials selection and design geometry bode well for significant improvement over these initial results. Last but not least, we proposed a novel nanostructured photovoltaic desalination system, which comprises: a solar cell, configured to receive solar radiation, including an n-doped semiconductor layer, a p-doped semiconductor layer, the two semiconductor layers forming a p-n junction, and a nano-channel array, formed in the p-n junction; an input reservoir, coupled to the solar cell, the input reservoir configured to contain a salty fluid, and to release the salty fluid to the solar cell; an output fluid management system, coupled to the solar cell, the output fluid management system configured to receive an output fluid from the solar cell; wherein the channel array is configured to receive the salty fluid from the input reservoir, and to output the output fluid to the output fluid management system. This new design greatly reduces power consumption required for desalination.
