Engineering the surfaces of group IV materials for energy applications

Abstract

A majority of our energy needs today is met by fossil fuels. The combustion of fossil fuels leads to the release of greenhouse gases which adversely affect our environment. This has prompted significant efforts to harness the various sources of renewable energy available to us. Solar energy is the only source of renewable energy capable of meeting our ever-increasing energy demands. Of the various solar technologies, photovoltaic (PV) technology promises to be an attractive option for the generation of electricity due to no emissions of greenhouse gases and various additional socio-economic benefits. A major obstacle in the implementation of PV technology on a large scale is the cost. This has led to an increased interest in research directed towards reducing the cost of electricity generated through PV. Si based PV holds majority of the market share. Therefore, a major challenge in the field of PV is the reduction of cost of electricity generated via Si based PV technologies. This goal can be achieved by the development of high-efficiency solar cells based on Si. Surface passivation is one of the reasons often cited for low efficiencies observed in solar cells. Hence, as a part of the work completed in this dissertation, we have studied the surface passivation in c-Si solar cells. Specifically, we have used infrared spectroscopy techniques and minority carrier lifetime measurements to study the mechanism of surface passivation of c-Si by Al2O3. The passivation of the Si surface via Al2O3, deposited by atomic layer deposition (ALD), is achieved by a reduction in the defect density at the interface (D[subscript it]) (chemical passivation) and an increase in the fixed negative charge (Q[subscript f]) associated with the Al2O3 films (field effect passivation). A post-deposition annealing step is required to achieve this high level of passivation. We have investigated the effect of the annealing step in order to understand the mechanism of chemical passivation. Specifically, we have studied the role of H and O in the chemical passivation of c-Si by Al2O3. Our results indicate that it is the restructuring at the interface, and not the migration of H from Al2O3 to the c-Si interface, that contributes to the reduction in the interface defect density. In addition to Al2O3, TiO2 has also been proposed as a surface passivant for c-Si. TiO2, due to its optical properties, has the added advantage of acting as an anti-reflection coating in addition to the surface passivation. Atomic layer deposition (ALD) technique is often used to deposit these TiO2 thin films. However, the low growth per cycle observed for typical ALD processes makes it unfeasible to be used on a large scale. Therefore, we have developed a novel ALD chemistry for the deposition of TiO2 with a high growth per cycle. We use a metal alkoxide metal-chloride precursor combination to deposit the TiO2 thin films wherein the metal-alkoxide acts as the oxygen source. We have used in situ IR spectroscopy to study the surface reaction mechanism during the deposition process. We found that the reaction follows a alkyl-tranfer mechanism over the range of 150-250 °C. The use of metal-alkoxide as the oxygen source would also potentially mitigate the problem of interfacial oxide formation and hence, enable deposition of TiO2 on oxygen sensitive substrates. Renewable energy sources such as solar energy, have an intermittent nature which increases the importance of an efficient energy storage system. Due to their high energy and power density, low self-discharge and maintenance, lithium-ion batteries (LIBs) are attractive candidates to meet this challenge. However, the LIB technology needs significant improvements before it can be implemented as an effective storage system. One of the factors which would improve the capacity of the LIB is the anode material. Group IV materials such as Si, Ge and Sn, all have higher capacities than the current conventional anode material, graphite. The use of these materials as anodes in LIBs is however unfeasible due to the significant volume expansion these materials undergo upon lithiation. The volume expansion leads to electrode pulverization and a loss in battery capacity after just the first few charging and discharging cycles. A way to circumvent this issue is the use of nanomaterials. Specifically, carbon-coated Group IV nanomaterials have shown enhanced capacities as compared to graphite. To this end, we have developed a single-step technique to synthesize carbon-coated Si, Ge, and Sn nanoparticles. In this technique, we use two non-thermal plasmas in series to first synthesize the nanoparticles, and then coat them with carbon. We have studied the effects of varying the plasma parameters on the nature of the coating we employ on these nanoparticles. We have shown that the use of two plasmas allow us to independently control the synthesis and coating of these nanoparticles. Efficient use of energy also contributes to the reduction of fossil fuel emissions. Artificial lighting consumes a significant portion of the electricity we generate. Hence the use of LEDs, which have been shown to be more efficient that traditional lighting sources, can greatly impact the fossil fuel emissions. Silicon carbide, due to its high band-gap has been proposed as a material for the manufacture of blue LEDs. The use of nanomaterials has been shown to enhance the luminescence properties of silicon carbide. Therefore, as a part of the work completed in this dissertation we propose a low-temperature technique to synthesize silicon carbide nanoparticles. We use a dual-plasma setup similar to the one used to synthesize the carbon-coated Group IV nanoparticles. The Si nanoparticles are synthesized in the upstream plasma and then carburized in the downstream plasma to form crystalline silicon carbide nanoparticles.