Si-based photovoltaic and photoelectrochemical cells for high-efficiency solar energy harvesting

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2022-05

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Rapidly growing consumption of energy derived from carbon-based fossil fuel has caused energy crises, global warming, and air pollution. Therefore, the development of renewable energy is required. Solar energy has been considered as a promising renewable energy source since it is clean, inexhaustible, and abundant in nature. Therefore, many technologies have been studied for harvesting solar energy including photovoltaics, solar heating, concentrating solar power, and solar-driven water splitting. In this dissertation, we demonstrate a variety of approaches for improving solid-state devices for harvesting solar energy via the photovoltaic effect or photoelectrochemical water splitting. First, we demonstrate improved passivation quality for Si photovoltaic devices using i a-Si:H films with a gradient-layered structure consisting of interfacial, transition, and capping layers deposited on c-Si surfaces. The H₂ dilution ratio (R) during deposition was optimized individually for the interfacial and capping layers, which were separated by a transition layer for which R changed gradually between its values for the interfacial and capping layers. This approach yielded a significant reduction in surface carrier recombination, resulting in improvement of the minority carrier lifetime from 1480 μs for mono-layered i a-Si:H passivation to 2550 μs for the gradient-layered passivation approach. We then demonstrate approaches for design and fabrication of Si-based photoelectrochemical devices that demonstrate high performance and stability for solar-driven water splitting. Metal-insulator-semiconductor (MIS) structures are widely used in Si-based solar water splitting photoelectrodes to protect the Si layer from corrosion. Typically, there is a tradeoff between efficiency and stability when optimizing insulator thickness. In this study, we demonstrate improved Si-based MIS photoanodes with thick insulating layers fabricated using thin-film reactions to create localized conduction paths through the insulator and electrodeposition to form metal catalyst islands. These fabrication approaches are low-cost and highly scalable, and yield MIS photoanodes with low onset potential, high saturation current density, and excellent stability. By combining this approach with a p⁺n-Si buried junction, further improved oxygen evolution reaction (OER) performance is achieved with an onset potential of 0.7 V versus reversible hydrogen electrode (RHE) and saturation current density of 32 mA/cm² under simulated AM1.5G illumination. A two-step Ni/NiFe electrodeposition process is then demonstrated to create more efficient OER catalysts. The Ni/NiFe catalyst layers increase Schottky barriers between Si and metal catalyst and lower the photoanode onset potential, improving applied bias photon conversion efficiency (ABPE) to a Ni catalyst, to 3.5%.

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