Atomic layer deposited protection layers for metal-insulator-silicon water splitting cells [electronic resource]

In pursuit of a fully sustainable energy economy, there has been increased effort to develop artificial photosynthesis technologies capable of creating clean fuels and chemicals from greenhouse gases with only the energy of the sun. Photoelectrochemical cells have been developed since the 1970's as all-in-one devices capable of exactly that task, but progress has been impeded by fundamental challenges. Principal among these is the trade-off that the most chemically stable materials due to a wide bandgap are necessarily the most inefficient at converting solar energy. In 2011, our group proposed a novel solution to this problem with a nano-layered composite structure capable of achieving both ends: atomic layer deposited (ALD) metal oxide protected cells where a top layer is chemically passivating and a bottom layer is efficient at solar energy conversion. In this work, I have carried out a rigorous study of ALD-TiO2 protection for metal-insulator-silicon devices, interrogating each layer of the structure and building state-of-the-art analytical models for performance of each component, as well as for the cell as a whole. The catalyst layer was studied first, demonstrating that this protection layer technology is a general solution, applicable with a wide range of catalysts. Second, the protection layer was probed and I discovered bulk-limited leaky conduction for the ALD-TiO2. I proposed a model of trap-mediated hopping conduction in the TiO2 in series with tunneling through the ultrathin SiOx, a model which has become the center point of much ongoing research and remains the accepted theory in the field for the anomalous, hole-conductive TiO2. Third, I probed the SiOx/Si interface, showed that conduction across the SiOx was well described by tunneling, and developed models that investigate the transition from leaky to capacitive structures. From here I developed engineering solutions including ALD-SiO2 and oxygen scavenging methods for fabricating ultrathin SiOx layers in these water splitting devices. In studying the device as a whole, I have developed a general theory for a so-called 'leaky capacitor' and applied this to understand the photovoltage loss observed in insulator-protected devices. This understanding allowed for the development of general design principles for maximizing photovoltage in MIS cells of varying architecture between so-called Type 0 photoelectrochemical cells and fully separated PV-electrolyzer systems. In applying these design principles, I have demonstrated the highest reported photovoltage to date for single junction silicon water splitting cells both in a so-called Type 1 Schottky junction and Type 2 pn junction photoanode. Taken altogether, this work represents a complete scientific investigation and understanding of protected silicon photoelectrode operation, and the corresponding engineering advances culminating in record photoanode performance.

Keywords: Materials Science and Engineering.
Advisor: McIntyre Paul Cameron primary advisor. Thesis advisor
Chidsey Christopher E. D. (Christopher Elisha Dunn)
Salleo Alberto