Investigating Unexpected but Advantageous Integrated Systems for Solar Water Splitting

Citation

Nuñez, Paul Daniel (2021) Investigating Unexpected but Advantageous Integrated Systems for Solar Water Splitting. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8q5x-9d27. https://resolver.caltech.edu/CaltechTHESIS:01162020-234409466

Abstract

Advantageous systems are frequently utilized for solar water splitting sometimes are not the most well-understood. Based on fundamental understandings of semiconductor physics, certain combinations of materials should never be advantageous for applications in solar water splitting. (Un)fortunately, these expectations are unrealized. Specifically, we address herein how photogenerated current is able to pass through a-TiO₂ from n-Si as well as the formation of a barrier height from electroless-deposited Pt on p-Si.

Chapter 2 addresses how charge is able to pass through thick layers of atomic layer deposited a-TiO₂ even though the deep valence band of the a-TiO₂ should make the a-TiO₂ act as a blocking layer. It was found that the presence of mid-gap defect states, observable by x-ray photospectroscopy (XPS) valence band spectrum and electron paramagnetic resonance (EPR), in the a-TiO₂ act as a channel for current to pass. The implications of the current traversing through the mid-gap defect states are that global and local changes to the mid-gap defect concentration will strongly affect the amount of current able to pass at all potentials. Thus, the choice of top contacts is limited to metals that have a work-function less than ~5.2 eV else the resistivity would increase.

Chapter 3 focuses on explaining the origin of the barrier height for electroless Pt on p-Si during hydrogen evolution. The work function of Pt should create an ohmic contact which is observed when Pt is e-beam deposited to p-Si. The origin of the barrier height was found to be dependent on the route for charge transfer. Facile redox couples showed that the solution potential of the redox couple controlled the barrier height as charge transferred occurred favorably at the h-terminated surface. While performing hydrogen evolution reaction, the barrier height is formed through the formation of a hydrogen dipole layer that occurs at the interface of the SiOₓ|Pt interface.

Thesis Files