High-Efficiency Solar Fuel Devices: Protection and Light Management Utilizing TiO2

Global climate change coupled with increasing global energy consumption drives the need for renewable and carbon-neutral alternatives to fossil fuels. Photoelectrochemical devices store solar energy in chemical bonds, and have the potential to provide cost-effective fuel for grid-scale energy storage as well as to serve as a feedstock for the production of carbon-neutral transportation fuels. A widely recognized goal is the demonstration of a monolithically-integrated solar-fuels system that is simultaneously efficient, stable, intrinsically safe, and scalably manufacturable. This thesis presents the development of three separate high-efficiency solar fuel devices protected by thin films of amorphous TiO2, and develops light management strategies to increase the performance of these devices. First, high-efficiency monolithic cells were designed to perform solar water-splitting and CO2 reduction. These designs are driven by high-quality single-crystalline III-V semiconductors that are unstable when placed in direct contact with aqueous electrolytes but can be protected against corrosion by hole-conducting amorphous films. Experimental fabrication and characterization of this tandem device was realized in the form of a fully-integrated water-splitting prototype with a solar-to-hydrogen efficiency of 10% showing stability for over 80 hours of operation. This was followed by the demonstration of water-splitting and CO2 reduction devices enabled by bipolar membranes, which increased stability and alleviated materials-compatibility constraints by creating a pH difference between the anolyte and catholyte, maintained at steady-state. Finally, universal light management strategies were developed using high-aspect-ratio TiO2 nanocones, resulting in an increase in catalyst loading with ultrahigh broadband transmission.