First Principles Evaluation of Nickel Oxide and Other Materials for Solar Energy Conversion

Abstract

Global climate change and pollution caused by fossil fuels necessitate the search for inexpensive, clean, renewable energy sources. Photocatalytic and photovoltaic solar energy conversion to fuels and electricity, respectively, are among the possible solutions to this challenge. Engineering devices that can efficiently achieve these tasks requires fundamental understanding of the materials involved, identification of ways to improve these materials, and discovery of new materials that could help achieve higher efficiencies and lower costs. The work presented in this dissertation contributes to these fronts via first-principles quantum mechanical calculations.

In particular, we extensively study nickel oxide (NiO), an inexpensive semiconductor, with the desired potentially carrier-lifetime-extending charge-transfer property. We identify and devise various theoretical models that accurately describe NiO’s electronic structure. We use these models to show that alloying NiO with Li2O could decrease NiO’s band gap from ~4 eV to ~2 eV, making it an appropriate light absorber for use in various solar energy conversion devices.

We study hole transport in NiO and NiO alloys. We show that hole conductivity in NiO can be enhanced by forming homogeneous LixNi1-xO alloys with high enough Li concentration, making LixNi1-xO alloys suitable for use as p-type hole conductors. We further find that hole transport in NiO is confined to two dimensions. We predict that forming MgxNi1-xO and ZnxNi1-xO (which we find to be transparent to visible light) disrupts this confinement and leads to three-dimensional hole transport, thereby increasing conductivity. This makes MgxNi1-xO and ZnxNi1-xO alloys suitable for use as transparent conducting oxides.

We introduce CoO and Co0.25Ni0.75O alloy as new intermediate band semiconductors (IBSCs), capable of absorbing light across multiple band gaps and enhancing light absorption in IBSC-based solar cells. Finally, we investigate the spatial concentration of hole and electron states in methylammonium (MA) lead iodide (MAPbI3), a promising material for photovoltaic devices. We show that although some orientations of the MA ion in the crystal structure may lead to spatial separation between hole and electron states, this spatial separation does produce lower overlap between these states and therefore we conclude it is not responsible for the long carrier lifetimes in MAPbI3.