Strongly Correlated Insulators for High-Efficient Photovoltaics

Strongly correlated insulators (SCIs) are proposed as a promising material for photovoltaics because the strongly correlated \textit{d} electrons can facilitate the creation of multiple electron-hole pairs per incoming photon through a process called \textit{impact ionization}, which would lead to increased device efficiency. The material of interest in this document LaVO$_3$ is among the SCIs that are shown to have a fast impact ionization rate with a narrow bandgap. In this dissertation, we report the growth of off-stoichiometric LaVO$_3$ thin films using pulsed laser deposition, and the characterizations that are to study how deviating La:V stoichiometries affect the electronic properties of LVO thin films. We find that the off-stoichiometry clearly alters the physical properties of the films. Structural characterization shows that both La-rich and V-rich films have different levels of structural distortion, with La-rich (V-rich) films showing a larger (smaller) out-of-plane lattice parameter compared to what one would expect from epitaxial strain effects alone. Both types of films show deviation from the behavior of bulk LVO in optical measurement, i.e., they do not show signatures of the expected long-range orbital order, which can be a result of the structural distortions or the presence of structural domains. In transport measurements, La-rich films display clear signatures of electronic phase separation accompanying a temperature-induced metal-insulator transition, while V-rich films behave as Mott insulators. The out-of-plane lattice parameter plays a crucial role in determining the transport properties, as the crossover from Mott-insulating to disorder-induced phase-separated behavior occurs around a lattice parameter value of 3.96~\AA, quite different from what has been previously reported. This dissertation also includes the study on strongly correlated metal SrVO$_3$, which is planned to work as electrodes in the prototype photovoltaics. We successfully grow SrVO$_3$ thin films at different thicknesses. We find it maintains good conductivity at a thickness of 15 nm. These studies have laid the foundation for future research on designing heterostructures of strongly correlated oxides for photovoltaics.