During the past century, impacts of climate change on both natural and human systems have been observed worldwide. Numerous scientific investigations suggest a strong correlation between the global warming and greenhouse gas (GHG) emissions. CO2 emission accounts for 78% of the GHG emissions, and 35% of this CO2 comes from electricity generation. Reduction of GHG emission from the electricity generation would therefore be beneficial to decrease global warming.

As a renewable energy source, in comparison with conventional fossil fuels, solar energy has limitless supply, is accessible in most geographic locations, and is much cleaner. Currently, solar energy is economically viable in areas where the infrastructure is limited, or the GHG emissions are restricted by policy. To further facilitate the ubiquitous deployment of solar energy on a tera-watt utility scale, further increases in power conversion efficiency and reductions in cost are still required of solar cell technology.

Third generation solar cells are emerging solar cell technologies, which are predicted to operate beyond the Shockley-Queisser limit for single bandgap cells. Nanostructured materials are under investigation as potential candidates for next generation photovoltaic technologies. In this dissertation, one type of nanostructured material, semiconductor quantum dots (QDs), were studied for their potential applications for next-generation photovoltaics.

Epitaxial self-assembled InAs/GaAsSb QD solar cells are investigated for applications as intermediate band solar cells. These systems have a theoretical efficiency of ∼ 50% with a simple single junction design. Two sets of optical InAs/GaAsSb QD samples grown by Molecular Beam Epitaxy (MBE), one set with various InAs deposition thicknesses, and the other set with different percentages of Sb composition in the barrier, were studied to determine the optimal growth conditions in terms of QD density and uniformity. A deposition thickness of 3 monolayers (ML) and 14% Sb matrix composition were shown to yield uniform QDs with the highest QD density ~ 3.5 X 10^11 /cm^2$ and a quasi-flat valance band alignment.

Four p-i-n GaAs solar cells with different intrinsic region designs were then grown by MBE. An unusually large reduction of the Voc and a complex behavior of Jsc were both observed. A detailed experimental investigation of these devices supports the hypothesis that thermal activation of defects or ionization of impurities in the lattice induces a transition from that dominated by radiative recombination to non-radiative processes. This results in a quenching of the photoluminescence and electroluminescence intensity and a decrease the z-factor from 2 to 1, with increasing temperature. The 1.1% lattice mismatch between the GaAs substrate and GaAsSb matrix contributes to the defect formation, which serves as the main limitation of the performance of InAs/GaAsSb quantum dot solar cells presented in this work.

PbS/ZnO colloidal QD solar cells are investigated for thin film solar cell applications. A suite of transport characterization techniques including current-voltage, capacitance-voltage, and impedance spectroscopy were used to study the effect of the interfaces and intrinsic surface states in a standard ITO/ZnO/PbS/Au colloidal quantum dot solar cell, without any passivation. An unintentional Schottky barrier formed at the PbS/ZnO interface results in Fermi level pinning and induces a non-linearity in the diode characteristic of this solar cell. Losses associated with Shockley-Reed-Hall recombination processes through interfacial and midgap states associated with the surface states on the PbS QDs contributes to a low minority carrier diffusion length, serving to inhibit the performance of the CQD solar cells.