Achieving high efficiency solar cells and concurrently driving down the cell cost has been among the key objectives for photovoltaic researchers to attain a lower levelized cost of energy (LCOE). While the performance of silicon (Si) based solar cells have almost saturated at an efficiency of ~25%, III-V compound semiconductor based solar cells have steadily shown performance improvement at approximately 1% (absolute) increase per year, with a recent record efficiency of 46%. However, the expensive cost has made it challenging for the high efficiency III-V solar cells to compete with the mainstream Si technology. Novel approaches to lower down the cost per watt for III-V solar cells will position them to be among the key contenders in the renewable energy sector. Integration of such high-efficiency III-V multijunction solar cells on significantly cheaper and large area Si substrate has the potential to address the future LCOE roadmaps by unifying the high-efficiency merits of III-V materials with low-cost and abundance of Si. However, the 4% lattice mismatch, thermal mismatch polar-on-nonpolar epitaxy makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this dissertation is to systematically investigate heterogeneously integrated III-V multijunction solar cells on Si substrate. Utilizing a combination of comprehensive solar cell modeling and experimental techniques, we seek to better understand the material properties and correlate them to improve the device performance, with simulation providing a very valuable feedback loop. Key technical design considerations and optimal performance projections are discussed for integrating metamorphic III-V multijunction solar cells on Si substrates for 1-sun and concentrated photovoltaics. Key factors limiting the “GaAs-on-Si” cell performance are identified, and novel approaches focused on minimizing threading dislocation density are discussed. Finally, we discuss a novel epitaxial growth path utilizing high-quality and thin epitaxial Ge layers directly grown on Si substrate to create virtual “Ge-on-Si” substrate for III-V-on-Si multijunction photovoltaics. With the plummeting price of Si solar cells accompanied with the tremendous headroom available for improving the III-V solar cell efficiencies, the future prospects for successful integration of III-V solar cell technology with Si substrate looks very promising to unlock an era of next generation of high-efficiency and low-cost photovoltaics. / Ph. D.