The ability to tune optical excitation (exciton) energies and direct their motion to a specific location will allow for unprecedented control over energy propagation and conversion in optoelectronic materials. In this talk, I will present our recent density functional theory and many-body perturbation theory studies aimed at understanding the nature and energetics of excitons within two classes of materials: organic and defective semiconductors. First, I will present our recent calculations aimed at understanding the spectroscopic properties of organic crystalline semiconductors, and tuning these properties for enhanced photovoltaic performance. By introducing a new analysis of the electron-hole correlation function, we demonstrate that excitons within organic crystals are delocalized with a degree of charge-transfer character, which can be controlled through solid-state morphology or change of conjugation length, suggesting a new strategy for the design of optoelectronic materials. Additionally, I will present investigations of the influence of point defects on the optoelectronic properties of bulk and monolayer semiconducting materials. For bulk GaN and monolayer GeSe, the predicted bandstructure and optical absorption spectrum indicate that introduction of the point defect can result in significant modification of the optoelectronic properties, particularly in 2D. A similar analysis of the electron-hole correlation function as above demonstrates how the Wannier exciton is perturbed by the presence of a defect.