"First-principles design of excited-state properties in organic materials" - Organic semiconductors are a highly tunable and diverse class of optically active materials that are promising for next-generation photovoltaic devices. To effectively harness these materials for light-harvesting applications, we need to fundamentally understand their excited-state electronic structure, i.e. how light absorption, charge transfer, and charge transport relate to the properties of their molecular components and are influenced by solid-state morphology. Here I will present recent theoretical studies to understand the spectroscopic properties of organic semiconductors and tune these properties for improved photovoltaic device performance. First-principles many-body perturbation theory calculations of prototypical organic semiconductors provide a quantitative perspective on recent controversial interpretations of photoemission/inverse photoemission data, and on the nature and binding energy of solid-state optical excitations (excitons). Moreover, for experimentally-synthesized bulk crystals, I will discuss how the energy of low-lying excitons can be understood through an electrostatic model, and their nature tuned through structural control. Lastly, I will describe how these studies can provide important new insight into the performance of complex donor-acceptor interfaces utilized in working solar cells.