Jeff Neaton Lawrence Berkeley Laboratory Organic semiconductors are a highly tunable, diverse class of cheap-to-process materials that are promising active materials for next-generation solar energy conversion. The development of more efficient and durable organic materials for light-harvesting applications requires new intuition that links molecular-scale morphology and chemical composition to underlying excited-state properties – e.g. exciton binding energies – governing energy conversion. Here, I will discuss the use of a first-principles many-body perturbation theory approach for computing and understanding spectroscopic properties of organic semiconductors pentacene, perfluoropentecene, PTCDA, and related co-crystals in the gas phase and solid state. Our quantitative calculations compare well with the magnitude of transport gaps extracted from photoemission and inverse photoemission data, and with measured optical absorption spectra. Using a new analysis, we explain the nature of low-lying solid-state excitons, which have significant binding energies and charge-transfer character in these systems. We further demonstrate how the excited-state properties of these organic semiconductors are tunable via structural control. Implications for active materials of interest for organic photovoltaics are discussed.