Research in the past decade on organic photovoltaics (OPV) has led to the rapid achievement of several remarkable milestones: ~100% internal quantum efficiencies have been obtained (1:1 ratio of photons absorbed to carriers collected in the external circuit), a 2-3 fold increase in the efficiency of champion devices, and an explosion in the number of high-performing materials (there are now almost 20 donor polymers that obtain >5% efficiency when blended with fullerene, and some promising signs of alternative acceptors). However, the materials design that has led to these achievements is almost exclusively based on simple energy level considerations or similarity to already established chemical units. A compelling case can be made that the field has already extracted most of the gains that are possible from energy level tuning alone. The next challenge for the field is to move beyond simple energetic, monomolecular descriptions (N>1) in the understanding and design of these materials. The talk will focus on mesoscopic OPV phenomena that are intrinsically multimolecular in nature: exciton diffusion, charge generation, charge collection, and the impact of different forms of disorder on the efficiency of each process. Emphasis will be given to promising methodologies for describing these phenomena on the 1-100 nm length scale and where improvement is required.
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