Molecular semiconductors offer intriguing electronic properties. In particular, singlet-exciton fission, the nonradiative decay of one singlet exciton into two triplet excitons effectively doubles the amount of carriers available for, e.g., photovoltaic current generation, thereby effectively surpassing the Shockley-Queisser-limit. An efficient use of singletexciton fission in actual devices, however, requires a detailed understanding of the decay dynamics in donor-acceptor heterostructures. We present a quantitative study on model single-crystalline perfluropentacene at cryogenic temperature and related heterostructures to reveal the intricate interplay between singlet-exciton fission and the nanoscopic molecular arrangement, the role of charge-transfer into and out of molecular systems and discuss the potential for functionalizing inorganic semiconductors. Finally, the potential implications in heterosystems and for functionalization of inorganic semiconductor devices are discussed.