Exciton fission is a process that occurs in certain org. materials whereby 1 singlet exciton splits into 2 independent triplets. In photovoltaic devices these 2 triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies >40%. Fission dynamics were measured using ultrafast photoinduced absorption and present a 1st-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is nonadiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, fission yields near unity were demonstrated even when monomers are sepd. by >5 \AA. For efficient solar cells, fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring mol. properties like soly. and energy level alignment while maintaining the high fission yield required for photovoltaic applications. [on SciFinder(R)]