Through a remarkable series of advances in material design, the efficiency of organic solar cells has risen from 1% to over 20% within two decades, surpassing most predictions. Key to reaching this milestone has been the development of nonfullerence acceptors (NFAs) which, when paired with suitable polymer donors, yield high internal quantum efficiencies (IQEs) despite having a significantly smaller driving force for exciton splitting than is present in typical polymer:fullerene systems. There is some evidence that the best performing NFAs can generate charge separated states within single material domains, and it has been suggested that this process is what underlies their ability to efficiently generate charge pairs across a heterojunction. In this work, we address this question using an experimentally validated computational model of charge pair generation in molecular crystals. Our model can account for the significant effects of excitonic and electronic delocalisation on both excited state lifetimes and on charge and energy transfer rates. Using this model, we track the time evolution of excited states in single-component systems and demonstrate how both intramolecular parameters, such as reorganisation energy, and intermolecular parameters, such as electronic coupling, influence the yield of charge separated states. By analysing our results within a thermodynamic framework, we can identify the main loss pathways and thus identify the states which limit the overall yield of free charges. Additionally, we investigate how the same set of parameters affect charge generation efficiency in heterojunctions. Our results indicate that, although there is a correlation between the ease of charge generation in single-component and heterojunction systems, it is unlikely that charge pairs generated in the acceptor domains contribute significantly to the photocurrent in a heterojunction architecture. Finally, we apply our model to real crystal structures to understand how seemingly small morphological changes can lead to significant differences in the yield of separated charges. Taken together, our simulation results rationalise the experimentally observed differences between the ease of charge generation in different families of NFAs and suggest design criteria for future materials to further boost organic solar cell performance.