The staggered alignment of quasi-particle energy levels is widely regarded to be the key criterion necessary for electron-hole charge separation to occur at heterogeneous material interfaces. However, staggered energy levels at nano-scale interfaces, such as those between organic molecules and inorganic quantum dots, do not necessarily imply charge separation across the interface because the excitonic effect is often significant. Using quantum Monte Carlo calculations, we perform a detailed study of the role of the excitonic effects on charge separation across a representative set of interfaces between organic molecules and quantum dots. We find that the exciton binding energy of charge transfer excitons is significantly larger than would be estimated from a simple Coulombic analysis and, at these nano-scale interfaces, can be as significant as that of Frenkel excitons. This implies that charge transfer excitons can act as a trap states for electron-hole recombination instead of facilitating charge separation. We conclude that, in general for nanoscale interfaces, high-fidelity quantum many body calculations are essential for an accurate evaluation of the detailed energetic balance between localized and delocalized excitons and are thus crucial for the predictive treatment of interfacial charge separation processes.