Charge separation of excitons in materials is one of the most important physical processes to utilize the solar energy in diverse devices including solar cells and photo-catalysts. Heterogeneous interfaces with the so-called type-II character are often employed to infer the interfacial charge transfer in this context. As a simple criterion for designing such an interface, the energy alignment of the quasi-particle states together with the exciton binding energy of electron-donating materials is often discussed in the literature. However, an accurate description of the effect of exciton binding at the interface has not been investigated extensively. Although density functional theory (DFT) is a powerful method to investigate various elec-tronic properties of materials, incomplete description of many-body interactions often leads to an incorrect interpretation of the energy level alignment. While Many-Body Perturbation Theory and Quantum Monte Carlo (QMC) are promising in this context, much more work is necessary to assess how well these methods perform in practice. In this talk, I will discuss how significant the excitonic effect can affect charge separation process at heterogeneous interfaces between PbS quantum dot and thiophene oligomers by employing QMC calculations. Our study shows that charge transfer excitons can act as a trap states for electron-hole recombination instead of facilitating charge separation. This work has been published in Nano Letters.