The recent development of nonfullerene acceptors (NFAs) has resulted in an abrupt increase in the organic photovoltaic (OPV) efficiency. It remains unclear why charge separation (CS) can occur at a high yield in NFA OPVs despite the small energy-level offset at the donor-acceptor (D-A) interface. $\mathrm{In}$ order to resolve this issue, zinc phtholacyanine (ZnPc) and its fluorinated derivative (${\mathrm{F}}_{8}\mathrm{Zn}\mathrm{Pc}$) are used to build a model D-A interface for understanding the mechanism underlying the efficient CS in NFA OPVs. At this interface, we find that bound charge-transfer (CT) excitons can undergo a heating process in which the enthalpy increases at a surprisingly fast rate. Specifically, the CT exciton gains 0.3 eV from the environment in merely 10 ps. This fast exciton heating, which eventually leads to CS, resembles the CS behavior observed in NFA bulk heterojunctions (BHJs). Interestingly, when the pair of D-A molecules switches from a face-on to an edge-on orientation, the CT-exciton heating process is reversed to the typical hot CT-exciton cooling process in which the enthalpy decreases with time. By using sub-band-gap excitation, we further find that the exciton heating can occur directly from cold CT excitons without the need of excess energy provided by the energy-level offset. The competition between the exciton heating and cooling is explained by a model that incorporates the effect of entropy, electron delocalization, and structural anisotropy. It is found that for the face-on orientation, the small contact area between the delocalized electron and hole within the CT exciton significantly reduces the number of cold CT states, which increases the entropic driving force and drives the reaction into the heating direction. This finding provides insight on why the entropy-and-delocalization-driven CS would preferentially occur in NFA BHJs. The relative orientation of the NFA molecular stack and the polymer chain, together with bulky side groups, make the delocalized electron and hole in contact with each other via isolated pointlike junctions, which is the key for activating the exciton heating and subsequent CS.