The adapted Su−Schrieffer−Heeger (aSSH) model Hamiltonian is extended in this work to incorporate the interchain π−π stacking and dynamical electron−phonon coupling effects so that the photoinduced charge-transfer mechanism can be directly probed at the π-conjugated heterojunction interfaces. It is found that excitons generated in the bulk poly(p-phenylene vinylene) (PPV) phase require an activation energy of 0.23 eV to reach the heterojunction interfaces before getting their charges separated. Electron transfers from the D1 state of PPV to the t1u state of C60 follow the nonadiabatic mechanism, which is accelerated by three major factors including the 0.95 eV energy drop between the two states, close vicinity of the electron-donating D1 state to the C60 phase, and suppressed inversion symmetry of C60 at the interfaces. The irreversible phonon relaxation energy associated with the nonadiabatic electron transfer is estimated to be 0.3 eV, which explains the widely accepted empirical energy offsets between the measured open-circuit voltage and the theoretical built-in potential.