In this paper, a comprehensive study is carried out on the dissociation mechanism of excitons in bulk-heterojunction organic solar cells. It is proposed that at the donor-acceptor interface, a Frenkel exciton relaxes to a charge transfer exciton and then dissociates into free charge carriers with the aid of molecular vibrational energy. The interaction operator between the charge transfer exciton and molecular vibrational energy is derived and used to formulate and calculate the rates of dissociation of singlet and triplet excitons into free charge carriers. The dissociation rates are found to be dependent on the binding energy, lowest unoccupied molecular orbital offset between the donor and acceptor, the phonon energy, reduced excitonic mass, excitonic Bohr radius, and the dielectric constant of the organic material. Using the proposed dissociation mechanism, three points have also been highlighted that could provide possible reasons as to why the performance of bulk-heterojunction organic solar cell is low.