Study of the mechanism and rate of exciton dissociation at the donor-acceptor interface in bulk-heterojunction organic solar cells

Abstract

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. V

A Theoretical Study on the Operation Principle of Hybrid Solar Cells

“…The mechanism of exciton dissociation has been proposed in our previous study [7] and occurs through two steps: (1) As the energy of acceptor's conduction band energy level, , is lower than that of donor's LUMO, , a Frenkel exciton relaxes to a charge transfer (CT) exciton state where electron gets transferred to the acceptor's CB [19]. It is to be noted that the CT exciton is not yet dissociated because for dissociation it requires an external energy at least equal to the binding energy [20].…”

“…The rate of exciton dissociation, Rd, for singlet and triplet excitons in hybrid OSCs is derived using the interaction operator between CT exciton and molecular vibration energy, transition matrix element and Fermi's Golden rule and can be written as [7]:…”

“…where ωv is the phonon frequency and and are the singlet and triplet exciton binding energies, respectively, given as [4]: After exciton dissociation, the free electron is transported to cathode and hole to anode with the aid of the electric field generated due to the workfunction difference between the electrodes and a force, F, of [7]:…”

“…Hybrid materials normally consist of inorganic nanoparticles as acceptor [3] and organic materials (small molecules/polymers) as donor materials. Together they form the photovoltaic active layer where photon absorption, Frenkel exciton formation [4][5][6], diffusion [4] and dissociation [7,8] take place. The free charge carriers are transported and collected at opposite electrodes [2,7,9].…”

“…At such offset, dissociation of both singlet and triplet excitons is ensured because it surpasses their binding energies, however, the singlet dissociation rate is five orders of magnitude faster than that of triplet. Both the dissociation rates are higher than the corresponding recombination rates which implies that the dissociation of an exciton may be faster than its non-radiative recombination, which enhances the performance of the hybrid solar cells [6,7].…”