Monishka Rita Narayan

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

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.

Effect of simultaneous excitation of singlet and triplet excitons on the operation of organic solar cells

Primary steps of exciton formation in organic solar cells are presented here. The rates of absorption of photons to excite singlet and triplet excitons are derived using exciton-photon and exciton-spin-orbit-photon-interaction, respectively, as perturbation operators. In both singlet and triplet absorptions, the rates are found to depend on the absorption energy, excitonic Bohr radius, and the dielectric constant of the donor organic material. Incorporation of heavy metal atoms enhances the exciton-spin-orbit-photon interaction and hence the rate of excitation of triplet excitons because it depends on the square of the heaviest atomic number. The new exciton-spin-orbit-photon interaction operator flips the spin to a singlet form leading to faster dissociation into charge carriers and resulting in higher photon to electron-hole pair conversion efficiency in organic solar cells.

Dissociation of charge transfer excitons at the donor–acceptor interface in bulk heterojunction organic solar cells

It is emphasized that the formation of charge transfer (CT) excitons in bulk heterojunction (BHJ) organic solar cells (OSCs) does not automatically lead to the dissociation of excitons and generation of charge carriers due to the built-in electric field created by the difference in the work functions of the electrodes. Like Frenkel excitons, CT excitons are also charge neutral excited entities and cannot be efficiently dissociated by such electric field. A condition of the dissociation of CT excitons is proposed and used to derive the rates of exciton dissociation in BHJ OSCs. Results are calculated and compared with experiments for the BHJ OSC of PTB7: PC71BM and found to agree very well qualitatively.

Photovoltaic contribution of photo-generated excitons in acceptor material of organic solar cells

Contribution of exciton generation in acceptor material to the photovoltaic performance of three bulk-heterojunction organic solar cells (BHJ OSCs), PTB7:PC71BM, P3HT:ICBA and P3HT:PC61BM are studied. Singlet and triplet rates of absorption and dissociation and diffusion lengths are calculated and compared with those when excitons are generated in the donor of these BHJ OSCs. It is found that the rates of absorption and dissociation and diffusion lengths are comparable whether excitons are generated in donor or acceptor of these BHJ OSCs. Therefore, it is proposed that the contribution of exciton generation in acceptor may not be negligible.

Excitonic Processes in Organic Semiconductors and Their Applications in Organic Photovoltaic and Light Emitting Devices

This chapter discusses the excitonic processes in organic semiconductors and their applications in photovoltaic and light emitting devices fabricated from these materials. The mechanisms of excitonic absorption, diffusion and dissociation of excitons at the donor-acceptor interface are presented in bulk-heterojunction organic solar cells. After the formation of Frenkel excitons upon photon absorption, excitons must diffuse to the interface to dissociate into free charge carriers which are then collected at their respective electrodes. The interface must be in close proximity, of the order of the diffusion length, and for efficient dissociation the offset of the lowest unoccupied molecular orbital energy between the donor and acceptor must be at least equal to the exciton binding energy. The Forster and Dexter energy transfer mechanisms are used to calculate the exciton diffusion coefficients and exciton diffusion lengths for singlet and triplet excitons, respectively. The newly derived interaction operator between charge transfer exciton and molecular vibration energy is used to understand the mechanism and derive the rate of dissociation of excitons into free charge carriers. The exciton diffusion and dissociation in bulk-heterojunction organic solar cell are presented first followed by the radiative recombination of exciton in organic light emitting devices (OLEDs).

Diffusion of excitons in materials for optoelectronic device applications

The diffusion of singlet excitonsis known to occur through the Forster resonance energy transfer (FRET) mechanism and that of singlet and triplet excitonscan occur through the Dexter carrier transfer mechanism. It is shown here that if a material possesses the strong exciton-spin-orbit-photon interaction then triplet excitonscan also be transported /diffused through a mechanism like FRET. The theory is applicable to the diffusion of excitonsin optoelectronic devices like organic solar cells, organic light emitting devices and inorganic scintillators.

Exciton dissociation and design optimization in hybrid organic solar cells

The process of exciton dissociation in hybrid organic solar cells is studied and the rates of singlet and triplet exciton dissociations in P3HT:SiNW hybrid solar cell are calculated. Possible loss mechanisms during the dissociation process have also been highlighted. Design optimization in flexible P3HT:SiNW hybrid solar cell is performed and a power conversion efficiency of 4.70% is obtained which is a notable enhancement from its current experimental power conversion efficiency.