Kinetic Modeling of the Electric Field Dependent Exciton Quenching at the Donor–Acceptor Interface

Abstract

In this work, the electric field dependence of the exciton quenching efficiency at the donor/acceptor (D/A) heterojunction was studied. The concentration of singlet excitons and charge transfer states at the D/A interface was modeled by a system of coupled differential equations with transition rates obtained from Marcus theory. We applied then the model to determine the angular dependence (expressed by a parameter, θ) of the local exciton quenching induced by the orientation of the dissociation process relative to the direction of the electric field (F). We found that the exciton diffusion to the D/A heterojunction is not homogeneous for every value of θ, but it is higher toward regions where the dissociation rate is greater. The consequence of this effect is that only small values of this parameter will effectively contribute to the average quenching efficiency. There is then a gradual increase of the quenching efficiency with F, a fact that was verified experimentally. Following this principle, we were able to fit the experimental data measure in a bulk heterojunction device. In addition, we studied the field dependence of the PL quenching in a bilayer device that presents a very useful structure to test the theory. We found that the model explains the field-induced quenching efficiency not only when F has a favorable orientation that enhances the charge transfer but also when F tends to inhibit this process. In addition, our analysis might give some hints on the degree of mixing between the donor and acceptor in the active layer in this kind of device architecture. We believe that the model may clarify the processes that influence the dynamics of exciton dissociation at D/A interfaces. It is also useful to explore the effects of temperature, energy disorder, site disorder, and exciton binding energy in the photovoltaic effect of organic solar cells. Overall, it opens the possibility to deeply understand the effect of an electric field in new D/A heterojunctions with low driving force and efficient exciton dissociation.