Our findings demonstrate the importance of kinetic factors in determining the overall charge separation efficiency in D/A systems.

No 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.

Organic photovoltaics (OPVs) hold promise for a cost-effective, ecofriendly, and sustainable technology to harvest solar energy. However, the widespread application of OPVs has been hindered mostly due to their intrinsic narrow absorption band and environmental impacts caused by toxic solvents required during the synthesis. In this work, we investigated energy transfer dynamics in water-dispersed polymeric nanoparticles (NPs) of F8T2, MDMO-PPV, and their mixtures (bicomponent) synthesized by the miniemulsion technique. Using femtosecond transient absorption and time-resolved fluorescence spectroscopy, we showed that Forster resonance energy transfer (FRET) can be promoted in the bicomponent NPs on an ultrafast time scale and modulated upon the variation in polymer concentrations. The higher FRET efficiency achieved for the bicomponent NPs compared to reports for single-component donor− acceptor systems implies that this design strategy may be utilized in primary building blocks of ternary architecture. Our results suggest that the water-dispersed polymeric bicomponent NP of F8T2/MDMO-PPV is a promising candidate to be incorporated in ternary structures of OPVs in combination with acceptors, such as fullerene C 60 , to extend the absorption band, increase the energy transfer efficiency, and facilitate exciton dissociation.