Limits...
Role of coherence and delocalization in photo-induced electron transfer at organic interfaces

View Article: PubMed Central - PubMed

ABSTRACT

Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces.

No MeSH data available.


Related in: MedlinePlus

Ensemble-averaged evolution of electron probability density in the plane perpendicular to the donor (grey area) and acceptor (8 × 16 × 16 nm3 domain) interface at different times following photoexcitation for the indicated values of inter-acceptor coupling JA.The rightmost column shows the corresponding absolute charge separation distance dabs(t) (black traces) and delocalization radius l(t) (blue traces). Filled blue circles illustrate the extent of electron coherence at a given time. The color scale at the bottom describes the probability of finding the electron at the indicated distance from the interface.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5015064&req=5

f1: Ensemble-averaged evolution of electron probability density in the plane perpendicular to the donor (grey area) and acceptor (8 × 16 × 16 nm3 domain) interface at different times following photoexcitation for the indicated values of inter-acceptor coupling JA.The rightmost column shows the corresponding absolute charge separation distance dabs(t) (black traces) and delocalization radius l(t) (blue traces). Filled blue circles illustrate the extent of electron coherence at a given time. The color scale at the bottom describes the probability of finding the electron at the indicated distance from the interface.

Mentions: Figure 1 shows the simulated temporal evolution of electron density for the indicated values of inter-acceptor electron coupling JA. Data are projected in the plane perpendicular to the donor/acceptor interface. External electric field is not applied to highlight the effects of delocalization. Our results show that the electron is transferred from the donor site to a nearby pool of coherently coupled acceptor sites within ~300 fs. The number of accessible sites in a given time interval grows with increasing intermolecular (inter-acceptor) coupling, allowing for the electron to transfer to more distant sites already at very early times. Electron transfer is quantitatively characterized in the rightmost column of Fig. 1, where the kinetics of the absolute e-h separation distance, dabs(t) (Fig. 1 black traces) and the electron delocalization radius, l(t) (Fig. 1, blue traces), are shown (details on these parameters are given in the Methods section). The extent of electron delocalization is visualized by the blue circles. These results only weakly depend on the donor excitation energy εD (see Supplementary Information), in agreement with experimental studies of charge separation efficiency versus excitation energy13.


Role of coherence and delocalization in photo-induced electron transfer at organic interfaces
Ensemble-averaged evolution of electron probability density in the plane perpendicular to the donor (grey area) and acceptor (8 × 16 × 16 nm3 domain) interface at different times following photoexcitation for the indicated values of inter-acceptor coupling JA.The rightmost column shows the corresponding absolute charge separation distance dabs(t) (black traces) and delocalization radius l(t) (blue traces). Filled blue circles illustrate the extent of electron coherence at a given time. The color scale at the bottom describes the probability of finding the electron at the indicated distance from the interface.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5015064&req=5

f1: Ensemble-averaged evolution of electron probability density in the plane perpendicular to the donor (grey area) and acceptor (8 × 16 × 16 nm3 domain) interface at different times following photoexcitation for the indicated values of inter-acceptor coupling JA.The rightmost column shows the corresponding absolute charge separation distance dabs(t) (black traces) and delocalization radius l(t) (blue traces). Filled blue circles illustrate the extent of electron coherence at a given time. The color scale at the bottom describes the probability of finding the electron at the indicated distance from the interface.
Mentions: Figure 1 shows the simulated temporal evolution of electron density for the indicated values of inter-acceptor electron coupling JA. Data are projected in the plane perpendicular to the donor/acceptor interface. External electric field is not applied to highlight the effects of delocalization. Our results show that the electron is transferred from the donor site to a nearby pool of coherently coupled acceptor sites within ~300 fs. The number of accessible sites in a given time interval grows with increasing intermolecular (inter-acceptor) coupling, allowing for the electron to transfer to more distant sites already at very early times. Electron transfer is quantitatively characterized in the rightmost column of Fig. 1, where the kinetics of the absolute e-h separation distance, dabs(t) (Fig. 1 black traces) and the electron delocalization radius, l(t) (Fig. 1, blue traces), are shown (details on these parameters are given in the Methods section). The extent of electron delocalization is visualized by the blue circles. These results only weakly depend on the donor excitation energy εD (see Supplementary Information), in agreement with experimental studies of charge separation efficiency versus excitation energy13.

View Article: PubMed Central - PubMed

ABSTRACT

Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces.

No MeSH data available.


Related in: MedlinePlus