Coherent and Incoherent Contributions to Charge Separation in Multichromophore Systems

Abstract

In organic materials, exciton dissociation into free charges requires overcoming an electron-hole Coulomb interaction that exceeds the thermal energy and may still be large after charge transfer at a donor/acceptor interface. We analyze the factors affecting efficiency of charge separation and subsequent removal of electrons and holes from such a donor-acceptor interface and suggest strategies for optimizing these processes. Energy transfer, charge separation and charge transfer in the vicinity of the donor-acceptor interface are studied within a common theoretical framework based on a quantum master equation, for a model system with realistic excitation energies and electronic couplings. We find that enhancing the efficiency of both charge transfer from the donor to the acceptor and of charge removal from the donor-acceptor interface requires an intricate balance between the extent of electronic delocalization throughout the material and rates of energy dissipation. For very large exciton binding energies, cascade charge separation in systems with more than one donor and one acceptor species, such as molecular polyads, is found to greatly facilitate the dissociation of geminate pairs. Our calculations predict charge separation on sub-picosecond timescales for several parameter combinations, leading to design principles for enhancing charge separation in multi-chromophore systems.

Spontaneous Exciton Dissociation at Organic Semiconductor Interfaces Facilitated by the Orientation of the Delocalized Electron–Hole Wavefunction

“…Detailed parameters of this model can be found in the method section. Similar models have been used by others23,60 to study exciton delocalization and CS.…”

“…Detailed parameters of this model can be found in the method section. Similar models have been used by others [23,60] to study exciton delocalization and CS. Figure 5a shows the energy of eigenstates as a function of the average electron-hole separation, r, for the C 60 /ZnPc (yellow) and the F 8 ZnPc/ZnPc (purple) interfaces.…”

Origin of space-separated charges in photoexcited organic heterojunctions on ultrafast time scales

“…Even though similar theoretical models have been lately proposed, 38,56 we believe that our theoretical treatment goes beyond the existing approaches, since it treats both the exciton generation and their further separation on equal footing and it deals with all the relevant interactions on a fully quantum level. Namely, the vast majority of the existing theoretical studies on charge separation at heterointerfaces does not treat explicitly the interaction with the electric field which creates excitons from an initially unexcited system, 17,19,38,45,56 but rather assumes that the exciton has already been generated and then follows its evolution at the interface between two materials. If we are to explore the possibility of direct optical generation of space-separated charges, we should certainly monitor the initial process of exciton generation, which we are able to achieve with the present formalism.…”

Charge Dynamics in Organic Photovoltaic Materials: Interplay between Quantum Diffusion and Quantum Relaxation

“…[13][14][15][16][17][18][19][20][21][22] For example, Tamura and Burghardt 13 attributed fast CS to the two main factors, (i) electron delocalization within the fullerene condensate and (ii) the role of vibronically hot CT states, by carrying out a combined approach of electronic structure calculations and quantum dynamical analysis. Sun To obtain a complete description of charge dynamics it is therefore important to model the charge dynamics of both processes, "low-energy CT to CS states" and "CS to low-energy CT states", which would occur on two different timescales.…”