By employing a femtosecond electric pump pulse, we theoretically investigate the re-excitation dynamics of a “cold” charge transfer (CCT) state at organic donor/acceptor (D/A) interfaces. It is demonstrated that a relaxed CCT state can be pushed to different “hot” CT (HCT) states via experiencing electron (HCT1 state) and/or hole (HCT2 state) higher-energy transitions, where the transition modes and probabilities are primarily determined by the pulse energy. Without the assistance of a charge driving field, both the two HCT states relax to the initial CCT state through different internal conversion processes, whose dynamics are clearly clarified in this work. However, after a driving field is applied, we find that both of the HCT states can be dissociated into free charges before their relaxations. In particular, the HCT2 state is very easily dissociated compared to the HCT1 state, as well as the CCT state, due to the more delocalized hole charge distribution along the donor. In addition, by enhancing the pulse intensity, we can further improve the hole delocalization along the donor so that the pulsed HCT2 state is more favorable to be dissociated. This work underlines the importance of charge delocalization for the interfacial charge dynamics, including both the internal conversion and charge separation, mediated by different intermediate HCT states in organic solar cells.
As a promising strategy, strain engineering has been employed on different semiconductor materials, aiming to modulate their photoelectric properties, thus innovate device applications. An interesting experimental result is the directional exciton migration induced by creating some local nonuniform strains in materials, such as “exciton funneling.” In this Letter, to clarify the dynamical mechanism and the impacting factors, we theoretically investigate the migration dynamics of excitons/biexcitons along organic polymers induced by a funnel-like nonuniform compression strain. First, the migration dynamics of an exciton/biexciton are separately demonstrated right after the strain created. It is found that both of them will migrate toward the strain center with the speeds up to 7–10 nm/ps, comparable with the experimental observations in a bending ZnO microwire, where the difference between the exciton and biexciton migration dynamics is also emphasized. We attribute the present exciton/biexciton migration mechanism to the strain-induced gradient of the exciton/biexciton creation energy along polymers. Furthermore, some typical factors impacting the exciton migration dynamics are considered, such as the strain gradient, the initial ratio of the exciton located in the strain region, the electron-lattice interaction, etc. Finally, based on these findings, we briefly discuss the possible results about the strain-modulated luminescence of organic polymers.
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