Ultrafast Charge Separation from a “Cold” Charge-Transfer State Driven by Nonuniform Packing of Polymers at Donor/Acceptor Interfaces

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Understanding the mechanism of how a “cold” charge-transfer (CCT) state is dissociated into free charges and how morphologies of bulk heterojunction thin films affect such a process is of critical importance for improving the overall efficiency of polymer solar cells (PSCs). Here, by modeling a polymer/fullerene-based donor/acceptor interface in the transition region from an intermixed polymer/fullerene phase to a pure polymer phase, we demonstrate that the nonuniform packing of polymers can provide a driving force to dissociate the CCT state in a time scale of no more than 200 fs. Importantly, charge separation is shown to be achieved by two successive processes, that is, “charge delocalization process between polymers” and “charge migration process along polymers”, and there exists an optimal packing of polymers that is most efficient for charge separation. Also, we demonstrate that the charge separation process can be further promoted by aggregation or crystallinity of fullerenes as a result of charge delocalization. These findings are in agreement with the morphology-dependent ultrafast charge separation phenomena reported experimentally in numerous high-performance PSCs.