Exciton-to-Charge Dynamics Driven by the Nonuniform Polymer Packing at Donor/Acceptor Interfaces

Although the encouraging power conversion efficiency of over 15% has been achieved for polymer solar cells (PSCs), the details of exciton-to-charge dynamics and the corresponding energy loss are yet to be understood and still under intensive debate. Herein, we present a model study on the exciton-to-charge dynamics by constructing a polymer/small molecule (SM)-based donor/acceptor (D/A) interface in the transition region from an intermixed polymer/SM phase to a pure polymer phase. During dynamical simulations, the exciton is supposed to be initially photogenerated in SM, and the evolution follows a nonadiabatic method. It is found that for a given polymer packing, the charge dynamics is mainly determined by the energy offset ΔEₕ of hole charge transfer. Specially, a minimum value of ΔEₕ is required for hole charge transfer when the exciton arrives at the D/A interface, by which the exciton can be dissociated and a charge-transfer (CT) state is simultaneously created. When the energy offset ΔEₕ is above the critical value for long-range charge separation, the formed CT state can be further separated into free charges. As a result, exciton-to-charge dynamics can be completed, and the visualization results show that the whole process successively experiences charge transfer, delocalization, relaxation, and separation. In addition, we demonstrate that the energy loss during the exciton-to-charge dynamics is linearly correlated with ΔEₕ, and its critical value for long-range charge separation can be decreased by the aggregation or crystallinity of SMs so as to reduce the energy loss of charge generation in PSCs.