Efficient Charge Dissociation of Triplet Excitons in Bulk Heterojunction Solar Cells

Understanding the sources of energy loss in bulk heterojunction (BHJ) solar cells is of outstanding importance for increasing power conversion energy. Herein, we employ a full quantum mechanics approach to determine the rates of charge transfer processes at the acceptor–donor interface for a prototypical BHJ blend. It is shown that charge recombination of a singlet charge transfer (CT) state at the acceptor/donor (A/D) interface is very fast, being the most effective process which prevents efficient charge migration toward the electrodes. Triplet CT states can also undergo charge recombination by fast decay to a low lying triplet state of the donor and/or of the acceptor, but the backward process is also fast enough to allow efficient charge dissociation. Slowing down the fast decay process of singlet CT states appears to be hardly practicable, since it would reflect either on a reduced spectral absorption window or on a slow rate of photoinduced electron transfer, so that an alternative and possibly more viable route for increasing organic solar cell efficiency could be that of favoring the formation of triplet CT states over singlet ones.