Evidence for Entropy-Driven Charge Separation in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved power conversion efficiencies close to 20%. These NFA OPVs can efficiently generate free carriers at the donor/acceptor (D/A) interface despite a very small energy level offset. However, why efficient charge separation can occur with a minimal energy loss is still not clear. We will present our recent time-resolved two-photon photoemission (TR-TPPE) spectroscopy measurements on exciton dynamics in a representative NFA/polymer blend, the Y6/PM6 bulk heterojunction (BHJ). We find that charge transfer (CT) excitons are formed within a few picoseconds after the photoexcitation. Subsequently, an energy uphill process occurs in which the energy of the CT exciton is increased by 0.15 eV on the 10-100 ps timescale. We attribute this energy uphill process to the conversion of bound CT excitons to free electron-hole pairs. The observed energy uphill process is rather anormal because excited electrons typically lose their energy to the environment when no external electric field is applied. We propose that the energy uphill charge separation process is driven by entropy. The entropy-driven charge separation would be promoted by the specific nanostructure of NFA BHJs. This work is based upon work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award No DE-SC0024525.