Efficient Exciton Dissociation Enabled by the End Group Modification in Non-Fullerene Acceptors

For organic photovoltaic (OPV) cells, in order to overcome the larger Coulombic binding energy between holes and electrons, an extra driving force is required for efficient exciton dissociation. Here, we report two nonfullerene acceptors named IO-4H and IO-4F for OPV cells. By employing the polymer PBDB-TF as a donor, the PBDB-TF:IO-4H-based device only shows a power conversion efficiency (PCE) of 0.30% with a charge dissociation probability ( P diss ) of 13.3%. On the contrary, the PBDB-TF:IO-4F-based device demonstrates a PCE of 7.85% with a P diss of 81.3%. The photoelectric processes demonstrate that both devices have similar charge transport and charge recombination properties. The limitation of photovoltaic performance is the low exciton dissociation efficiency in the PBDB-TF:IO-4H-based device. The theoretical studies show the electrostatic potential (ESP) of IO-4H is negative in the end groups and similar to the ESP of PBDB-TF, whereas ESP of IO-4F is positive. PBDB-TF and the IO-4F may form a strong intermolecular electric field to assist the exciton dissociation. Our results suggest that increasing the ESP difference between donor and acceptor may be beneficial to promote exciton dissociation, thus improving photovoltaic performance.