A strategy that employs the central-core regiochemistry to develop two isomeric perylene diimide (PDI)-based small molecular acceptors (SMAs), BPT-Se and BPT-Se1, is introduced, and the effect of the central-core regiochemistry on the optical, electronic, charge-transport, photovoltaic, and morphological properties of the molecules and their devices is investigated. The PDBT-T1:BPT-Se1-based device delivers a power conversion efficiency (PCE) of 9.54% with an excellent fill factor (FF) of 73.2%, while the BPT-Se-based device yields a PCE of 7.78%. The large improvement of PCE upon isomerization of BPT-Se should be ascribed to the concurrent enhancement of FF, short circuit current ( J SC), and open circuit voltage (V OC) of the PDBT-T1:BPT-Se1 devices. The higher FF of the organic solar cells (OSCs) based on PDBT-T1:BPT-Se1 can be attributed to the higher charge dissociation and charge collection efficiency, less bimolecular combination, more balanced µ h/µ e, better molecular packing and a more favorable morphology. It is worth mentioning that the FF of 73.2% is the highest value for PDI-based SMAs OSCs to date. The result shows that regiochemistry of the central core in PDI-based SMAs greatly affects the physicochemical properties and photovoltaic performance. The success of the isomerization strategy offers exciting prospects for the molecular design of PDI-based SMAs.
Keywords: fill factor; isomerization; organic solar cells; perylene diimide; small molecular acceptors.