Theoretical Study of Excited State Charge Transfer Characteristics based on A-D-A and A-DA′D-A Type Nonfullerene Acceptors

The excited state charge transfer (CT) characteristics of nonfullerene acceptors play an important role in determining the photo excited CT mechanisms and performance for organic solar cells (OSCs). Here, we systematically investigated the structure-property relationship and excited state CT characteristics based on A-D-A and A-DA′D-A type acceptors by density functional theory (DFT) and time-dependent DFT (TD-DFT). Comparing to the A-D-A type acceptors IT-4F and DTPC-DFIC, the A-DA′D-A type acceptor Y6 exhibits distinct structural and electronic characteristics, such as featuring larger Mulliken electronegativity, molecular polarity index (MPI), and electrostatic potential (ESP). Besides this, the larger hole and electron delocalization index (HDI, EDI) proves more effective charge separation for Y6. Their excited state CT characteristics are all mainly from the intermediate cores to the terminal units, while the CT in the opposite direction of Y6 is stronger than that of IT-4F and DTPC-DFIC, which provides theoretical support for the formation of multielectron transport channels in 3D network structure. These results may explain why Y6 and its derivatives show such efficient photovoltaic performance and provide a theoretical basis for the design and screening of novel acceptors as well as further deepen the understanding of the excited states charge transfer in OSCs.