Extended π-conjugated perylene diimide dimers toward efficient organic solar cells

A series of PDI-based small molecular acceptors (FPDI-I, FPDI-II, F2PDI-I and F2PDI-II) using 9H-fluorene as the central core and two PDI units or two PDI dimer units (PDI2) as the terminal groups have been synthesized. The optical, electrochemical and photovoltaic properties of these four acceptors have been systematically investigated. These acceptors exhibit twisted structures mainly due to the dihedral angles between the central fluorene core and the two terminal PDI units. The twisted molecular structures can effectively prevent the acceptor molecules from forming oversized aggregates. The two terminal fused PDI dimer units in F2PDI-I and F2PDI-II can enhance their molar absorption coefficients. Using PBDB-T as the donor polymer, F2PDI-I and F2PDI-II based devices exhibit higher and more balanced charge mobilities. As a result, organic solar cells (OSCs) based on PBDB-T:F2PDI-I give the best device performance with a power conversion efficiency (PCE) of 6.82%, a Voc of 0.90 V, a Jsc of 11.32 mA cm−2 and an FF of 66.65%.