PDI-based heteroacenes as acceptors for fullerene-free solar cells: importance of their twisted geometry

Two perylenediimide (PDI) derivatives (FP 4 TT2T and FP 4 3T) with extended conjugation have been successfully synthesized. Due to their long fused D-A (D: donor and A: acceptor) molecular structures, PDI-based heteroacenes have rigid backbones and exhibit strong light absorption in the 300-600 nm range. With the contribution from the PDI moiety with strong electron affinity, the molecules possess low energy levels of LUMOs ( ca. −3.7 eV), fitting well as acceptors for fullerene-free organic solar cells (OSCs). OSC devices containing FP 4 TT2T and FP 4 3T were fabricated and fully characterized. It is found that the geometric twist in PDI-based heteroacenes as acceptors could enhance the performance of OSC devices. OSCs based on FP 4 3T with a more twisted geometry achieved a power conversion efficiency of 6.05%, which is higher than those based on the FP 4 TT2T counterpart. Two novel PDI-based small molecules with twisted geometry were designed and synthesised as acceptors for organic solar cells.