Nonfullerene acceptors with an N-annulated perylene core and two perylene diimide units for efficient organic solar cells

We have designed and synthesized a series of perylene based A-D-A type chromophores, which are composed of two electron deficient perylene diimides (PDI) and one electron rich N-annulated perylene (NP). By virtue of the bi-axial rigid skeleton of the flat NP unit, the intermolecular aggregation is efficiently suppressed, leading to a very small red-shift of absorption being observed in going from solutions to films. In addition, the molar extinction coefficients have been significantly elevated in case of sulfur or selenium annulation at the bay positions of the PDI units. Quantum calculation is employed for the geometry optimization to further understand the contributions of different excitations to the UV-vis absorption spectra. Morphology studies demonstrate that the twisted conformations of these acceptors are favorable for the formation of suitable phase separation in the as-cast films. The heteroatom annulation could force the PDI plane to take on a more planar conformation, which is favorable to have closer pi-pi stacking for efficient electron transportation. Finally, the as-cast devices based on PBDB-T:NP-diPDI-Se display a power conversion efficiency (PCE) of 6.25% with a V-oc of 0.98 V, a J(sc) of 11.73 mA/cm(2) and an FF of 54.13%.