The Twisted Benzo[ghi]-Perylenetriimide Dimer as a 3D Electron Acceptor for Fullerene-Free Organic Photovoltaics.

A series of twisted N,N-linked benzo[ghi]-perylenetriimide dimers (t-BPTI) with various lengths of the α-branched alkyl side chain at the six-membered imide ring position was designed, synthesized, and characterized. These compounds showed the low-lying LUMO energy level of -3.78 eV, which was similar to that of PC61 BM (-3.71 eV), but with intensive optical absorption in the range 350-500 nm. The twisted molecular geometry with two nearly perpendicular BPTI planes achieved a favorable nanoscale phase separation by relieving the self-aggregation of rigid BPTI units in blend films. The acceptor t-BPTI-3 unit with the longest alkyl side chains has been demonstrated to be an efficient electron acceptor in solution-processed bulk heterojunction organic photovoltaics (OPV), giving a power conversion efficiency of 3.68 % when using conjugated polymer PTB7-Th as the donor and without additional treatments.