Near-Infrared Absorbing Non-Fullerene Acceptors With Unfused D-A-D Core For Efficient Organic Solar Cells

Two non-fullerene acceptors based on D-A-D-type unfused central units, i.e., BCPDT-1 and BCPDT-2, were synthesized, employing 3-bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2ethylhexyl)benzo-[1,2:4,5-c?]-dithiophene-4,8-dione (BDD) unit as the A moiety and 4,4-dialkyl-4H-cyclopenta[2,1-b:3,4-b?]dithiophene (CPDT) unit as the D moiety. The two molecules possess identical backbones, but carry different side chains (octyl for BCPDT1 and 2-ethylhexyl for BCPDT-2) on CPDT units. Both BCPDT-1 and BCPDT-2 presented broad absorption extending to near-infrared region with optical band gaps of 1.36 and 1.39 eV, respectively. Organic solar cells (OSCs) were fabricated with PBDB-T as donor and BCPDT-1 or BCPDT-2 as acceptor. The devices based on BCPDT-2 exhibited efficient exciton dissociation and charge collection as well as weak charge recombination, attributed to the proper film morphology with nano-scale phase separation and favorable molecular orientation. Consequently, the BCPDT-2 based device displayed a higher power conversion efficiency (PCE) of 10.65%, while the BCPDT-1 based device showed an inferior PCE of 7.54%.