Designing indacenodithiophene based non-fullerene acceptors with a donor–acceptor combined bridge for organic solar cells

Non-fullerene small acceptor molecules have gained significant attention for application in organic solar cells owing to their advantages over fullerene based acceptors. Efforts are continuously being made to design novel acceptors with greater efficiencies. Here, optoelectronic properties of four novel acceptor–donor–acceptor (A–D–A) type small molecules (A1, A2, A3 and A4) were studied for their applications in organic solar cells. These molecules contain an indacenodithiophene central core unit joined to different end capped acceptors through a monofluoro substituted benzothiadiazole (FBT) donor acceptor (DA) bridge. The different end capped acceptor groups are; 2-2(2-ethylidene-5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (A1), 2-2(2-ethylidene-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (A2), 2-(5-ethylidene-6-oxo-5,6-dihydrocyclopenta-b-thiophene-4-ylidene)malononitrile (A3), and 2-2(2-ethylidene-5,6-dicyano-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (A4). The calculated optoelectronic properties of the designed molecules were compared with a well-known reference compound R, which was recently synthesized and reported as being an excellent A–D–A type acceptor molecule. All designed molecules showed the appropriate frontier molecular orbital diagram for a charge transfer. A4 shows the highest absorption maximum (λ(max)) of 858.6 nm (in chloroform solvent), which was attributed to the strong electron withdrawing end-capped acceptor group. Among all of the designed molecules, A3 exhibits the highest open circuit voltages (V(oc)) which was (1.84 V) with PTB7-Th and (1.76 V) with the P3HT donor polymer. Owing to a lower value of λ(e) with respect to λ(h,) the designed molecules demonstrated superior electron mobilities when compared with reference R. Among all of the molecules, A4 shows the highest electron mobility owing to the lower value of λ(e) compared to R.