Design of Nonfused Nonfullerene Acceptors Based on Pyrido- or Benzothiadiazole Cores for Organic Solar Cells

Nonfullerene acceptors (NFAs) have received tremendous attention due to their significant contributions to the field of organic solar cells (OSCs). However, the synthetic complexities of large fused-ring conjugated structures in typical NFAs limit their commercial applicability. Thus, a low-cost design strategy with synthetic simplicity and admirable absorption characteristics is highly desirable. In the present work, we report the synthesis of two nonfused NFAs, namely, 2,2'-((([1,2,5]thiadiazolo[3,4-c]pyridine-4,7-diylbis(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b']dithiophene-6,2-diyl))bis-(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (WHC-1) and 2,2'-(((benzo[c][1,2,5]thiadiazole-4,7-diylbis (4,4-bis(2-ethylhexyl)- 4H-cyclopen ta [2,1-b: 3,4-b']dithiophene-6, 2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (WHC-4), based on altering the central acceptor core (A') with a pyrido- or benzothiadiazole (PT/BT) ring, keeping identical donor (D) bridges and end-capping acceptors (A's). The tuning of the central core with PT/BT altered the optical properties, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy levels, and fine-tuning in the molecular crystallinities. Moreover, a significant difference in the photovoltaic performance with decent power conversion efficiencies of 6.6% and 9.3% was achieved by WHC-1 and WHC-4, respectively, using a well-known polymer donor PM6. The results demonstrated the utility of A-D-A'-D-A type nonfused NFAs with broadband absorption spanning from 300 to 960 nm for high-performance OSCs. Furthermore, the scope of the development in A-D-A'-D-A type NFAs using appropriate building blocks can lead to a great breakthrough in OSCs.