New Medium Band Gap Nonfullerene Acceptors Based on 4H-Dithieno[3,2 b:2′,3′-d]Pyrrole with Different Aliphatic Side Chains for Organic Solar Cells

The alkyl side chains in the core in the A-D-A nonfused ring nonfullerene acceptors significantly influence the photocurrent generation and overall power conversion efficiency (PCE) in the organic solar cells (OSCs). Here in, we have designed two A-D-π-D-A unfused NFAs featuring the identical central π-ethynylene linker and BTA terminal acceptor (A) unit but utilizing dithiophenepyrrole (DTP) with different side chains, i.e., octyldodecyl (OD) (NFA-8) and 2-hexyldecyl (HD) (NFA-9). The investigations were focused on their optical and electrochemical properties. When paired with PBDB-T as the donor, the OSCs utilizing PBDB-T:NFA-8 and PBDB-T:NFA-9 attained PCE of about 9.95 and 13.90%, respectively. The improved PCE for NFA-9-based OSCs is primarily attributed to the increased short circuit current and fill factor (FF). The higher short circuit current and FF in the NFA-9-based OSCs could be attributed to the prolonged exciton lifetime for NFA-9, effective separation of exciton into free charge carriers, and their successive collection in the NFA-9-based OSCs. Due to the absorption spectra of these bulk heterojunction active layers being restricted to 810 nm, these NFAs can have a potential for indoor OSCs and guest acceptors for ternary and front subcells for tandem OSCs.