Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1,2,5]thiadiazole Unit for Polymer Solar Cells

We designed and synthesized two isomeric nonfullerene acceptors, IFBR-p and IFBR-d. These molecular semiconductors contain indacenodithiophene (IDT) as the central unit, adjacent asymmetric 5-fluorobenzo[c] [1,2,5]thiadiazole units, and are flanked with rhodanine as the peripheral units. The orientation of the two fluorine atoms (proximal, p, or distal, d), relative to IDT impacts most severely the film morphologies when blended with the electron-donating polymer PTzBI. Polymer solar cells based on PTzBEIFBR-p give rise to a power conversion efficiency (7.3 +/- 0.2%) that is higher than what is achieved with PTzBI:IFBR-d (5.2 +/- 0.1%). This difference is attributed to the lower tendency for (over)crystallization by IFBR-p and the resulting more favorable morphology of the photoactive layer. These results highlight the subtle impact of substitution regiochemistry on the properties of nonfullerene acceptors through modulation of their self assembly tendencies.