Regioregular dithienosilole- and dithienogermole-based small molecules with symmetric distal/distal orientation of F atoms

This study describes the synthesis and characterization of a series of regioregular dithieno(3,2-b;2′,3′-d)-silole (DTSi)- and dithieno(3,2-b;2′,3′-d)-germole (DTGe)-based small molecules—7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(5-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTSi(BTFTh₂)₂), 7,7′-(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(5-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTGe(BTFTh₂)₂), 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bi-s(4-(benzofuran-2-yl)-5-fluorobenzo[c][1,2,5]thiadiazole) (DTSi(BTFBFu)₂), and 7,7′-(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(4-(benzofuran-2-yl)-5-fluorobenzo[c][1,2,5]thiadiazole) (DTGe(BTFBFu)₂) (DTSi(BTFTh₂)₂, DTGe(BTFTh₂)₂, DTSi(BTFBFu)₂, and DTGe(BTFBFu)₂ in which two F atoms are in a symmetric distal/distal configuration relative to the central core units. The optical and electrochemical properties are strongly influenced owing to changes of the terminal units (BFu and Th₂) rather than by the tuning of the core units (DTSi and DTGe). In addition to a high fraction of the edge-on orientation in the neat films of all of the molecules, the presence of hexyl side chains in the Th₂ terminal units are found to play a critical role in forming high-quality films, leading to homogeneous and smooth surfaces in the films of Th₂-containing molecules. When used as donor materials in small-molecule:fullerene solar cells, the best power-conversion efficiency of 3.26% was achieved with the DTSi(BTFTh₂)₂-based system. Our study herein provides a relevant step toward understanding such isomorphic small molecules.