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A series of four polymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole (2FBT), PBDT2FBT, PBDT2FBT-O, PBDT2FBT-T, and PBDT2FBT-T-O, are synthesized with their four different side chains, alkyl-, alkoxy-, alkylthienyl-, and alkoxythienyl. Experimental results and theoretical calculations show that the molecular tuning of the side chains simultaneously influences the solubilities, energy levels, light absorption, surface tension, and intermolecular packing of the resulting polymers by altering their molecular coplanarity and electron affinity. The polymer solar cell (PSC) based on a blend of PBDT2FBT-T/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibits the best photovoltaic performance of the four PBDT2FBT derivatives, with a high open-circuit voltage of 0.98 V and a power conversion efficiency of 6.37%, without any processing additives, post-treatments, or optical spacers. Furthermore, PBDT2FBT-T-O, which has a novel side chain alkoxythienyl, showed promising properties with the most red-shifted absorption and strong intermolecular packing property in solid state. This study provides insight into molecular design and fabrication strategies via structural tuning of the side chains of conjugated polymers for achieving highly efficient PSCs. [ABSTRACT FROM AUTHOR]

Structural optimization of the side chains of conjugated polymers is essential for highly efficient organic solar cells. In article number 1400087, Kilwon Cho and co‐workers describe the importance of side chains by investigating polymer solar cells based on PBDT2FBT low‐bandgap polymers containing four types of side chains. The effects of the side chains on the optoelectronic and morphological properties of the bulk‐heterojunction photoactive layer are explored and found to be strongly correlated with the power conversion efficiency of the organic photovoltaic devices. [ABSTRACT FROM AUTHOR]