Effect of bisalkylthio side chains on benzo[1,2-b:4,5-b′]dithiophene-based polymers for organic solar cells

Abstract

The length and conformation of solubilizing side chains in π-conjugated polymers impact polymers' self-assembling properties in thin-film devices. To reveal the effect of different bisalkylthio side chains, four copolymers containing bisalkylthio side chain modifying two-dimensional benzo[1,2-b:4,5-b′]dithiophene (BDT) donor with (5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BDff) acceptor have been developed and studied in this report. The different absorption maxima (λmax) of PBDTT-DS1-BTff, PBDTT-DS2-BTff, PBDTT-DS3-BTff and PBDTT-DS4-BTff are 578, 574, 591 and 594 nm in CHCl3 solution, respectively, indicating different aggregation behaviors. The devices based on PBDTT-DS1-BTff didn't reveal high PCEs due to its poor miscibility with PC71BM compared to those with other polymers. The devices blending PBDTT-DS2-BTff with PC71BM exhibited a highest PCE of 5.82%, with an enhanced JSC of 11.06 mA cm−2, a VOC of 0.92 V and a FF of 57.2%. This work demonstrates that suitable side chain lengths in π-conjugated polymers are of great significance in optimizing the performance of polymers based solar cells.