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

The length and conformation of solubilizing side chains in pi-conjugated polymers impact polymers' self assembling properties in thin-film devices. To reveal the effect of different bisallcylthio 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 (lambda(max)) 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 these with other polymers. The devices blending PBDTT-DS2-BTff with PC71BM exhibited a highest PCE of 5.82%, with an enhanced J(Sc) of 11.06 mA cm(-2), a V-OC of 0.92 V and a FF of 57.2%. This work demonstrates that suitable side chain lengths in pi-conjugated polymers are of great significance in optimizing the performance of polymers based solar cells. (C) 2016 Elsevier Ltd. All rights reserved.