Active Layer Morphology Engineering of All-polymer Solar Cells by Systematically Tuning Molecular Weights of Polymer Donors/Acceptors

In all-polymer solar cells (APSCs), number-average molecular weights (M(n)s) of polymer donors and polymer acceptors play an important role in active layer morphology and photovoltaic performance. In this work, based on a series of APSCs with power conversion efficiency of approaching 10%, we study the effect of M(n)s of both polymer donor and polymer acceptor on active layer morphology and photovoltaic performance of APSCs. We select poly[4-(5-(4,8-bis(5-((2-butyloctyl)thio)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b ']dithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-7-(5-methylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole] (CD1) as the polymer donor and poly[4-(5-(5,10-bis(2-dodecylhexadecyl)-4,4,9,9-tetrafluuoro-7-methyl-4,5,9,10-tetrahydro3a,5,8,10-tetraaza-4,9-diborapyren-2-yl)thiophen-2-yl)-7-(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PBN-14) as the polymer acceptor. The M(n)s of polymer donor CD1 are 14.0, 35.5 and 56.1 kg/mol, respectively, and the M(n)s of polymer acceptor PBN-14 are 32.7, 72.4 and 103.4 kg/mol, respectively. To get the desired biscontinueous fibrous network morphololgy of the polymer donor/polymer acceptor blends, at least one polymer should have high or medium M-n. Moreover, when the M-n of polymer acceptor is high, the active layer morphology and APSC device performance are insensitive to the M-n of polymer donor. The optimal APSC device performance is obtained when the M-n of both the polymer donor and the polymer acceptor are medium. These results provide a comprehensive and deep understanding on the interplay and the effect of M-n of polymer donors and polymer acceptors in highperformance APSCs.