Controlling molecular weight of naphthalenediimide-based polymer acceptor P(NDI2OD-T2) for high performance all-polymer solar cells

A widely-used naphthalenediimide (NDI) based electron acceptor P(NDI2OD-T2) with different number-average molecular weight (Mn) of 38 (N2200L), 56 (N2200M), 102 (N2200H) kDa were successfully prepared. The effect of molecular-weight on the performance of all-polymer solar cells based on Poly(5-(5-(4,8-bis(5-decylthiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b']dithophen-2-yl)thiophen-2-yl)-6,7-difluoro-8-(5-methylthiophen-2-yl)-2,3-bis(3-(octyloxy)phenyl)quinoxaline) (P2F-DE):N2200 was systematically investigated. The results reveal that N2200 with increased Mn show enhanced intermolecular interactions, resulting in improved light absorption and electron mobility. However, the strong aggregation trend of N2200H can cause unfavorable morphology for exciton dissociation and carrier transport. The blend film using N2200 with moderate Mn actually develops more ideal phase segregation for efficient charge separation and transport, leading to balanced electron/hole mobility and less carrier recombination. Consequently, all-polymer solar cells employing P2F-DE as the electron donor and N2200M as the electron acceptor show the highest efficiency of 4.81%, outperforming those using N2200L (3.07%) and N2200H (3.92%). Thus, the Mn of the polymer acceptor plays an important role in all-polymer solar cells, which allows it to be an effective parameter for the adjustment of the device morphology and efficiency.