Revisiting Conjugated Polymers with Long-Branched Alkyl Chains: High Molecular Weight, Excellent Mechanical Properties, and Low Voltage Losses

High-molecular-weight conjugated polymers exhibit excellent mechanical properties due to long-chain entanglement, but it is challenging to increase molecular weight since conjugated polymers with rigid backbones show low solubility and tend to be precipitated during polymerization. In this work, we have successfully obtained high-molecular-weight polymers through the introduction of long-branched alkyl chains and optimization of polymerization conditions. Three conjugated polymers with long 2-octyldodecyl side units were obtained with number-average molecular weights (M(n)s) of 38.8 kDa (OD-PM6_L), 81.2 kDa (OD-PM6_M), and 178.3 kDa (OD-PM6_H), while the conjugated polymer PM6 with short 2-ethylhexyl units showed an M-n of 43.8 kDa. The branched side units and molecular weight have a significant influence on the mechanical properties, in which the elastic ranges and the crack-onset strains were enhanced to >20% and >170% in OD-PM6_H thin films. In addition, due to the suppressed non-nonradiative recombination via increasing the donor-acceptor spacings, organic photovoltaic devices based on OD-PM6_H showed an efficiency of over 13% with low voltage loss. These results demonstrate that introducing longer side units into conjugated polymers provides the opportunity to realize high molecular weight, resulting in excellent mechanical properties and high efficiencies in organic solar cells toward flexible applications.