Effect of Broken Conjugation on the Stretchability of Semiconducting Polymers

Increasing the flexibility of polymer chains is a common method of increasing the deformability of solid polymeric materials. Here, the effects of “conjugation-break spacers” (CBSs)—aliphatic units that interrupt the sp2-hybridized backbone of semiconducting polymers—on the mechanical and photovoltaic properties of a diketopyrrolopyrrole-based polymer are described. Unexpectedly, the tensile moduli and cracking behavior of a series of polymers with repeat units bearing 0%, 30%, 50%, 70%, and 100% of the CBS are not directly related to the percent incorporation of the flexible unit. Rather, the mechanical properties are a strong function of the order present in the film as determined by grazing-incidence x-ray diffraction. The effect of the CBSs on the photovoltaic performance of these materials, on the other hand, is more intuitive: it decreases with increasing fraction of the flexible units. These studies highlight the importance of solid-state packing structure—as opposed to only the flexibility of the individual molecules—in determining the mechanical properties of a conjugated polymer film for stretchable, ultraflexible, and mechanically robust electronics.