Despite the importance of mechanical compliance in most applications of semiconducting polymers, the effects of structural parameters on the mechanical properties are typically not emphasized during the design and evaluation of the new materials. This paper examines the effects of the length of the alkyl side-chain�from butyl to dodecyl�on the tensile modulus and the brittleness for a series of regioregular poly(3-alkylthiophenes) (P3ATs: P3BT, P3HT, P3OT, P3DDT) and their blends with a soluble fullerene derivative (PC61BM). The tensile modulus decreases significantly with increasing length of the alkyl side-chain, from 1.87 GPafor butyl side chains to 0.16 GPa for dodecyl chains (Figure 1a). The large effect on the mechanical properties from small structural changes agrees well with the calculations from theory. The results enable the selection of ultra-compliant materials for solar cells that can withstand tensile strains up to 10%, at which the popular P3HT:PC61BM cannot (Figure 1b to 1e). Our conclusion suggests a different perspective in optimization of organic semiconductors on the basis of mechanical compliance, as opposed to purely electronic performance, which should be considered for the engineering of flexible and stretchable electronics.

Elasticity and ductility of conjugated polymers and polymer-fullerene composites as a function of molecular structure

247th National Meeting (2014)

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