The fragility of many modern organic semiconductors might limit their applicability in applications demanding mechanical compliance. For example, many pure polymers, polymer-fullerene blends, and the transparent conductor indium tin oxide all fracture at tensile strains of two percent or less on compliant substrates. The design of organic semiconductors that can be deformed significantly would facilitate roll-to-roll production, mechanical robustness for potable applications, conformal bonding to curved surfaces other than cylinders, and would enable large-scale solar farms based on ultra thin organic modules that can survive forces of the outdoor environment.
This paper describes our efforts to understand and control the structural parameters that influence the mechanical properties of modern conjugated polymers. Our conclusions include the effect of the side chain in determining the elasticity, ductility, and adhesion of polymers and their blends with fullerenes, and how this effect can be predicted by theory. Ultra-compliant materials are used for the first time in solar cell that can be stretched and conformed to hemispherical surfaces without damage. We also describe the synthesis of all-conjugated block copolymers whose goal is to maximize both electronic properties and mechanical compliance. A new polymerization based on cross-coupling of conjugated “macromonomers” enables block-copolymer-like materials from step-growth polymerizations. Mechanical and photovoltaic properties of these segmented copolymers are also reported. Our results should inform the engineering of new semiconducting polymers for flexible and stretchable applications.