ABSTRACT

Semiconducting polymers have always been associated with flexible applications, such as solar cells, active-matrix displays, and biomedical sensors. 1 , 2 Mechanical flexibility of these materials, however, is not automatic. The mechanical properties of semiconducting polymers must be engineered by tuning the chemical structure, molecular weight, processing conditions, and interactions with other materials in the device stack. 3 , 4 Despite the importance of mechanical deformability in essentially all applications of semiconducting polymers, mechanical properties have, until recently, been an afterthought. For example, the mechanical stability of organic solar cells has often been overlooked in favor of improving power conversion efficiencies. However, the development of semiconducting polymers that can endure the rigors of roll-to-roll coating, survive long term against mechanical deformations in the outdoor environment, and withstand packing and transportation in portable devices demands an understanding of their mechanical properties. These properties—including elasticity, extensibility, strength, and toughness—are critically dependent not only on the molecular structure of the materials but on the ways these structures pack in the solid state, which are, in turn, mediated by the conditions of processing. Prediction of the mechanical behavior of materials in a device is confounded by the fact that the properties of materials measured in the laboratory can depend on testing conditions, such as temperature, strain rate, and choice of substrate.