Structure, Thermodynamic and Thermomechanical Characteristics of Semiconducting Polymers: Nomenclature and Property Conundrums

Organic semi-conducting polymer thin films are used in many organic electronic devices and have shown promising results in applications ranging from organic light emitting diodes to solar cells and biosensors. The molecular packing of the polymers controls a number of optical and electronic properties and the thermal and thermomechanical properties are critical for devices stability in blends, processing strategies, and mechanical failure modes. Due to the molecular design, semi-conducing polymers exhibit a continuing range of paracrystalline disorder, yet at the same time, they do not behave like classic paracrystalline materials. In fact, the classic structure and property paradigms and associated nomenclature don’t seem to apply. Classic material design labels such as random coil, rigid rod, hairy rod and bottlebrush seem to be delimiting structural characteristics, with semi-conducing polymers occupying the concept and parameter space in-between these limits, yet with characteristics that appear to be not just linear combination of the characteristics found in the classical limits. We review the current understanding and the various conundrums. For example, paracrystallites in a semi-conducting thin film are usually assumed to have a certain size and shape in three dimensions, which then can have a preferred edge-on or face-on orientation relative to the substrate with a random orientation in-plane. X-ray analysis indicates though that most materials exhibit only two-dimensional paracrystalline platelet ordering and might often resemble a layered glass. Thermomechanical characterization frequently indicate multiple relaxation transitions without the presence of a clear glass transition, yet a high temperature liquid-crystalline-like phase. Furthermore, the phase diagrams of semi-conducting polymers with ‘small molecule’ acceptors such as fullerenes or modern non-fullerene acceptors frequently exhibit reentrant phase boundaries that reflect upper and lower-critical solution temperature characteristics. The totality of information suggests that a new reference frame if not new paradigm and associated nomenclature is required to facilitate understanding and communication about the order, molecular packing, texture, thermomechanical properties, and structure-properties relations in semi-conducting polymers.