UC San Diego

The promise of π-conjugated (semiconducting) polymers is to combine the electronic functionality of semiconductors with the processability and mechanical robustness of plastics. However, most research done on the mechanical properties of polymeric semiconductors (e.g., for organic photovoltaics, thin-film transistors, wearable sensors) has had the underlying goal of increasing the “stretchability”: the deformability and softness. Yet, softness is the wrong characteristic for many applications envisioned for organic semiconductors, including electronic interconnects, conductive paints and coatings, conductive adhesives, touch screens and displays, electronic textiles and fabrics, chemical sensors, and distributed sources of solar energy at risk of damage by indentation, scratching, and abrasion. For example, for organic photovoltaics, solar cells that can be integrated into surfaces already modified by human artifacts (e.g., rooftops, roads, and painted outdoor surfaces) comprise a greater potential source of renewable energy than the niche uses envisioned for portable and wearable solar cells. As such, a focus on modulus and ultimate extensibility—properties characteristic of “stretchability”—at the expense of mechanical robustness (e.g., strength, toughness, elasticity, and adhesion)—leaves many potentially lucrative applications on the table. Thus, the mechanical performance of a semiconducting polymer film must be tailored to the mechanical requirements for the device in which it is incorporated. Likewise, a semiconducting polymer film must also serve a structural role, as a glue that holds the device stack together; it must have sufficient cohesive and adhesive energy to retain electronic function of the device even when deformed. As such, the polymeric semiconductor also must function as an adhesive, despite this not being the primary function. This dissertation discusses two methods for controlling the mechanical properties of semiconducting polymer films that are common to conventional polymers, yet rarely applied to conjugated polymers. The first is the processing of the polymer film using continuous deposition methods common for industrial manufacturing of plastic materials. The second is the use of a crosslinker to introduce covalent bonds between polymer chains (common for commercial rubbers and elastomers). Finally, this dissertation discusses how changes to the side chain length of a model conjugated polymer affect the adhesive function of the polymer film.