Beyond Stretchability: Increasing the Strength, Hardness, and Survivability of Semiconducting Polymers by Crosslinking

Essentially all research done on the mechanical properties of polymeric semiconductors for organic photovoltaics has had the underlying goal of increasing the “stretchability”: that is, the deformability and softness. However, softness is the wrong figure of merit for many applications envisioned for organic semiconductors, including distributed sources of solar energy subject to damage by indentation, scratching, and abrasion. A focus on modulus and fracture strain at the expense of strength, toughness, and elastic range – i.e., properties characteristic of hardness and resilience – leaves many potentially lucrative applications on the table. For example, in organic photovoltaics, applications in which materials can be integrated onto surfaces already modified by human artifacts (e.g., rooftops, roads, and painted surfaces) comprise an enormous potential source of renewable energy. Additionally, the predominant focus on tensile properties fail to account for other forms of mechanical injury these devices will face (e.g., compressive forces). For photovoltaic applications that require high mechanical robustness, the mechanical properties of semiconducting polymers must be optimized to improve their strength, toughness, resilience, and hardness while maintaining their stretchability. Crosslinking is a well-understood technique for tuning the mechanical properties of conventional polymers. In this talk, we discuss our progress in increasing the mechanical robustness of semiconducting polymer films by using a four-armed azide-based crosslinker (“4Bx”). We show that low loadings of 4Bx can be used to improve the strength, toughness, and hardness of semiconducting polymer films, while maintaining their stretchability (i.e., fracture strain). Additionally, low loadings of 4Bx can be used to increase the mechanical robustness of the bulk heterojunction in organic solar cells, resulting in devices with increased survivability and stability (i.e., abrasion resistance, thermal ageing, solvent resistance).