Conjugated polymers have unique physical properties imparted by their relatively stiff donor-acceptor backbone and soft hairy side chains. This structural motif can give rise to complex thermomechanical behavior that is difficult to characterize. Yet, the thermal relaxations have important implications for morphological formation and mechanical behavior of electronic devices. In this talk, we will report on the thermal relaxations observed in PBnDT-FTAZ using DMA, and how the relaxation temperatures vary with subtle changes to the molecular structure of the polymer. We will show that there are multiple relaxation features are due to local relaxations such as side-chain relaxations to those that are more cooperative including aggregate relaxation. We will then discuss how the various relaxations impact the stress-strain behavior of the film, and how the observed behavior can be used to guide stretchable polymer semiconductor design. Furthermore, the thermal relaxations inform molecular ordering of films in the solid-state. We show the connection between the observed thermal relaxations and films crystallinity in PBnDT-FTAZ. The talk will then move onto polymer: small molecular acceptor (SMA) blends used in high-performance organic solar cells (OSCs). To study the effect of morphology on relaxation behavior we introduce a kirigami-inspired test (KIT) method for use in a DMA. This approach uses a patterned polyimide substrate that effectively transfers an applied load to the thin film of interest. This allows the study of frustrated relaxations due to a non-equilibrium mixed-phase morphology commonly found in OSCs. We use the KIT method to show how the thermal relaxation behavior influences the mechanical and morphological stability of OSCs.

Thermal relaxations in conjugated polymers and implications for flexible and stretchable device performance

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ACS Fall 2022

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