Dynamic Mechanical Analysis of Bulk-Heterojunction Active Layers Using a Kirigami-Inspired Substrate Support to Gain Insights into the Mechanical Stability of Organic Solar Cells

Current state-of-the-art organic solar cells (OSC) has an active layer comprised of a conjugated polymer-small molecule acceptor (SMA) blend. The material selection and local morphology is not only critical for power conversion efficiency, these factors also dictate the thermomechanical behavior of the film. Understanding the thermomechanical behavior of the active layer is important to understand the expected mechanical stability as well as inform processing strategies and device stability. Given the films are typically quenched into non-equilibrium morphology, it is important that the mechanical behavior is characterized under these same conditions. Here, we introduce a kirigami-inspired thin-film (KIT) test method to test the dynamic mechanical behavior of bulk heterojunction films in device relevant morphologies. In this approach a kirigami inspired substrate is used to support that active layer. This substrate has a cut pattern that effectively transfers the applied load to the film of interest. Using this method we show that the thermomechanical behavior of the active layer is dependent on processing conditions which has important implications for mechanical stability. We then use this method to highlight a number of thermal relaxation events of neat polymers, SMAs and polymer:SMA blends. In particular, we show how the SMA structure impacts the polymer relaxation behavior and how it ultimately relates to mechanical stability.