Rubber-Toughened Organic Solar Cells: Miscibility–Morphology–Performance Relations

Tough organic solar cell (OSC) active layers are necessary to achieve robust, flexible, and stretchable devices. A major challenge is that the brittle small molecule acceptor (SMA) in polymer/SMA bulk heterojunctions results in films prone to mechanical failure. To improve mechanical toughness, we investigate the use of a thermoplastic elastomer (styrene-b-ethylene-butylene-styrene) (SEBS) as an additive in high-performance photoactive layers. We find a consistent transition of all measured parameters [e.g., fracture energy (Gc) and power conversion efficiency (PCE)] at a SEBS concentration of 5–10 wt %, which is driven by the miscibility of the SEBS. We use this insight to optimize the SEBS loading for PCE and toughness. Optimized OSCs are found to increase Gc significantly with a marginal decrease in PCE, resulting in a record Gc·PCE metric, considering all OSC photoactive layers. The pronounced miscibility–function relationship provides a framework to optimize elastomer addition in OSCs for performance and toughness.