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

Toughorganic solar cell (OSC) active layers are necessary to achieverobust, flexible, and stretchable devices. A major challenge is thatthe brittle small molecule acceptor (SMA) in polymer/SMA bulk heterojunctionsresults in films prone to mechanical failure. To improve mechanicaltoughness, we investigate the use of a thermoplastic elastomer (styrene-b-ethylene-butylene-styrene) (SEBS) as an additive in high-performancephotoactive layers. We find a consistent transition of all measuredparameters [e.g., fracture energy (G (c))and power conversion efficiency (PCE)] at a SEBS concentration of5-10 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 G (c) significantly with a marginal decrease in PCE, resulting in a record G (c)& BULL;PCE metric, considering all OSC photoactivelayers. The pronounced miscibility-function relationship providesa framework to optimize elastomer addition in OSCs for performanceand toughness.