Impact of Elastomer Composition on Rubber-Toughened Organic Solar Cell Performance

Adding a thermoplastic elastomer to an organic solar cell (OSC) active layer has been shown to improve its mechanical resilience while largely maintaining the optoelectronic performance. However, a guide to select from the many available elastomers is needed for systematic optimization. Here, we delineate how the structure and composition of the elastomer impact the active layer morphology, fracture toughness, and OSC performance when applied in a high-efficiency polymer/small-molecule acceptor active layer. We compare styrene-ethylene-butylene-styrene (SEBS) and styrene-ethylene-propylene-styrene (SEPS) elastomers with varying molecular weight (M_w) and styrene content. The morphology was probed in detail by resonant soft X-ray scattering (RSoXS) and grazing incidence wide angle X-ray scattering (GIWAXS). We reveal that the compliant elastomer is more impactful than M_w in improving the fracture toughness. The compliance increases when the styrene content is reduced and when the EP block is employed instead of the EB block. In addition, the improved miscibility of the low-styrene-content SEPS leads to the least disruption of the molecular packing, resulting in maintaining power conversion efficiency (PCE) the best. These factors lead to the emergence of a Pareto front along the fracture toughness and OSC efficiency dimensions. The optimal combination of mechanical resilience and OSC efficiency was found when incorporating a mass fraction of 7.5% low-styrene-content SEPS in a PM6/BTP-eC9 active layer. Such OSCs maintained nearly 95% of the neat binary active layer PCE while more than tripling the fracture energy. Based on these results, guidelines for elastomer selection and loading are provided to advance rubber-toughened OSCs.