Organic solar cells (OSCs) that combine high photovoltaic efficiency with mechanical resilience are critical for wearable devices. However, prevalent acceptors often act as stress concentrators, leading to film embrittlement. A general toughening approach remains elusive. Here, a broadly applicable strategy is introduced using SEEPS, an elastomeric agent with finely‐tailored miscibility with the acceptor to toughen OSCs. A toughening parameter η , derived from dynamic mechanical analysis is defined, that quantitatively correlates with elastomer‐acceptor miscibility with mechanical enhancement. SEEPS induces pronounced secondary relaxations that dissipate strain energy, yielding an over 11‐fold increase in fracture strain. In situ grazing‐incidence X‐ray scattering reveals that SEEPS preserves molecular packing and suppresses phase separation under strain. The resulting intrinsically stretchable OSCs retain four‐fifths of starting efficiency after 500 stretch‐release cycles at 40% strain, and sustain its four‐fifths efficiency at 52% strain. This work achieves record‐breaking efficiency over 16% while preserving exceptional mechanical stretchability, offering insights for high‐performance stretchable photovoltaics.
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