Control of Effective Elastomer Density Enables Mechanically Robust and High‐Efficiency Intrinsically Stretchable Organic Solar Cells

Intrinsically stretchable organic solar cells (IS‐OSCs) are highly promising for next‐generation wearable electronics. The incorporation of thermoplastic elastomers (TPEs) provides a cost‐effective strategy to improve mechanical compliance. However, the influence of TPE structural diversity on device performance has been largely overlooked. In this work, the concept of effective elastomer density ( D e ) is introduced as a unified molecular descriptor to quantitatively evaluate how elastomer structures affect IS‐OSC morphology and functionality. It is demonstrated that increasing D e enhances stretchability by inducing domain coarsening and surface roughening in amorphous regions, but simultaneously prolongs exciton lifetimes and suppresses charge extraction and transport. Notably, IS‐OSCs achieve an optimal balance at a critical D e of 1.5 mol m −3 , delivering a high initial power conversion efficiency (PCE) of 14.3% and retaining 80% of the initial PCE at 30.6% strain, representing the best performance reported to date for IS‐OSCs employing the elastomer‐plasticization strategy. This descriptor‐based framework provides a predictive and generalizable guideline for the molecular design of elastomers in stretchable optoelectronic devices.