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.