3D Crystal Framework Regulation Enables Se-Functionalized Small Molecule Acceptors Achieve Over 19% Efficiency

Se-functionalized small molecule acceptors (SMAs) exhibit unique advantages in constructing materials with near-infrared absorption, but their photovoltaic performance lags behind that of S-containing analogs in organic solar cells (OSCs). Herein, two new Se-containing SMAs, namely Se-EH and Se-EHp, are designed and synthesized by regulating bifurcation site of outer alkyl chain, which enables Se-EH and Se-EHp to form different 3D crystal frameworks from CH1007. Se-EH displays tighter π–π stacking and denser packing framework with smaller-sized pore structure induced by larger steric hindrance effect of outer alkyl chain branched at 2-position, and a higher dielectric constant of PM6:Se-EH active layer can be obtained. OSCs based on PM6:Se-EH achieved very high PCEs of 18.58% in binary and 19.03% in ternary devices with a high FF approaching 80% for Se-containing SMAs. A more significant alkyl chain steric hindrance effect in Se-EH adjusts the molecular crystallization to form a favorable nanofiber interpenetrating network with an appropriate domain size to reduce rate of sub-ns recombination and promote balanced transport of carriers. This work provides references for further design and development of highly efficient Se-functionalized SMAs.