Chiral Bifacial Non‐Fullerene Acceptors with Chirality‐Induced Spin Selectivity: A Homochiral Strategy to Improve Organic Solar Cell Performance

Abstract

Abstract Asymmetric design has emerged as an effective strategy to enhance the performance of non‐fullerene acceptors (NFAs) in organic solar cells (OSCs). Although most asymmetric NFAs focus on lateral (i.e., left–right) asymmetry, out‐of‐plane anisotropy has remained underexplored, despite its ability to induce both a vertical dipole moment and molecular chirality, thereby potentially enabling functionalities such as chirality‐induced spin selectivity (CISS). With this perspective, we develop chiral and bifacial NFAs, ( S , S )‐IE4F, and ( R , R )‐IE4F, featuring an indacenodithiophene (IDT) core bearing a hydrophilic oligo(ethylene glycol) group and a hydrophobic phenyl group on the bridging sp 3 ‐carbons. Their racemic counterpart ( rac ‐IE4F) and a non‐bifacial achiral isomer ( meso ‐IE4F) are also synthesized as the controls. Compared to meso ‐IE4F, the bifacial isomers exhibit higher solubility, more favorable molecular packing for carrier transport, and larger dipole moments. Notably, neat ( S , S )/( R , R )‐IE4Fs and their bulk heterojunctions with PBDB‐T exhibit pronounced CISS effect, with spin polarizations of ∼70% and ∼50%, respectively. Consequently, OSCs based on homochiral bifacial IE4Fs achieve a power conversion efficiency of 8.17%, three times higher than that of meso ‐IE4F. This study highlights the impact of isomerization in NFAs and provides a new molecular design strategy for homochiral asymmetric NFAs.