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

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 sp3-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.