Ester-Alkyl Linker-Regulated Molecular Folding in Head-to-Head Dimers Enabling >20% Efficiency in Organic Solar Cells

Dimeric acceptors have demonstrated significant potential for the simultaneous realization of high efficiency, good stability, and even stretchability in organic solar cells (OSCs). However, most dimeric acceptors suffer from compromised efficiency due to insufficient morphological control. Herein, we design and synthesize three head-to-head flexible alkyl-chain-linked dimeric acceptors, CH-E2, CH-E6, and CH-E10, along with their monomeric counterpart CH-E1, by systematically varying the alkyl linker lengths via esterification. This synthetic approach avoids conventional metal-catalyzed coupling reactions, eliminating the need for expensive catalysts and toxic intermediates, such as organotin reagents. Results demonstrate that the linker lengths critically govern molecular conformations, packing motifs, and aggregation behavior. The binary PM6:CH-E6 and ternary PM6:CH-E6:CH-E1 devices, benefiting from favorable film microstructures, enhanced charge carrier dynamics, and reduced E_loss, achieve PCEs of 19.17 and 20.14%, respectively. Furthermore, the flexible alkyl chain linkage inhibits molecular diffusion, thereby stabilizing the active layer morphology under thermal and mechanical stress. Thus, CH-E6-based devices exhibit significantly improved MPP operational stability and thermal endurance. The crack-onset strain (COS) of the PM6:CH-E6 blend film reaches 20%, twice that of PM6:CH-E1.