Programmable Tethered Small-Molecule Acceptors for Efficient, Stable, and Stretchable Polymer Solar Cells

Recently, intensive research on donor-acceptor structured low-bandgap small-molecule acceptors (SMAs) has enabled power conversion efficiencies (PCEs) exceeding 21% in polymer solar cells (PSCs), a record-breaking milestone for the field. This success stems from their broad light absorption and minimized energy losses. For practical application, however, further progress requires simultaneous advances in efficiency, operational stability, and mechanical robustness. In PSCs, SMAs are blended with polymer donors to form a bulk-heterojunction (BHJ) morphology featuring finely mixed domains that facilitate exciton dissociation and efficient charge collection. However, the lower glass transition temperature of SMAs (Tg < 100 °C) compared to that of polymer donors raises concerns about their thermodynamic relaxation and crystallization within the mixed domains, which can drive progressive phase separation and morphological degradation, posing critical challenges for long-term operational stability. Moreover, the high rigidity and crystalline nature of state-of-the-art SMAs that benefit charge transport often embrittle the BHJ film, creating a bottleneck for stretchable device applications. Therefore, a fundamental challenge lies in simultaneously maximizing the PCE while ensuring long-term morphological stability and mechanical flexibility in PSCs. To address these challenges, our research program recently transitioned from individual SMAs to discrete tethered SMAs. In this molecular architecture, two or more SMA subunits are covalently attached via flexible spacers to an aromatic core. This modular design permits independent programming of each component, offering a powerful means to modulate aggregation behavior in both solution and thin films relative to free SMA subunits. Current designs have evolved from simple dimeric structures to more complex topologies including three-dimensional tetramers, dendritic hexamers, and star-shaped dodecamers. Importantly, such designs retain the synthetic precision and batch-to-batch reproducibility of small molecules while introducing polymer-like attributes, including an elevated Tg, suppressed molecular diffusion, and enhanced intermolecular entanglement. In this Account, we provide an overview of our efforts to design programmable tethered SMAs. We illustrate how modular control of the aromatic-core structure, spacer length, and subunit electronic structure enables precise manipulation of the molecular geometry, self-assembly, and intramolecular entanglement for achieving high PCEs while ensuring the morphological stability and mechanical flexibility of PSCs. These factors lead to acceptors delivering high device efficiencies (e.g., 20.2% PCE in a binary device), long operational lifetimes (e.g., T80 ≈ 35,000 h), and superior mechanical flexibility (e.g., 150% enhancement in crack onset strain). Finally, we outline current challenges and future directions for the tethered approach. We emphasize that tethered structures should be viewed not simply as molecular-weight factors governing Tg but as distinct topological entities, especially critical for higher-order structures. We hope this Account will offer broader lessons for the design of next-generation acceptors across multiple length scales, from a mere electronic chromophore to a versatile topological and thermodynamic platform enabling efficient, stable, and stretchable PSCs.