Ethylene oxide side chains and rhodanine end-groups incorporated in a star-shaped cathode interface molecule for high-performance organic solar cells

Cathode interface layers (CILs) are pivotal for optimizing the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we report novel non-ionic, CIL molecules—linear L1 and multi-arm star-shaped L2 featuring rhodanine terminals—as high-performance alternatives to dominant perylene diimide (PDI) and naphthalene diimide (NDI) derivatives. Synthesized via an atom-economical direct C H arylation, L1 and L2 incorporate 1,4-dibromobenzene and 1,3,5-tribromobenzene cores, respectively, coupled to cyclopentadithiophene arms functionalized with solubilizing alkoxy chains. These materials offer dual advantages over their alkyl-chain analogs: significantly enhanced alcohol-solvent solubility and effective reduction of the cathode work function. The star-shaped L2 exhibits superior electronic properties, including a deeper HOMO level (−5.66 eV) that effectively suppresses hole migration to the cathode. In PM6:Y6-based OSCs, L2 achieves a PCE of 17.54 %, significantly outperforming the benchmark PDINO (15.54 %). Furthermore, it maintains a high PCE of 17.41 % in PM6:BTP-eC9 systems, demonstrating broad applicability across different active layers. This work establishes the innovative design principle of CIL materials: strategic control of the core-arm geometry combined with polar side-chain engineering. This synergistic approach enables concurrent optimization of electronic properties and interfacial morphology, leading to the enhanced OSC performance.