Achieving Highly Efficient, Mechanically Robust and Thermally Stable Organic Solar Cells through Optimizing Branching Positions and Side Chain Length of Small Molecule Acceptors

Achieving high efficiency, mechanical robustness and long-term stability is crucial for practical application of organic solar cells (OSCs). Owing to the crystalline nature of small molecule acceptors (SMAs), high-efficiency OSCs typically exhibit low mechanical stretchability (crack-onset strain, COS <5%). Herein, we synthesized three SMAs: BTP-C3, BTP-EH and BTP-HD, which share an identical dithienothiophen[3,2-b]-pyrrolobenzothiadiazole core but vary in branching position on the pyrrole rings and branching alkyl chains length attached to the branching position. We systematically investigated the side chain impact on the photoelectric performance, mechanical properties and operational stability of OSCs. Especially, BTP-EH blend film exhibits more ordered packing and stronger crystallinity than BTP-C3 blend film, offering efficient charge transport and higher power conversion efficiency (PCE). While BTP-HD with longer side chains enhances miscibility with D18 donor, substantially improving mechanical stretchability. Consequently, the D18:BTP-EH device achieved a high PCE of 18.1% and remarkable mechanical stretchability (COS ~26%). The resultant intrinsically stretchable OSCs (is-OSCs) exhibited a record PCEs of 15.6%, which represent among highest values reported to date for is-OSCs. Additionally, BTP-EH based device maintained over 80% of its initial PCE at 85 °C for ~780 h. Our findings underscore the importance of SMAs’ side chain in the efficiency, mechanical stretchability and stability of OSCs.