Achieving highly efficient, mechanically robust and thermally stable organic solar cells through optimizing the branching position and side chain length of small molecule acceptors

Abstract

Achieving high efficiency, mechanical robustness and long-term stability is crucial for the 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 their branching positions on the pyrrole rings and branched alkyl chain lengths attached to the branching position. We systematically investigated the impact of the side chain on the photoelectric performance, mechanical properties and operational stability of OSCs. In particular, the BTP-EH blend film exhibited more ordered packing and stronger crystallinity than the BTP-C3 blend film, offering efficient charge transport and higher power conversion efficiency (PCE), while BTP-HD with longer side chains showed enhanced miscibility with the 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 PCE of 15.6%, which is among the highest values reported to date for is-OSCs. Additionally, the BTP-EH based device maintained over 80% of its initial PCE at 85 °C for ∼780 h. Our findings underscore the importance of the side chains of SMAs in the efficiency, mechanical stretchability and stability of OSCs.