Abstract:

Double-cable conjugated polymers consist of a donor backbone and acceptor side chains linked by a long, flexible spacer. While this molecular design offers a promising solution to address the thermal instability and phase separation issues inherent in conventional binary blend systems, the development of high-performance double-cable polymers faces multiple significant challenges. The synthetic complexity is considerably high, requiring multi-step functionalization to attach the spacer to the acceptor unit, which often results in low yields and difficulties in purification. Moreover, the selection of suitable acceptor materials is severely limited, as only a narrow range of acceptors possess appropriate sites for spacer attachment while maintaining their intrinsic electronic properties. This limitation greatly restricts the exploration of material combinations and the potential for performance breakthroughs. Another critical yet underexplored challenge lies in the precise engineering of the spacer itself. Although studies have investigated the effect of spacer length, the role of spacer attachment position—a subtle but crucial structural parameter—remains poorly understood. This study designed and synthesized two such polymers, ZP-1 (with the spacer attached at the para-positions of the indenone benzene ring) and ZP-2 (at the ortho-positions), through spacer isomerization engineering. Investigations reveal that shifting the substitution from the ortho-(ZP-2) to the para-position (ZP-1) optimizes favorable molecular packing and enhances film crystallinity. This structural optimization facilitates exciton dissociation and charge transport, resulting in a significant improvement in the short-circuit current density and fill factor for ZP-1-based devices. Consequently, a power conversion efficiency (PCE) of 10.43% has been achieved, outperforming ZP-2 (9.44%). Furthermore, both polymers exhibit excellent thermal stability, retaining over 80% of their initial PCE after 8, 000 h of continuous thermal aging. Notably, when incorporated as a third component into the D18:BTP-eC9 binary blend, ZP-1 effectively optimizes the donor-acceptor interface, leading to a simultaneous enhancement in short-circuit current density (J_SC) and fill factor (FF) and a high PCE of 19.81%. This improvement is attributed to its templating effect during film formation and the promotion of charge generation at the optimized interface. The ternary device also demonstrates significantly enhanced thermal stability compared to the binary counterpart. This work provides valuable insights for the rational design of efficient and stable single-component organic solar cells, as well as novel ternary systems, by highlighting spacer isomerization engineering—specifically, the precise control of linkage position—as a powerful strategy to tailor molecular packing, crystallinity, and ultimate device performance.

Key words: Organic solar cells, Double-cable conjugated polymer, Spacer isomerization engineering, Molecular packing