Non-fused core-linked star-shaped oligomer acceptors for stable binary organic solar cells with over 19% efficiency

Star-shaped oligomer acceptors are promising candidates for high-performance and robust organic solar cells (OSCs). However, the limited diversity of this community of acceptors leaves a significant knowledge gap regarding their structure–performance relationship. Herein, we designed two new star-shaped oligomer acceptors, namely 3BY and 3QY by introducing non-fused central units to bridge the Y-acceptor arms. The structural variability of the non-fused cores provides an available platform to finely regulate the aggregation properties of oligomers. In particular, the triazine center of 3QY allows multisite intramolecular non-covalent interactions, which can not only improve the molecular self-assembly, but also refine the pre-aggregation of the polymer donor and film-forming kinetics of the heterojunction blend. Finally, assembled PM6:3QY solar cells realized a very impressive efficiency of up to 19.27%, far outperforming that of PM6:3BY (17.75%) and ranking the highest efficiencies among the reported OSCs based on oligomer acceptors. Meanwhile, the considerable molecular sizes of the star-shaped molecules retard molecular diffusion, affording notable device stability with a large T_80% over 3000 h for PM6:3QY device under thermal stress. This study establishes a reliable structure-performance relationship and demonstrates the great potential of non-fused core-bridged star-shaped oligomers for the fabrication of high-efficiency OSCs with long-term stability.