Modulating π-Spacer Architecture Directs Linear Backbone Conformation and Ordered Molecular Packing toward Efficient Organic Solar Cells

Abstract The development of high-performance polymer donors is essential for advancing non-fullerene organic solar cells (OSCs). While the triazolo[4,5-f]isoindole-5,7(2H,6H)-dione (TzDI) unit has emerged as a promising electron-withdrawing building block, its synthetic accessibility remains limited by poor atom economy, high cost, and highly hazardous reagents. Herein, we develop a facile and low-cost synthetic protocol for brominated TzDI monomers. Furthermore, three D–π–A polymers (PTzDI, PTzDIT, PTzDITT) with the same donor but distinct π-spacers-none, thiophene, or alkylated thieno[3,2-b]thiophene (TT)-were designed and synthesized to elucidate the impacts of molecular structures on backbone conformation, solution aggregation, film-forming dynamics, and film crystalline structures. Density functional theory calculations reveal that the TT-based polymer PTzDITT adopts a more linear chain conformation with a lower rotational energy barrier, endowing it with pronounced solution pre-aggregation and accelerated film-formation kinetics. Consequently, benefiting from more ordered molecular packing and favorable fibrillar network morphology, PTzDITT affords a power conversion efficiency of 16.45% in OSCs, significantly outperforming its thiophene-bridged (12.45%) and spacer-free (2.97%) counterparts. This work not only provides a practical synthetic protocol for TzDI-based materials but also offers a new perspective on the role of conformational flexibility in directing molecular self-assembly for efficient OSCs.