Semicrystalline Unfused Polymer Donors with Backbone Halogenation toward Cost-Effective Organic Solar Cells^†

Developing novel unfused building blocks with simple synthesis and low cost is essential to advance and enrich cost-effective polymer donors; however, it remains a challenge due to the lack of efficient molecular strategies. Herein, a class of low-cost and fully unfused polymer donors with precisely regulated backbone planarity via halogenation was designed and synthesized, namely PDTBTBz-2H, PDTBTBz-2F, and PDTBTBz-2Cl. These polymer donors possess a four-step synthesis route with over 80% yield from cheap raw chemicals comparable to existing low-cost polymer donors, such as PTQ10. Benefitting from the planar backbone via incorporating the F···S non-covalent interactions, PDTBTBz-2F exhibits more robust J-type aggregation in solution and a long-ranged molecular stacking in film relative to PDTBTBz-2H and PDTBTBz-2Cl. Moreover, the systematical study of PDTBTBz-based organic solar cells (OSCs) reveals the close relationship between optimized molecular self-assembly and charge separation/transport regarding backbone halogenation when paired with the non-fullerene acceptor (Y6-BO-4F). As a result, the photovoltaic devices based on semicrystalline PDTBTBz-2F achieved a promising power conversion efficiency (PCE) of 12.37%. Our work highlighted the influence of backbone halogenation on the molecular self-assembly properties and a potential unfused backbone motif for further developing cost-effective OSCs.