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

Comprehensive Summary 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.