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Developing novel unfused building blocks with simple synthesis and low cost is essential to advance cost-effective polymer donors in the field of organic solar cells, however, remains challenging. Herein, a class of low-cost and fully unfused polymer donors with precisely regulated backbone planarity via halogenation was designed and synthesized, which possess a four-step synthesis route with over 80% yield from cheap raw chemicals comparable to existing low-cost polymer donors, such as PTQ10. More details are discussed in the article by Li et al. on page 752—759.