Decoding the halogenation cost-performance paradox in organic solar cells

The power conversion efficiencies of organic solar cells have now surpassed 20%, marking a considerable advance in performance. This progress raises an important question: which molecular or macromolecular modifications contribute most effectively to efficiency gains? Among these, halogenation — specifically fluorination and chlorination — has been a key driver of performance improvements, making it a particularly promising avenue for materials exploration. In this Perspective, we provide a comparative discussion of a broad range of non-halogenated and halogenated building blocks, acceptors and donors, evaluating the impact of halogenation on efficiency and scalability. We also examine critical challenges, including organic solar cell durability, large-scale manufacturability and the realistic costs associated with halogenation, positioning it as a central factor in performance optimization.