Precise Side‐Chain Engineering Optimizes Polymer Pre‐Aggregation and Crystallinity for Efficient Organic Solar Cells With Minimized Non‐Radiative Energy Loss

ABSTRACT Minimizing energy loss ( E loss ) to achieve high open‐circuit voltage ( V OC ) is essential for improving the efficiency of organic solar cells (OSCs). In addition to non‐fullerene acceptors, aggregation‐caused quenching in linear polymer donors also contributes to E loss . Although polymer donors with strong aggregation characteristics are beneficial for enhancing crystallinity and improving charge transport, such strong aggregation often leads to increased non‐radiative recombination losses (Δ E 3 ). Therefore, precisely optimizing crystallinity and aggregation is essential for reducing E loss while maintaining efficient charge mobility. Here, we designed and synthesized a series of wide‐bandgap polymer donors (P1–P6) based on chlorinated benzodithiophene (BDT) donor unit and diester‐functionalized thieno[3,2‐ b ]thiophene acceptor moiety (TT‐Th). By systematically optimizing the alkyl side chains on both the BDT and ester‐thiophene units, we achieved precise control over pre‐aggregation behavior. Our results demonstrate that extending the side chains on the TT‐Th unit progressively reduces polymer pre‐aggregation and Δ E 3 , but simultaneously weakens crystallinity and increases π–π stacking distance, thereby compromising charge transport. Among P1–P5, P4 with 2‐butyloctyl side chains exhibited the best balance between pre‐aggregation and Δ E 3 , yielding the highest efficiency. Further optimization by shortening the BDT side chain to 2‐ethylhexyl in P6 moderately enhanced both pre‐aggregation and crystallinity. Although this led to a slight V OC reduction, the improved charge transport properties enabled a champion efficiency of 15.74% with a low Δ E 3 of 0.22 eV. Notably, the efficiency of 15.74% is one of the highest values reported for D‐A alternating polymers based on ester‐bithiophene units. This work present an effective strategy to optimize pre‐aggregation and crystallinity, offering valuable insights into reducing E loss and enhancing OSC performance.