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

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 (ΔE₃). 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 ΔE₃, 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 ΔE₃, 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 ΔE₃ 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.