Balancing Solubility and Efficiency by BDF–FBTz Polymer Donors for Realizing 20% Non‐Halogenated‐Solvent‐Processed Organic Solar Cells

ABSTRACT Organic solar cells (OSCs) are promising next‐generation photovoltaic technologies, but their practical deployment is limited by polymer donors (PDs) that lack both high efficiency and compatibility with non‐halogenated solvents. Herein, we present a molecular design strategy that alleviates the trade‐off between performance and solubility by weakening polarizability and enhancing dipole moments to better match Hansen solubility parameters (HSPs). Guided by this concept, we design benzo[1,2‐b:4,5‐b′]difuran–difluorobenzo[d]thiazole (BDF–FBTz)‐based PDs, namely PBFuT‐F and PBFuT‐Cl, which show optical bandgaps and energy levels complementary to Y‐series non‐fullerene acceptors. PBFuT‐Cl exhibits optimized aggregation, a lower HOMO level, and good face‐on stacking. The PBFuT‐Cl:L8‐BO blends display excellent processability in non‐halogenated solvents (toluene, o ‐xylene, tetrahydrofuran, and dihydropyran), affording power conversion efficiencies (PCEs) of 19.49%, 19.25%, 19.17% and 17.59%, respectively. Moreover, the ternary blend OSCs based on PBFuT‐Cl:L8‐BO:BTP‐eC9 processed from toluene achieve a PCE of 20.30%, which is the highest value so far for BDF‐based PDs. The results demonstrate that BDF–FBTz copolymers can balance solubility and efficiency while broadening the non‐halogenated‐solvent processing window for OSCs.