Molecular Weight Engineering of D18 Polymer Enables 20.55% Efficiency in Non-Halogenated Solvent-Processed Organic Solar Cells via Controlled Film Formation Kinetics

• • D18 polymers with molecular weights from 41.6 kDa to 70.9 kDa were synthesized; the medium molecular weight (D18-M) achieved a PCE of 20.55%, among the highest for non-halogenated solvent-processed OSCs. • • Crystallization kinetics analysis shows that lower molecular weight accelerates film solidification, causing excessive aggregation, while higher molecular weight slows it, leading to insufficient phase separation; the optimal balance yields favorable morphology. • • Energy loss evaluation reveals that blend film emission is more dependent on exciton environment (morphology) than on intrinsic polymer luminescence, indicating that non-radiative loss is influenced by film morphology. • • The work provides a simple molecular weight tuning strategy to control film formation kinetics, enabling high-efficiency processing from non-halogenated solvents, which is critical for scalable production.