Tuning Molar Mass of the D18 Polymer via Stille Polymerization: Impact on Morphology and Large-Area Blade-Coated Organic Solar Cells

Achieving reproducible, high-performance organic solar cells (OSCs) requires precise control over the molar mass of donor polymers, as it governs film formation, morphology, and charge transport. Here, we systematically investigate the influence of Stille polymerization conditions on the molar mass, optoelectronic properties, morphology, and device performance of the benchmark donor polymer D18. By varying reaction time, catalyst type, and catalyst loading, we access D18 batches with weight-average molar masses (<i>M</i> _w) ranging from approximately 12 to 93 kg·mol^-1. Gel permeation chromatography, UV-vis absorption, cyclic voltammetry, optical microscopy, profilometry, and AFM indicate that higher-<i>M</i> _w polymers exhibit enhanced aggregation signatures, as well as smoother, more compact films, and improved donor-acceptor phase separation compared to low-<i>M</i> _w analogues. Bulk heterojunction OSCs with an inverted architecture (R2R-patterned PET/IMI)/ZnO/D18:Y6:PC₇₀BM/PEDOT:PSS/Ag) are fabricated by blade coating under ambient atmosphere over large active areas (0.55 cm²). Devices based on low-<i>M</i> _w D18 (<i>M</i> _w ≈ 12-14 kg·mol^-1) show poor performance (PCE < 2%), which correlates with low shunt resistance and unfavorable morphology, whereas high-<i>M</i> _w D18 samples (<i>M</i> _w ≈ 83-93 kg·mol^-1) reach power conversion efficiencies of 7.8-8.0%, approaching that of a commercial D18 reference (8.9%). A solvent study further reveals that halogenated solvents (chloroform, chlorobenzene) are required to fully realize the potential of high-<i>M</i> _w D18, while <i>o</i>-xylene yields more homogeneous films and competitive efficiencies primarily for low-<i>M</i> _w material. These findings highlight molar mass control and solvent selection as interdependent parameters for optimizing morphology and device performance in scalable, blade-coated D18-based organic solar cells.