Polymer Conformation and Persistence Length Govern Molecular Weight-Dependent Photovoltaic Performance in Organic Solar Cells

The molecular weight of conjugated polymers profoundly influences organic solar cell (OSC) efficiency, yet the structural mechanisms driving this relationship remain unresolved. Here, it is demonstrated that polymer conformation, governed by backbone rigidity and persistence length, is the critical determinant of molecular weight dependent performance in bulk-heterojunction OSCs. By analyzing the benchmark polymers PM6 and D18, opposing trends is uncovered: PM6 achieves peak power conversion efficiency (PCE) of 19.11% at low number-average molecular weight (M_n∼51 kDa), 18.47% at medium-M_n (68 kDa), and declining to 16.83% at high M_n (122 kDa), whereas D18 improves from 16.87% (41 kDa) to 17.75% (67 kDa) and 19.15% (83 kDa). Neutron scattering and computational modeling reveal that D18's bulky 2-butyloctyl side chains impose high rotational barriers, stiffening the backbone and extending its persistence length. This rigidity enables high-M_n D18 to form ordered crystalline domains that enhance charge transport. In contrast, PM6's flexible backbone shortens its persistence length, driving amorphous tie-chain formation at high M_n that disrupts crystallinity and exacerbates recombination. These results establish polymer conformation and persistence length as universal descriptors linking M_n to microstructure, resolving long-standing contradictions in polymer design.