Core-to-core overlap enables efficient interchain charge transport beyond crystalline domains in a conjugated polymer: high fill factors in thick organic photovoltaic cells.

Recent advances in π-conjugated polymers have achieved high charge carrier mobilities by enhancing intrachain transport, particularly in devices such as organic field-effect transistors (OFETs), where charge transport occurs predominantly in-plane. However, for vertical devices such as organic photovoltaics (OPVs) where out-of-plane transport is required, efficient interchain charge transport becomes essential yet remains challenging. We previously reported a π-conjugated polymer PTNT2T, incorporating a newly developed π-extended dithienonaphthobisthiadiazole (TNT) unit, and primarily attributed its high charge carrier mobility in OFETs-despite its low crystallinity and wider π-π stacking distance-to enhanced intrachain transport arising from backbone rigidification. In this study, we demonstrate that, in addition to this intrachain contribution, the superior mobility of PTNT2T also arises from enhanced interchain transport enabled by strong core-to-core overlaps between TNT units. Importantly, these molecular-level contacts create intermolecular electronic connectivity that enables charge transport extending beyond crystalline domains into amorphous regions. Notably, PTNT2T exhibits substantially more efficient out-of-plane charge transport than the more crystalline reference polymer. This dual-pathway mechanism leads to exceptionally high fill factors in both fullerene- and nonfullerene-based OPVs, even with active layers as thick as 400 nm. Our findings demonstrate that strong core-to-core overlap enables efficient interchain charge transport even without high crystallinity, offering a new design strategy for π-conjugated polymers.