Solvent-Polarity-Induced Preaggregation for Edge-On Orientation and Enhanced Electron Mobility in NDI-Based Conjugated Polymers

Controlling molecular organization is critical for achieving efficient charge transport in n-type conjugated polymers. Herein, we report a solvent-polarity-induced preaggregation strategy for enhancing electron mobility in the naphthalene diimide–naphthodithiophene (NDI–NDT)-based conjugated polymer, p(NDI-NDT). The introduction of polar auxiliary solvents induced solution-state preaggregation, with the aggregation extent strongly dependent on solvent polarity. UV–vis spectroscopy and atomic force microscopy confirmed progressively enhanced aggregation with increasing solvent polarity and methanol content. Despite the continuous increase in aggregation, the highest electron mobility was obtained at an intermediate methanol content of 10 vol %, representing an improvement of more than 2 orders of magnitude compared with films prepared from chlorobenzene alone. Two-dimensional grazing-incidence X-ray diffraction revealed that moderate preaggregation promoted the formation of highly ordered edge-on molecular packing and maximized the crystalline coherence length, whereas excessive preaggregation suppressed the development of long-range structural ordering. These results demonstrate that charge transport is governed not by the overall aggregation level itself but by the formation of interconnected edge-on domains with high structural coherence. This study provides an effective solution-processing strategy for controlling molecular orientation and enhancing charge transport in n-type conjugated polymers.