A Balanced Face-On to Edge-On Texture Ratio in Naphthalene Diimide-Based Polymers with Hybrid Siloxane Chains Directs Highly Efficient Electron Transport

Abstract

Structure property relationships associated with a hybrid siloxane-terminated hexyl chain (SiC6), photophysics, molecular packing, thin-film morphology, and charge carrier transport are reported for two novel naphthalene diimide (NDI)-based polymers; P(NDI2SiC6-T2) consists of NDI and bithiophene (T2) repeating units, while for P(NDI2SiC6-TVT), the (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) units are introduced into the NDI-based backbone. The analysis of the optical spectra shows that the preaggregation of these polymers in solution is highly sensitive to the choice of solvent such that the films prepared by using different solvents can be "tuned" with regard to their degrees and types of the aggregates. In-depth morphology investigations (atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD), and near-edge X-ray absorption fine structure (NEXAFS)) combined with device optimization studies are used to probe the interplay between molecular structure, molecular packing, and OFET mobility. It is found that the polymer films cast as a coating from chloroform (CF) solvent favor a mixed face-on and edge-on orientation, while 1-chloronaphthalene (CN)-cast films favor an almost entirely edge-on orientation, resulting in a difference in mobility between CF- and CN-cast devices. Within this work, the annealed P(NDI2SiC6-T2) device fabricated from CF, despite showing a less densely packed organization, shows the highest electron mobility of up to 1.04 cm(2)/V. s due to a highly balanced face-on to edge-on ratio. This work, for the first time, advances our understanding for how the balanced face-on to edge-on ratio plays a dramatic role in facilitating charge transport, opening a new charge-transport mechanism in electronic devices.