Structure−property relationships associated with a hybrid siloxaneterminated 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.
■ INTRODUCTIONSignificant efforts have been made toward the development of solution-processable polymeric materials for organic field-effect transistors (OFETs) because of their facile processability in solution at a relatively low temperature, which can open a new paradigm in application for flexible electronics and device manufacturing through cost-effective graphic art printing processes. 1−9 Current state-of-the-art polymers have been developed for use in p-channel OFETs with hole mobilities surpassing 10 cm 2 /V·s; 1,10−12 however, the distinct lack of highperformance, ambient-stable, solution processed n-channel OFETs has hindered the development of low cost organic complementary circuits. 13−23 Therefore, not only is the development of reliable n-channel polymeric semiconductors a crucial issue but also a challenge for this type of polymers is a deep understanding and control of the layer morphology in organic electronics.A breakthrough in n-channel polymers occurred with the development of P(NDI2OD-T2) polymer, containing naphthalene diimide (NDI) and bithiophene (T2) repeating units, by Facchetti and co-workers, demonstrating unprecedented OFET characteristics with electron mobility of up to 0.85 cm 2 / V·s in the device architecture. 24,25 Interestingly, P(NDI2OD-...