Influence of Simultaneous Tuning of Molecular Weights and Alkyl Substituents of Poly(thienoisoindigo-<i>alt</i>-naphthalene)s on Morphology and Change Transport Properties

To simultaneously assess the impact of molecular weight (<i>M</i><sub>n</sub>) and alkyl substituent variations of polymers on the structural and optoelectronic properties, herein, we conduct a systematic study of a series of poly­(thienoisoindigo-<i>alt</i>-naphthalene) (PTIIG-Np)-based polymers containing different alkyl substituents (2-hexyldecyl (HD), 2-octyldodecyl (OD), and 2-decyltetradecyl (DT) chains) and <i>M</i><sub>n</sub>’s (low (L) and high (H)). All of the polymers produce almost identical energy levels, whereas their optical spectra show a clear dependence on <i>M</i><sub>n</sub>’s and the alkyl substituents. Interestingly, increasing the alkyl substituent sizes of the polymers steadily increases the lamellar <i>d</i>-spacings (<i>d</i><sub>100</sub>), ultimately leading to a densely packed lamellar structure for PTIIGHD-Np. In addition, both H-PTIIGOD-Np and H-PTIIGDT-Np exhibit larger π-stacking crystallites than the corresponding low-<i>M</i><sub>n</sub> polymers, while for PTIIGHD-Np, their size increases in the low-<i>M</i><sub>n</sub> batch. Ultimately, L-PTIIGHD-Np shows the best hole mobility of 1.87 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> in top-gate and bottom-contact organic field-effect transistors (OFETs) with a poly­(methyl methacrylate), which is nearly 1 order of magnitude higher than other polymers tested in this study. Our results demonstrate that the simultaneous <i>M</i><sub>n</sub> and alkyl substituent engineering of the polymers can optimize their film morphology to produce high-performance OFETs.