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

To simultaneously assess the impact of molecular weight (M_n) and alkyl substituent variations of polymers on the structural and optoelectronic properties, herein, we conduct a systematic study of a series of poly­(thienoisoindigo-alt-naphthalene) (PTIIG-Np)-based polymers containing different alkyl substituents (2-hexyldecyl (HD), 2-octyldodecyl (OD), and 2-decyltetradecyl (DT) chains) and M_n’s (low (L) and high (H)). All of the polymers produce almost identical energy levels, whereas their optical spectra show a clear dependence on M_n’s and the alkyl substituents. Interestingly, increasing the alkyl substituent sizes of the polymers steadily increases the lamellar d-spacings (d₁₀₀), 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-M_n polymers, while for PTIIGHD-Np, their size increases in the low-M_n batch. Ultimately, L-PTIIGHD-Np shows the best hole mobility of 1.87 cm² V^–1 s^–1 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 M_n and alkyl substituent engineering of the polymers can optimize their film morphology to produce high-performance OFETs.