Effect of branching-point-extended alkyl chain on structural and electrical properties in NDI-based conjugated polymers

Abstract

Branching point of naphthalene diimide (NDI)-based conjugated polymers was systematically controlled by incorporating four different side chains, i.e. 2-hexyloctyl (PNDI1-T), 3-hexylnonyl (PNDI2-T), 4-hexyldecyl (PNDI3-T), 5-hexylundecyl (PNDI4-T). The effect of branching point engineering on their properties in terms of optical, electrochemical, structural and charge transport ability was investigated. Interestingly, NDI-based polymers showed enhanced intermolecular interaction in solution as branching point became far away from conjugated backbones. As-cast film of P(NDI3-T) and P(NDI4-T) exhibited ordered lamellar diffraction up to third order. Also, the stacking distance of branching-point-extended polymers became shorter (3.8-4.0 A), than that of P(NDI1-T) (4.1 A). After annealing film, intermolecular ordering was gradually developed in both face-on and edge-on directions. Consequently, P(NDI4-T) exhibited the highest electron mobility of 0.58 cm^2 V^{-1} s^{-1} among four different polymers, which is 10 fold higher than that of P(NDI1-T). This results were attributed to strengthened intermolecular interaction and 3-D conduction pathway driven by well-ordered bimodal textures.