Effect of Oxyethylene Chains on Characteristics of N-Type Semiconducting Polymers

Polymeric materials have been extensively utilized as electron transport (n-type) materials in organic solar cells instead of conventional fullerene derivatives due to their superior optoelectronic properties and stability. Among them, polymers with a structure comprising fused heterocyclic multi-ring aromatic units, and branched alkyl side chains ensuring solubility have been focused on. However, from the perspective of electron transport, the branched alkyl chains might hinder the stacking of conjugated systems. We proposed the molecular design, where highly flexible oxyethylene groups as side chains are incorporated aiming to achieve an optimal balance between electron mobility and solubility. We synthesized the conjugated polymer, P(NDI2O4C6-T2) consisting of naphthalene dimide as the main chain, and linear hexyl tetra(oxyethylene) group as the side chain (3, 6, 9, 12-tetraoxaoctadecyl group), and evaluated the basic properties including electron mobility. For comparison, analogues with linear octyl and branched 2-octyldodecyl groups, P(NDI2C8-T2) and P(NDI2OD-T2) were also prepared. Consequently P(NDI2O4C6-T2) exhibited the highest electron mobility and solubility in conventional organic solvents. Quantum chemical calculations were also carried out to verify the experimental results based on the intra- and intermolecular interaction in the polymer.