(Invited) Conjugated Polymer Brushes: A New Platform to Investigate Intramolecular Charge Transport?

Abstract

The intra-molecular transport along conjugated polymers has long been obscured by the dominant inter-molecular transport. To understand this important aspect of conjugated polymers, we have grown poly(3-methylthiophene) polymer brush films on indium tin oxide up to 120 nm thick. The morphological, structural, and electrical properties of such P3MT films were investigated. Optical and X-ray spectroscopy experiments indicate that the polymer chains have a nearly isotropic average orientation at low (< 10 nm) and high (> 30 nm) thicknesses, with mild vertical anisotropy in between. Atomic force microscopy shows that the film surfaces consist of column-shaped domains with an average cross-sectional area (2.3 × 10-3 µm2) and density (200 µm-2) that are independent of film thickness. Analysis of the contact between printed Au electrodes and the polymer brush film shows that a small fraction of the film surface (i.e., the tallest columns) makes contact with the electrodes. The measured bulk resistivity along the columns is 1.4 × 105 Ω·cm, two orders of magnitude lower than typical values for spuncast poly(3-alkylthiophene) films, while the resistance along individual polymer chains in the columns is estimated to be 360 GΩ/nm per molecule, comparable to molecular wires that exhibit charge transport by intramolecular processes. The enhanced conduction is likely due to additional intramolecular transport pathways enabled by the electrode-polymer brush-electrode device architecture, establishing conjugated polymer brushes as a platform for studying the interplay between synthesis, morphology, and charge transport phenomena. Figure 1