Surface Segregated Monolayers of Fluoroalkylated Rod-like Molecule for Asymmetric Charge Transport at Organic Semiconductor Interfaces

A rod-like molecule composed of biphenyl and a fluoroalkyl chain was synthesized, and its surface segregation behavior was investigated. This molecule spontaneously organized into a dense two-dimensional monolayer, termed surface segregated monolayer (SSM), on the surface of poly(3-hexylthiophene) (P3HT) film during spin-coating. The formation of SSM with high density and specific molecular orientation was confirmed by X-ray photoelectron spectroscopy, water contact angle measurement, and atomic force microscopy. The orientation of the molecular dipole moments within the SSM significantly increased the ionization potential, which was attributed to a vacuum level shift at the surface, as detected by ultraviolet photoelectron spectroscopy. The introduction of the SSM at the interface of organic semiconductor films led to an interfacial vacuum level shift, which could cause asymmetric current–voltage characteristics in the vertical hole conduction that varied with the direction of the interfacial dipole moments.