Surface-Segregated Monolayers: A New Type of Ordered Monolayer for Surface Modification of Organic Semiconductors

We report a new type of ordered monolayer for the surface modification of organic semiconductors. Fullerene derivatives with fluorocarbon chains ([6,6]-phenyl-C-61-buryric acid 1H,1H-perfluoro-1-alkyl ester or FCn) spontaneously segregated as a monolayer on the surface of a [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) film during a spin-coating process from the mixture solutions, as confirmed by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) showed the shift of ionization potentials (IPs) depending on the fluorocarbon chain length, indicating the formation of surface dipole moments. Surface-sensitive vibrational spectroscopy, sum frequency generation (SFG) revealed the ordered molecular orientations of the C-60 Moiety in the surface FCn layers. The intensity of the SFG signals from Fc(n) on the surface showed a clear odd-even effect when the length of the fluorocarbon chain was changed. This new concept of the surface-segregated monolayer provides a facile and versatile approach to modifying the surface of organic semiconductors and is applicable to various organic optoelectronic devices.