Photoelectrochemical generation of electronically conducting polymer-based hybrid junctions on modified Si(111) surfaces

The modification of Si(111) surfaces with Si-C linked alkyl monolayers terminated by electrochemically polymerizable di(2-thienyl)carbinol moieties has been investigated by different spectroscopic techniques including IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-resolution electron energy loss spectroscopy (HREELS). These surfaces were prepared from the photochemical reaction of ethyl undecylenate with hydrogen-terminated Si(111), followed by the conversion of the produced terminal ester groups into the tertiary alcohol using 2-thienyllithium. Despite the steric constraints induced by the aromatic rings in the monolayer, the absolute surface coverage of the organic chains was found to be high, ca. 40%. Differential capacitance measurements revealed that the derivatization procedure introduced surface states probably due to some oxidation of the Si(111) surface occurring during the photochemical step. The photopotentiodynamical oxidation of the modified Si(111) surfaces in the presence of thiophene yielded strongly adherent and smooth conducting polythiophene films. These photoelectrogenerated hybrid polythiophene-silicon junctions were then characterized by ex situ electrical measurements (dc transport, ac impedance, and surface potential measurements). The expected diode behavior due to the formation of a Schottky barrier at the alkyl/Si(111) interface was clearly observed although the rectification was rather weak and the characteristics highly nonideal. The surface states at the silicon interface were found to play a major role in determining the electrical properties of such junctions.