ABSTRACT
Molecular thin films (MTFs) have conductivities of positive temperature coefficients and generate Schottky barriers or depletion layers upon contact with a metal having an appropriate work function. The formation of Schottky barriers at MTF/metal or MTF/electrolyte junctions has been speculated by photovoltaic effect and rectifying behavior observed at these interfaces [15]. Because of these characteristics, MTFs are often referred to as molecular semiconductors or organic semiconductors [6-9]. Electrical properties of molecular semiconductors have been studied intensively, and most of the films exhibit a p-type or an n-type conductance depending on the chemical structure of the molecule [10,11]. Importantly, various properties of an individual molecule are retained in these semiconducting films, implying that a single molecule in the solid behaves as a minimum active element showing a variety of functionalities, such as absorption and emission of light, catalytic activity, redox property, and so on. A combination of the (semi)conductive nature of the solid and the molecular properties retained in the solid leads to novel applications of these MTFs. However, up to the present time, their electrical and energetic properties had not been fully understood because most traditional measuring techniques developed for inorganic semiconductors were not necessarily applicable to the MTFs due mainly to their high resistivities. Currently, deeper insight into energy structures of MTFs and junction properties at MTF/metal contacts is urgently needed for optimization of molecular (opto)electronic devices. The low processing costs and the
possibility of continued improvement in performance through synthetic variation make MTFs useful materials for a number of applications such as electroluminescent devices, solar cells, light-to-light up conversion devices, and gas sensors [12-18].
