ADVANCED MATERIALS Communications Molecular Rectification in Oriented Polymer Structures""

International audiencePolymeric semiconductor devices are receiving increasing attention in view of potential applications requiring low-cost processing over large areas.'"*] In this respect, unlike with evaporated molecules, the wet-processing capability of polymers offers total compatibility with other complemen-tary technologies. The concepts from which organic-semi-conductor devices are designed are mostly derived from in-organic-semiconductor physics and technology.''] In order to build efficient organic-semiconductor devices such as electroluminescent or photovoltaic solar cells, a rectifying junction is required. Such junction can be of two main typed4] Schottky junctions between an organic semi-conductor and a metal, and p-n junctions between two p-and n-type organic-semiconductors. The Schottky junctions often suffer from degradation problems originating from electrochemical potential differences at the rectifying con-tacts, which induce diffusion of electrode material into the polymer.'" Such a drawback is corrected with p-n junctions in which metal to polymer contacts are ohmic. However, p-n junctions require the use of two polymer layers with wet-processing compatibility. Moreover n-type organic-semiconductors are less common than p-type ones owing to their lower stability under oxygen which is a consequence of their reductive behavior. We report here a different approach in which oriented diode-like molecules, the so-called push-pull molecules used in nonlinear optics,[6] are contained inside a polymer binder. Oriented molecules induce a rectifying effect, be-having as a distributed homojunction within a single poly-mer thin film. In order to demonstrate the principle, we start with an initially symmetric structure: the polymer film is sandwiched between two identical electrodes. The cur-rent-voltage characteristics of the device are also symme-trical. By application of a static electric field through the polymer film, while heating near the glass transition tem-perature (T,,) dopant molecules are oriented in the field with an order parameter (cosO).['] The current-voltage characteristics then become strongly asymmetric. Experimentally, molecular order is controlled using sec-ond harmonic generation (SHG): it appears tha