Role of Oxygen in the Space Charge Layer Formation in Organic-Metal Junctions: Trans-Polyacetylene and Metallo-Phtha-Locyanine

Abstract

Junctions based on trans-polyacetylene and different metallo-phthalocyanines have been studied. The dark conductivity as a function of the frequency and of the applied voltage of sandwich cells of the type M₁/t-(CH)_x/M₂ or M₁/MPc/M₂ demonstrate that these different organic materials present very similar properties. If all these compounds show an apparent Schottky behavior, the standard model cannot be applied. The properties of the space charge region below the surface charge layer are dramatically different from those found in monocrystalline inorganic semiconductors (constant thickness, constant capacity and variable equivalent resistance on applied voltage). The O₂ adsorption is clearly shown to be responsible of the formation of the depletion layer. Indeed O2 not only acts as a dopant of t-(CH)_x and MPc but is also responsible for the formation of the space charge region near blocking electrodes with trapped charge carriers. The I-V relationship seems to indicate Frenkel Poole mechanism of charge detrapping.