The effects of oxygen doping and annealing on the surface and bulk electrical conductivity in planar copper phthalocyanine thin films for gas-sensing applications

In the development of gas-sensing devices based on phthalocyanine materials, it has become evident that the current drawn depends on previous exposure to oxygen, which acts as an acceptor impurity, and on the relative importance of the bulk and surface regions. The present work focuses on the effects of oxygen on ct-phase thin films of copper phthalocyanine (CuPc) having a planar configuration. In unannealed samples both ohmic and space-charge-limited conductivity (SCLC) regions were observed in films of thickness 0.1 mu m, whereas for films of thickness 1 mu m, SCLC was not observed below a field strength of 2 x 10(6) V m(-1). These variations may be explained in the context of the differing effects of oxygen on the surface and bulk regions of the films. Exposure to oxygen has the effect of enhancing the conductivity and increasing the threshold voltage between the ohmic and SCLC regions. In the thinner films, samples lightly doped with oxygen had a trap distribution exponentially distributed in energy, whereas long exposure to oxygen resulted in the establishment of a discrete dominant trap level. While annealing was found to reduce the conductivity due to oxygen desorption, the resulting activation energy was found to be dependent on the concentration of oxygen within the films. Observations of oxygen desorption from films of different thickness revealed that the binding energy of oxygen to the CuPc molecule is lower on the surface than in the bulk of the films. It was concluded that the adsorbed or absorbed oxygen molecules resulted in the establishment of a partial charge-transfer complex, and that the hole concentration on the surface of the films is lower than that in the bulk under vacuum. (C) 1998 Elsevier Science S.A.