Effects of water in phthalocyanine layers

The molecular water concentration inside zinc phthalocyanine (ZnPc) thin films was measured. After exposure to air, gas effusion experiments show that the ZnPc layers contain (1.7±0.4)×10²⁰ water molecules per cm³, which corresponds to 1 H₂O per 10 ZnPc units. We can distinguish a mobile and an immobilized population of H₂O in ZnPc films. The mobile part effuses out at room temperature when exposing the films to a low pressure of 10⁻² mbar, whereas temperature activation is needed to reach a complete out-diffusion of water. The effusion process was observed to proceed with a diffusion coefficient D_H2O of (1.3±0.3)×10⁻¹⁰ cm² s⁻¹ at 296 K. The rate of water effusion directly correlates with the timescale of the decrease of surface conductivity when exposing the layers to an equally low pressure. This indicates the existence of an electrically active surface layer of water molecules, which is refilled from the bulk of water molecules during the effusion process.