Electrical properties of nickel phthalocyanine (NiPc) sandwich devices incorporating a tetracyanoquinodimethane (TCNQ) layer

Metal-substituted phthalocyanines are a class of organic dye materials that are weakly semiconducting. Many of these materials have been investigated for molecular switching, gas sensing and photovoltaic properties. In particular, incorporating a strong electron acceptor such as iodine or tetracyanoquinodimethane (TCNQ) has been shown to substantially increase conductivity. In this work sandwich structures of aluminium-alpha-nickel phthalocyanine (NiPc)-silver have been fabricated by a deposition process that did not break the vacuum from phthalocyanine deposition to electrical characterization. The effect of incorporating TCNQ by spin coating onto the aluminium electrode prior to NiPc deposition is reported. Spin coating is a simple technique for thin film deposition, and results for these devices are found to be similar to previously reported work in which TCNQ was deposited by co-sublimation with the NiPc. The effect of exposure to air is shown to increase the conductivity by an order of magnitude; the conductivities of the devices incorporating TCNQ being approximately 20 times that of the devices without TCNQ. The devices without TCNQ are shown to continue to be doped by the air throughout a 94 h measurement period while the devices with the TCNQ layer are fully doped within 2 h. Thermally stimulated current measurements indicate a thermal activation energy Delta E for devices incorporating TCNQ of Delta E = 1.3 eV and Delta E = 1.9 eV for the non-TCNQ devices.