Modulating the Electrical Properties of Organic Heterojunction Devices Based On Phthalocyanines for Ambipolar Sensors.

Although ambipolar materials are highly studied in organic electronics, they are rarely used in gas sensors. In the present work, we studied ammonia sensing on organic heterojunctions in a bilayer configuration composed of octachlorinated metallophthalocyanines (M(Cl8Pc); M: Co, Cu, and Zn) as a sublayer and lutetium bis-phthalocyanine (LuPc2) as a top layer. Despite the small effect of metal atom in M(Cl8Pc) on the device current and the interfacial energy barrier, a strong effect on the NH3 sensing behavior was found such that Co(Cl8Pc)-, Cu(Cl8Pc)-, and Zn(Cl8Pc)-based devices exhibited n-type, p-type, and ambipolar charge carrier transport, respectively. Variable carrier transport has been explained by charges hopping at the interface and subsequent heterojunction formation. In particular, the ambipolar transport regime in Zn(Cl8Pc)-based devices is triggered by the chemical doping from NH3 and water when the device is exposed longer under NH3 at high humidity turning it n-type. Gas sensing studies performed in a wide concentration range of NH3 at a variable relative humidity (rh) exhibited very high sensitivity of these devices. The best performance is obtained with Co(Cl8Pc)-based devices demonstrated by a very high relative response (13% at 10 ppm NH3) and sensitivity (1.47%.ppm-1), sub-ppm limit of detection (250 ppb), and negligible interference from rh. Such superior sensing characteristics based on a new heterojunction device make it an ideal NH3 sensor for real application.

Solvent induced aggregation of protoporphyrin and octacarboxylphthalocyanine of zinc deposited on gold surface.

In this paper, we studied the influence of solvent on the morphology of zinc protoporphyrin and zinc octacarboxylphthalocyanine films transferred onto gold surface by dipping. In these films, carboxylic acid groups borne in periphery of macrocycles allow anchoring to gold via ionic interaction. First, we followed by UV-Visible absorption spectroscopy the solvation state of these conjugated macrocycles in pure DMF, in pure ethanol and in various ethanol/DMF mixtures. We show that the increase in ethanol proportion promotes interactions between macrocycles. Second, molecular layers of macrocycles spontaneously adsorbed from these various solutions onto gold surface were analyzed by ellipsometry, water contact angle measurements, UV-Visible absorption spectroscopy and atomic force microscopy. Results evidenced the layers were mainly composed of grains whose size of a few nanometers was directly related to the solvation conditions of molecules. In addition, Q band splitting was observed in the absorption spectrum of zinc octacarboxylphthalocyanine grain films which indicates specific organization of those molecules. Therefore solvent is shown to have a profound influence on the nanostructuration of as-prepared macrocycle layers on gold surface by promoting pre-organization in solution, and its composition enables to better control the morphology of those films by tuning the solubilization of macrocycles.