Explanation of the low oxygen sensitivity of thin film phthalocyanine gas sensors

The conductivity of phthalocyanine (Pc) is affected upon exposure to an atmosphere containing oxidizing molecules like oxygen, ozone, nitrogen oxides or chlorine. This phenomenon finds application in the use of thin phthalocyanine films as resistive gas sensors. Such sensors have shown that the sensitivity to oxygen is orders of magnitudes lower than that for the other oxidizing species. We explain this fact by quantifying the oxygen uptake and doping density of copper-, zinc-, manganese- and free base phthalocyanine films. The oxygen inside the film is found in molecular form as O2. Only the metal phthalocyanines acquire significant doping levels upon exposure to oxygen. The number of charge carriers is four orders of magnitude lower than the uptake of molecular oxygen in the material. It explains the observation that resistive phthalocyanine gas sensors have low sensitivity for the detection of oxygen.