Chemical gas sensors based on organic semiconductor polypyrrole

Polypyrrole (PPy) is one of the most studied conducting polymers. It is well known that the mechanical, physical, and chemical properties of PPy strongly depend on the nature of the dopant anion. In this context, on overwiev of the influence of small anions and metallomacrocycles, for example, metallophthalocyanines (MPcTS), on the chemical transducer behavior of the doped PPy layer studied by the Kelvin Probe method is given. The gas-polymer interaction changes the work function of the polymer, which can be explained by a secondary doping of the polymer by the adsorbed gas molecule. Studies have revealed that the central metal ion in MPcTS plays an important role in the chemical sensing properties toward NO2, chlorinated hydrocarbons, and organophosphorous compounds in the low ppm concentration range. Furthermore, the competitive doping method of PPy with a mixture of small anions and MPcTS strongly enhances the selectivity and reduces the response time of the conducting polymer, which are general problems in the use of conducting organic polymers for gas and vapor detection. Chemical sensitive PPy layers have been used to build chemical sensors in which the organic semiconductor works both as the active component in the electronic device structure and as the chemical sensing transducer element, for example, polymer field-effect transistors, PPy/Au-Schottky, and PPy/Si-heterojunction diodes. The junction parameters of the polymer-based diodes, as the ideality and the rectification factor, are strongly influenced by the dopant of the conducting polymer. In both forward bias and reverse bias, these junctions exhibit a significant, fast, and reversible response to NO2 gas, which can be explained by changes in the barrier height and in the charge carrier concentration of the PPy layer due to NO2 interaction.