NO2 sensing characteristics of copper phthalocyanine films: Effects of low temperature annealing and doping time

Low temperature annealing was performed on CuPc films to evaluate the possible structure transformation of the sensing films during the gas sensing period. The effects of heat annealing, as well as the doping time of NO2, on the sensing characteristics of CuPc films were investigated. The result shows that the structure transformation cannot be avoided even at a temperature as low as 100 °C. The structure transformation causes a decrease in film resistance and sensitivity to NO2, but an increase in response rate. After the NO2 doping period, the CuPc films cannot recover completely to the original resistance before doping due to the possible structure transformation and the deep diffusion of NO2 into the bulk crystal. The incomplete recovery problem of CuPc films can be solved by using responsivity instead of film resistance. The doping curves for the succeeding sensing cycles are nearly coincident, indicating that the change in film characteristics, resulted from the irreversible doping or structure transformation, can be eliminated by the present treatment. The experiments on the NO2 doping time show that a longer doing period causes a higher responsivity but a slower recovery rate.