Applications of Organic Solids to Chemical Sensing

Abstract

Experiments to evaluate several factors affecting the performance of organic semiconductors as gas sensors are reported. The influence of the strength of surface charge-transfer interactions on the magnitude, rate and reversibility of conductivity changes is reviewed and data presented on reversal rate as a function of temperature for NO₂ on lead phthalocyanine films, yielding a desorption energy of 76 kJ mol⁻¹. For copper phthalocyanine, surface heterogeneity prevents analysis of desorption rates in terms of a single desorption energy. Effects of the physical form of organic semiconductor surfaces on their gas sensing properties are reviewed and data presented showing these properties independent of film thickness for lead phthalocyanine on rough substrates. Studies of the effect of humidity on the response of lead phthalocyanine films to No₂ are reported. At 12% relative humidity the conductivity increase is smaller than in dry conditions and proportional to NO₂ concentration up to 0.1 ppm, saturating thereafter. At 33% relative humidity, increases are smaller and saturate at lower NO₂ concentrations. These results are interpreted in terms of reaction of water with NO₂ in the gas phase and blocking of surface sites by water molecules, and demonstrate the need for careful calibration procedures.