The implication of Meyer-Neldel behaviour for oxidising gas detection in phthalocyanine thin-films

The dark conductivity of phthalocyanine thin-films are often sensitive to the presence of oxidising gases such that changes in conductivity magnitude are accompanied by shifts in the associated thermal activation energy for conduction. It is noted that the conductivity pre-factor and activation energy are invariably related by the Meyer-Neldel rule so that the conductivity becomes independent of the oxidising gas concentration at a finite temperature T0. Data illustrating such a relationship are presented for evaporated metal-free phthtalocyanine films containing various amounts of absorbed oxygen where T0 is estimated to be about 1161 K. Independent work has proposed that T0 may be related to the energetic width of localised band tail states that are exponentially distributed and it is therefore argued that the T0 magnitude for a particular phthalocyanine will dictate its potential application as a gas-sensing medium. Phthalocyanines that exhibit a low T0 may thus offer low threshold detection possibilities at the expense of response speed, whereas high T0 materials should be considered for situations demanding the rapid detection of high concentrations of oxidising gases.