Thermistor behaviour in a semiconducting polymer-nanoparticle composite film

Carbon nanoparticle-polyimide (BTDA-ODA) composite thin films were shown to have semiconducting behaviour with a pronounced negative temperature coefficient of resistance (NTC) over a temperature range 300-425 K, unlike conventional polymer-based composite films that show predominantly positive temperature coefficients of resistance. Electrical resistivity of the nanocomposite thin films decreased progressively by six orders of magnitude as the carbon volume fraction was increased from 1% to 10% and showed a characteristic composite behaviour in the smearing region prior to the percolation threshold. The dominant conduction mechanism was shown to be fluctuation induced tunnelling in the Coulomb gap with an optimum hop energy (W-opt) and a hop distance (R-opt) of about 23.1 +/- 0.05 meV and 15.8 +/- 0.04 nm, respectively for a 1 vol% nanocomposite, consistent with the T-1/2 behaviour in the model of Adkins (1989 J. Phys.: Condens. Matter. 1 1253) for cermets. The NTC parameters, characteristic of thermistor performance of these thin films, indicated that polymer based nanocomposites can potentially be considered for NTC thermistor applications. The materials presently studied, when compared with commercially available ceramic thermistors, were shown to exhibit smaller thermal sensitivity parameters, which originate from their smaller activation energy for charge transport. Polymer based semiconductive composites show promise for use in flexible and light-weight electronics, particularly since they can be easily mass manufactured and imprinted on microelectronic substrates.