The Role of Electrodes in Conduction and Breakdown Phenomena in Solid Dielectrics

The molecular nature of most insulating solids of practical importance is emphasized. As a consequence of weak molecular interactions, charge transport through such materials occurs by tunnelling between localized molecular electron states. The energies of these states are modified by local polarization and this is particularly important at a metal electrode - molecular solid interface where charge transfer to the molecular states will occur, even under zero-field conditions. Under high field conditions, the charge transfer processes are emphasized. The details of these processes, which are inherently time-dependent, provide a microscopic alternative to Schottky and Fowler-Nordheim mechanisms. The possibility of "hot" electrons being generated via the localized tunnelling processes and of cascades of energy being released as stored charge is induced to move in the solid under a high field is considered. The latter can lead to breakdown which, by reason of the localized nature of the tunnelling process, will be filamentary in nature. The importance of considering the electrochemistry of likely breakdown processes on a molecular scale is stressed.