Electronic transport properties: I Conduction in delocalised, localised and polaronic states

Abstract

In this Chapter we shall establish the different laws required for electrical conductivity within a band scheme for organic solids (Appendix A-6 summarises laws for thermoelectrical power). Organic solids can be deemed semiconductors, or in more general terms conductors, especially when considering certain π-conjugated polymers undergoing development at the present time. The laws depend upon the nature of the conduction states involved, which in turn may be due to:

• valence and conduction band delocalised states (HOMO and LUMO bands in strong bonds);

• localised states associated with disorder that governs the depths of wells;

• states localised around the Fermi level in which transport mechanisms by hopping (Hopping to Nearest Neighbour, or HNN) or tunnelling effects (Variable Range Hopping, or VRH) appear; and

• polaronic states due to the localisation of carriers in wells with depths dependent on the ability of the lattice to deform, and with different laws appearing as a function of temperature domain.