Rotational Energy Barriers and Electron Tunneling in Organic Solids

It is concluded that nearly all charge transfer occurs by tunneling rather than by hopping in organic solids. However, the rate determining step cannot be the electron transfer per se in high resistivity, high activation energy organic solids. It is proposed that the rate determining precondition for tunneling in these compounds is a sterically favourable alignment between an “active donor site” on one molecule and an “active receptor site” on another, neighbouring, molecule; such alignment will occur by a hindered rotation, which is likely to involve only a portion of the molecule. Since the frequencies of such internal rotations are of the order of 10 11 to 10 13 cps, mobilities of the observed order result. A comparison between the activation energies of dielectric relaxation and electric conductivity yields at least rough agreement for 12 compounds. Attempts to correlate the drift mobility with values of the dielectric relaxation time are inconclusive.