Theory of Hole Injection and Conductivity in Organic Materials

The relationship between current and applied voltage is calculated for a process in which a given number of charge carriers of one sign is injected into matter which contains only a negligible density of charges of the opposite sign. Diffusion effects are taken into account approximately. The current-voltage relationship depends strongly on the image force exerted on the injected carriers near the electrode. The calculated current-voltage curve agrees qualitatively with that observed in anthracene under light excitation. The voltage at which saturation is approached can be calculated and depends upon a recombination velocity of the injected charge with the injecting electrode, and upon the image force. Different saturation voltages for various electrodes can be explained by a difference in recombination velocities. The dependence of the saturation voltage upon the intensity of the saturation current is also explained. For anthracene, the recombination between holes and a water electrode is expressed as a velocity much greater than thermal velocities.