On the superlinear increase in conductivity with dopant concentration in excitonic semiconductors

We show that the same factors that cause exciton formation in organic (excitonic) semiconductors, the low dielectric constant and the localized wave functions of the charge carriers, also control their doping processes. We compare doping in organic and inorganic semiconductors and show that the superlinear increase in conductivity with doping density should be a universal characteristic of excitonic semiconductors. The binding energy of the dopant electron to its conjugate cation in highly ordered perylene diimide films controls the free carrier density. The binding energy decreases with increasing dopant concentration because the neutral dopants increase the polarizability of the film. (C) 2004 American Institute of Physics.