Charge transport in doped organic semiconductors

We report an unusual transition in the conductivity of an organic semiconductor upon doping: For low doping levels, the conductivity of N,N,N-',N-'-tetra-p-tolyl-4-4(')-biphenyldiamine dispersed polycarbonate increases with doping in a nearly linear fashion, and shows an activation energy of 0.2 eV. At high doping levels, a superlinear increase of conductivity with doping is observed, and the activation energy decreases, reaching a low of 0.12 eV. This behavior is understood in terms of broadening of the transport manifold due to enhanced disorder coming from the dopants.