Effects of Dopant Concentration on the Mobilities of Molecularly Doped Polymers

Hole mobilities have been measured for poly(styrene) (PS) doped with triphenylmethane (TPM) and triarylamine (TAA) derivatives with the same dipole moment. The results are described by a formalism based on disorder. According to the formalism. charge transport occurs by hopping through a manifold of localized states that are distributed in energy. The key parameter of the formalism is the width of the hopping site energies. For TPM doped PS, the widths decrease with increasing dilution while for TAA doped PS, the widths increase with dilution. The widths are described by a model based on dipolar disorder. The model is based on the assumption that the total width is comprised of a dipolar component and a van der Waals component. The selection of dopant molecules with the same dipole moment provides a method by which the van der Waals component can be determined from an analysis of the total widths of both. For TPM doped PS, the van der Waals component is constant while for TAA doped PS the van der Waals component increases with increasing dilution. The difference is described by a charge delocalization argument.